As others have pointed out. It is important to remember that if you float your scope and then connect probe ground to a hot connection the entire case and terminals of the scope and anything else connected to it are now hot. You only need to touch any of those points and ground and you are fried. That is why you normally never float the scope but rather the DUT.
An alternative to be extra safe is to float both the device and the oscilloscope (using separate transformers!). Still you have to be mindful that your ground lead can energize your oscilloscope w.r.t. the device under test, so don't handle both at the same time in that case.
Well yeah that's pretty obvious from even basic electrical theory. It's more about parasitic components of circuits that actually become an issue sometimes!! But yes I do generally aggree with you of course!!
This was one of the shortest but absolute best videos I've seen of yours. Safety is number one, two, and three. Without proper safety knowledge, us techs can not do our job. Would love to see more videos like this. -Mr. Carlson's biggest fan.
I have had Earth ground running through my isolation transformer for decades and never have had a problem. I never plug my test equipment into my isolation transformer only DUT. Many devices have components referenced chassis ground. Running the Earth ground all the way through to the DUT helps to eliminate noise while testing. That said, Mr Carlson is absolutely correct in his presentation and I am fully aware of how my bench is configured. Thanks Mr Carlson
Just a beginner here, but I thought the correct way was to hook up your oscilloscope normally, and to the ground and use an isolation transformer (with no ground) on the device you are testing.
Thank you Mr. Carlson. If I ever get an isolation transformer like yours I'll be sure to open it up and cut the ground connection to the secondary side.
Thank you so much for the info about these isolation transformers. I did not know they would connect the separate grounds. I can not believe it is called an isolation transformer. You may have saved some lives with this video. I honestly would assume an isolation transformer is ISOLATED.
I fixed TV sets for 40+ years but never knew about this! Thanks for sharing this information. I always used the Sencore isolation variac with no problems but now in my home workshop I have a B&K TR-110 isolation transformer and it never occurred to me that I may be not be isolated like I assumed I was. I'll have to check this out. You may have just saved my scope.
I made this exact mistake 2 days ago on my oscope. After the bright flash I started thinking about it and then realized -of course the ground clip is grounded - duh. Then I started looking into prices of variacs that have an isolated transformer and then found this video. BTW, my Oscope does not measure correctly anymore - both the voltages and wave forms are incorrect now. i have not yet tried to replace the probe so hopefully that will be the only issue. Just wanted to say I made this exact mistake and your video helped me understand and verify why. Totally makes sense now. I did not realized that the barrel was connected to earth ground and was treating it like a multimeter. Thanks for the Video!
Possible that the board foil to the input connector burnt and possibly you can fix that and with a little luck there will be no other damage. Good luck and God Bless!
For starters you would normally use the isolation transformer to isolate the device under repair, not the scope.My isolation transformer the test plug is only connected to the secondary, the ground prong is completely open on the transformer.Mine is a Hammond manufacturing co, made in Canada.
I have a Hammond model 171B isolation transformer. The ground prong of the output receptacle is definitely connected to the input cord ground and the transformer frame on mine (I just checked). In addition, Hammond's own data sheets for the product show the output ground terminal connected: www.hammfg.com/electronics/transformers/line/171 (My unit is an older model with no circuit breaker, and with an outlet instead of a pigtail for the output).
I knew about the problem with the ground, nice to know the reason why to. Good explanation. That said your demonstration with the bulb and the probe hovering so closely to exposed mains with your bare hands, made me shiver and that in a video about safety........ :)
+Bart Zuidgeest LOL, When I watched the video I chuckled, my hands look way closer to those open clips than they actually were. Ahh....the magic of video. Thanks for your comment Bart!
A very interesting video Mr Carlson, it made me look up how houses are wired here in the UK. Once upon a time you either had a ground rod (some rural areas still do) or you had the house ground bonded to the sheath of the mains supply which went back to the substation before grounding out. The single phase domestic supply was one of the three phases of the national grid to live and the ground at the transformer for neutral so the earth and neutral were connected at that point. New builds and I think anything that is re-wired now has a system just like you describe with earth connected to neutral in the company fuse box at the distribution board. The other thought that comes to mind is that the earth connection on the secondary side of an isolation transformer seems to be absolutely pointless as it doesn't offer the safety that it appears to. The earth connection in an appliance is to provide a route to earth should anything fail internally and ground out the live causing the fuse to blow somewhere - either in the appliance, in the wall plug or at the distribution board. With more sophisticated systems with earth leakage or residual current detection that would trip out first. None of those things will happen if the live and neutral are isolated. But on the other hand you are unlikely to suffer much harm if you touch both the live and earth. When mine arrives I must test what potentials are there.
@@MrDoneboy This particular design of isolation transformer only isolates the secondary from mains. There is a big difference between a bench test isolation transformer and this. All these video prove is there's a whole lot of people who have no business playing with electricity doing just that.
I think this is a very under rated topic . I don't believe it has been addressed fully here which is understandable because this video highlights the difference between filter conditioning transformers and isolation transformer. We need to highlight links or create videos on practical techniques to isolate techs and equipment . More discussion is needed to increase safety when working on mains . There seems to be two topics a) human safety techniques while working on electrical equip and b) safely using test gear on high voltage without destroying equipment . Equipment requiring mentioning : isolation clothing , isolation transformers , differential probes.
I've seen many comments here about only the DUT being isolated from ground, however, when I was a student learning troubleshooting and repair techniques, the instructors at the vocational school cautioned us to never have the test equipment grounded. They sent us out to the rest of the world having been taught the very same thing Mr. Carlson showed in this video. Every piece of test equipment in that shop had all of the ground pins on the power cords removed. Every one of them. Meanwhile in the electronics theory class down the hall, they blew up a brand new 540 series Tektronix scope because of said grounding issue! I don't remember anything else about the incident so I don't know what they were measuring, how it was hooked up or what- just that they mentioned it to one of our instructors and then we were told about that precaution. After that the electronics instructors were removing the ground pins from the power cords or using the adaptors as shown here. Anyone care to explain why it is bad to float your test equipment? Not looking of arguments or heated discussion, just good explanations.
I was taking an analog electronics class at the local community college back in the 90's. My lab partner was in the air force and she said usually stationed in an AWACS. I was taking notes and she was hooking up the circuit. so she had the power supply on and somewhere in the circuit she said lets see positive goes to negative and before i could say no!, she connected the connection. there was a bang and smoke came out the back of the power supply. The instructor walked over expressionless. Picked up the power supply, unlocked a closet. He exchanged the blown power supply for a new one, locked the closet and w/o a word put the new power supply down on the bench in front of us.
Hi Mr. Carlson, thank you very much for this informative video! 1. Is it safe to disconnect the oscilloscope's earth wire at its plug, so that it only has a live an neutral connection? 2. Is it safe to disconnect the earth wire running from the ac mains supply to the isolation transformer's output plug, so that there is only a live and neutral connection available? 3. When do you need to use an isolation transformer? 4. Can you please do a video on how to connect an isolation transformer to a DUT, like a TV, amplifier etc. and also to instruments like oscilloscopes, signal generators etc. so that all of them can be used safely together to probe and repair a product. Thank you very much for your channel. I have learnt so much from your videos!
Thank you Paul for bringing to our attention the need to consider the nature of the supply coming to,our "Isolation" transformer, and the way that itself is connected internally.You certainly made me reappraise my modelling activities wherein I replicate different types of supply and their earthing arrangements -Tierra-Tierra TT, Tierra y Neutro Separado TNS , Tierra y Neutro Combinado [Separado] TNC[S].I work mainly from a step-down transformer (240 V 50 Hz. Input to 110 V centre tapped output). This type of transformer is commonly empoyed on building sites to afford a measure of safety and are often referred to as isolation transformers when in fact, because of the earthed centre tap of the secondary winding are in fact in metallic connection with a current carrying conductor. The term Protective Extra Low Voltage is more appropriate, as for it to be a Safety Extra Low Voltage transformer, as according to the On Site Guide to BS 7671 Section 3.4.3,/ 414.3 " ... have no live part connected to earth or protective conductor of another system."This clearly means a double wound transformer with no part of the secondary circuit connected to earth.A common example found here in the UK would be the shaver socket outlet supplies for deployment in bath/shower rooms.Saludos.
Balanced isolation transformers, which are sometimes used for audiophile and home theater systems, usually have a center-tap on the secondary which is sometimes connected to the ground connection on the output and chassis. Thus the output is not always isolated.
I was tought to always use TWO isolation transformers with no ground connection: One for load and the other for measuring device. Floating load can be measured with earthed scope but when ground lead is connected it becomes mains referenced. If you touch the device, you get shocked because it is no longer isolated from mains earth. Having also the scope floated via a second isolation transformer (with no earth), you avoid the problem. With that setup it is safe to touch any single point of a powered device. That is also safer than just isolating the scope from ground because in case anything leaks from "live" to scope chassis, it is isolated "live" ie. will not shock you. Never ever float only the scope and connect it to a mains connected load. Earth lead connected to eg. live wire makes scope chassis live which is very dangerous. As a kid I also made the classic mistake of connecting scope earth to live wire with all the sound and light effects assosiated. :) Never repeated that mistake.
As a note, in the UK our supply typically includes a separate earth that is handled way back at the generator, so earth and neutral have no connection within our premises.
Gary Richardson : Attaching one of the two wires of the mains generating alternator physically to the earth ensures that this wire is at zero potential (relative to the earth, which you are also touching) and thus safe to touch by you. You could not touch the other wire from the alternator or your body would complete the circuit to the earth, with risk of electrocution. If neither of the two wires from the alternator were attached to the earth, there would in effect be no continuously neutral or live wire, as both of the wires would take it in turns to be either live or neutral (for a duration which is relative to the alternation frequency of the alternator). It is safe to touch both the neutral wire and the frame of any metal cased and earthed machine (supply alternator also connected to the earth), simply because it is at the same potential as you, relative to the (elected) live wire, so if the live wire inside the machine should break and then touch the metal frame of the machine, the frame of the machine would then conduct the live current to the earth, rendering the metal frame essentially harmless to the touch. This is due to the very large voltage drop that the earth imparts to the circuit and hence machine case. It must be said that this condition would normally result in a fuse blowing due to the inevitably large current flow now taking place, assuming the earth wire is substantially large enough to carry the now elevated current flow.
An excellent explanation of something I was aware of (35 + years in semiconductor testing) but as you mentioned newer and less experienced techs may miss. I also want to commend you about the danger of just randomly defeating grounds as well. They serve a critical safety issue AND also a key point in shielding extraneous noise and signals in sensitive measurements. I'll share a story about a time in trying to measure picoamp level input bias currents on an op-amp and having massive 60Hz noise problems. I discovered(after a lot of time I need to add) the problem was on our very expensive computerized tester. A maintenance technician was tired of disconnecting and then reconnecting an elaborate bonding wire system to make sure all metal parts of the chassis were grounded and shielded every time service was required. So he decided not to reconnect the bonding wires..leading to large parts of the chassis acting like antenna and radiating 60Hz into sensitive measurement modules. Again enjoy your channel a great deal. I'm not too proud to say that even after years of experience in electronics, there is still plenty too learn. I'm now retired and starting to get into restoring some old tube powered radios, a new universe for me!
+GrandsonofKong Great to hear! Thanks for the story, and glad your enjoying the channel. Sounds like the tech was a little lazy. That sort of thing causes future techs a lot of issues. Merry Christmas!
You have so much experience and knowledge in this field, and I trust you, so had to ask you about my next project: I'm thinking to make an a adjustable AC bench power supply. My concern is to have the AC stable, when something using it draws more than 1A (or more) of current. When I needed for my project AC, I just used 2 resistors, wired like voltage divider and also used 2 potentiometers - wire-wound style. It did the trick, but since I'm gonna play with more AC projects, I would like to make a nicer power supply that is stable. I do not need anything more than 10A (that would be at the most I can imagine for now). I have several transformers 120V to 30V, 16V, 12V outputs. One has both 12V and 30V in one. They came from a large UPS (Uninterruptible Power Supplies from 1200W and 1500W respectively). I'm not very experienced in electronics, just a new hobby after a 50+ years of other hobbies. I'm trying to rekindle something that started when I was a teenager. I do have 100Mgz Oscilloscope, 50Mgz Function Generator, several meters, 3D printer and lots of parts I salvaged from printers, UPSes ... I love to recycle and fix everything. So I'm not looking to make something that would require lots of knowledge that I lack of right now. I do play with Arduino's and simple projects..... I have several pictures of the transformers with their parts numbers and I know what voltage they produce. I could not find much about them on the Internet, not even how to connect the mains, but I figured it out - I think. What I like is a simple schema how to build the adjustable power supply and some protection for it; also what should I do to stabilize the output voltage.... I have these inexpensive meters from Harbor Freight that I can use to measure the output, so I don't have to even have a circuit for that. But if it has that, and it is simple and few parts, I am open to the idea also. You, or anyone else reading this, can contact me on my Gmail: czecht@gmail.com (Yes, I was born in Czech Republic), but I live here in the best country in the world! since 1981. Thank you Mr Carlson and thanks everyone else who would like to help me out!
Just a couple of comments: 1. You mentioned powering an oscilloscope from an Isolation Transformer (although, admittedly, you didn't actually suggest doing that). That should almost NEVER be done. (The only exception might be if you are actually repairing an oscilloscope, where the scope is the Device Under Test (DUT)) Only the DUT should be connected to an isolation transformer. 2. A standard Isolation Transformer can be converted to a "technician's isolation transformer" by internally disconnecting the transformer's secondary neutral from ground. This maintains the ground path all the way to the DUT (assuming that the DUT has a ground prong). It is much safer than using the cheater plug while also isolating the DUT from the mains. However, even with a "tech" Isolation Transformer, you still need to be careful not to do things like connect the secondary (output) to ground.
I won't name any names but there are 2 professional techs here on you tube that use 2 isolation transformers because it's safer. they plug both their scope and tv or whatever into an isolation transformer. They both work as professional techs servicing consumer electronics.
The isolation transformer I inherited from my uncle did not have a grounded outlet. As I recall, it had a 2 prong, non-polarized socket. Not sure when it was built, but I would guess in the 1950s, as most of the equipment I got from him seems to be from that era. Thanks for the great video!
Me Carlson, I’ve seen this warning/explanation many times but what I haven’t seen is a proper fix for the problem. I can disconnect the “earth” lead on the output side of my isolation transformer but I don’t think that’s the proper fix. What is?
I'm only new to this stuff and this is something that scares me because (although I've not had to use one yet) I know it's going to bite me in the ass some day... God I'm glad I watched this video I was going to go to bed but desided to watch one last video. Thank you Mr Carlson.🙏👏
Modern isolation transformers often have the neutral and ground tied together at the outlet on the secondary side of the transformer. It is a clip that can usually be removed.
Mr Carlson's Lab Hello and thanks for your videos! I am new to repairing electronics can you please explain why it's so important to keep the ground for safety. I keep hearing this but don't understand why it makes it more safe? Why is it more safe for the BNC connectors to be grounded or the chassis of the isolation transformer to be grounded? I am sorry for asking such a noob question I just don't see anyone explaining this... Thank you
The earth connection provides an alternative path for the power should a fault arise. Since it is always connected it will protect even with the appliance turned off or more importantly beleived to be turned off. The permanent connection also continues to work if the active and neutral lines become reversed as can happen when untrained people wire up plugs. A well made appliance will often switch both active and neutral lines to also protect against reversed plugs. Many modern products do not have metal exterior chassis and will employ what is called double insulation where the usually plastic chassis acts as a second insulating medium. Virtually all power tools are made this way these days and will not require an earth connection.
Thanks for that great tip Paul. It is always nice to be reminded of safety when it comes to electronics. You did a great job of explaining this.Have a safe and happy holiday and we will see you next year.
Great Saftey Video. Comment on Isolation Transformers, the older ones came with a non-grounded plug and outlet. One did not have to worry about grounding saftey issue. Keep em comming, Greg
Hi I have just come across this video I found it interesting as I have just this week been setting up my work bench with isolation transformer . You did a very good demonstration of what the problem is . But I am somewhat concerned about your proposed solution . What seems to be being disregarded is while you make your measurement if you have removed the earth wire from the scope power plug and you have connected the ground probe clip to the hot side At this point the steel case of your scope has become live in relation to anything else that is in your area that is connected to ground if you touched this case while adjusting a knob you would likely get a shock . If you had a data lead from your scope to a pc then you would then be putting high voltage down that lead on the ground into other equipment which you may damage . In the event that you are using as a demonstration I would use a DVM to measure the voltage between the conductor and the ground clip and ensure that it is a very low value possibly a volt or lower before making the connection with the ground clip I would not need to remove the earth from the scope . If one gets used to doing these basic checks before making a connection then it can save a lot of damaged equipment . I am in the UK so all out power is 240 / ref to earth regards bill
This video should be viewed in electronics schools today. I knew of this problem 20 years ago and didn't know how to get around without floating the scope. I think the should be followed up with a GFCI video.
Hi if I understand your diagram correctly. By connecting my equipment under test in the isolation transformer. I'm safe. Because there is no low ohm connection between neutral or hot on secondary of the isolation transformer and mains. And your videos are great.
I had recently bought a small "isolated transformer" for safety reasons per advise here (RU-vid) and in books on testing circuits with scopes and other earthed equipment. Concerned, I tested and was surprised and confused to discover the ground line connection on the IT. Thanks to your video and the "EEVblog #279 - How NOT To Blow Up Your Oscilloscope!", I now understand. VERY VALUABLE! Thx!! Using the light bulb also provided a safety check method. +Thx. Now, I need to understand why AC circuits/devices don't blow up since hot and ground (via neutral) are connected 60-times a second (USA). ;-)
Surging Circuits neutral does not go positive. The hot is alternating between positive and negative 60 times a second. The neutral is the reference potential. It does not change.
This is one of the most complex and complicated issues in electronics (for me). I've heard so many do's and don'ts and now I'm more confused than ever. So, HOW DO YOU HOOKUP YOUR WORKBENCH? Do you ONLY plug the DUT (Device Under Test) to the isolation transformer? I think that's what they recommend for working on old Ham radios that don't have a power transformer - that have a "live" or "hot" chassis. Or, do you ONLY plug your test instruments into the isolation transformer and have the DUT plugged into house power? Or, do you plug everything into the isolation transformer? I think I've read that that can be dangerous, too. In any case, I guess it's a good idea to have a GFCI (Ground Fault Circuit Interrupter) (like used in bathrooms) multiple outlet bar to have the fastest shutdown of power if an imbalance in circuit power is detected. Another question I have is - do I need (would it be useful to have) one of those Siglent SDS1202X-E scopes to troubleshoot a bunch of old Tektronix 465 oscilloscopes I have? I think I'm looking for an excuse to buy one. I just checked out your Tektronix TDS 340A scope, too. I can get one of those on eBay for about the same price as the Siglent. The advantage being that the face-plate is arranged in a more familiar pattern like other Tek scopes whereas the Siglent is somewhat confusing to my eye. My rationale is that I don't know how functional or accurate any of those old 465's are and the Siglent or the TDS 340A would be new(er) and better able to give me more accurate readings, to troubleshoot those old 465's, wouldn't it? Maybe? Or would I be better off spending my money on other things?
In my universal phase motor ECU, I've designed it so that the chassis is directly connected to the safety ground, thus meaning that I can effectively touch the chassis with both hands and not get a shock. It also means that any capacitors holding their charge should discharge to ground once the unit is unplugged.
My understanding was that you should pretty much never float your scope, either through an isolation transformer, or especially from ground. Keep the scope plugged into the wall, and pit the device under test on the isolation transformer.
PelDaddy that was my first thought, put the scope i the wall, and put the device under test into the iso.... if you have an older piece of equipment (like 98% or so of stuff on this channel) it probably only has a 2 prong plug anyway.... so the iso-ground issue isn't an issue....
Teb Tengri the reason is that with a floating earth scope the BNC connectors are still attached together and to the chassis of the scope. So if you would hook mains voltage to the ground clip of a probe, all the other probes/BNCs and the case would be live. Since you normally treat the ground clips etc. as safe to touch, and often clip it straight onto the nearest ground of a DUT, you make it very easy for yourself to mess up and ruin your day. If you want the convenience of not having to worry where you stick your probes, there is no substitute for a proper isolated probe.
I just got a topaz isolation transformer. Heavy! I'm considering installing this in my van conversion as I am running audio gear. This video is eye opening. Thanks!
I worked at a very small electronics store that had been around for 50 years. The hardest thing to get, vacuum tubes. None are make in the US. Some made in China, Russia, Poland. Some surplus found and tested. I've used a tube tester in my younger years. My local store was the same family that started it. Government surplus was its first product stock.
Good video! Even some test equipment containing a built-in isolation transformer and variac has the ground pin in the receptacles bonded to the ground in the line cord. My VectorViz ISO-V-AC II WP-30 is set up this way. The other option besides using a 3 to 2 prong adapter is to use a high voltage differential probe such as the Caltest CT2593-1. I use one of these when working on offline switching power supplies. The chassis will still be hot, and you will need to be careful, but you won't blow your scope or probes up as the differential probe is good to 1000V common mode.
Another informative video that reminds us to be safe! I had to go back to the schematic of my Heathkit IP-5220 Variable Isolated AC Supply to remind myself that it's configured just as you describe; always good to remember! Merry Christmas and Happy New Year Paul. 73 - Dino KL0S
I'm always surprised that people, (trades people I'm talking about) that work with mains/supply wiring, do not realize that neutral is referenced to ground. There are many issues that can creep up, especially with circuit boards/ICs, with this problem. Great video
I have that exact same model of variac. I also own a device called a one-AC line viewer, which is a transformer you plug in the wall and has isolated BNC outputs to hook to your scope to view Hot, Neutral, and Ground. More recently I purchased a fluke scope meter, model 99B I believe, but mostly because I needed a portable and battery operated scope but the isolation is icing on the cake as I work on high power broadcast transmitters. Anyway thanks for the informative videos, nothing here I didn't already know but many others don't and it could save a life, or a scope at least...
Great video. I have a variac and isolation transformer (no ground pin on the secondary of the isolation transformer) but most of the time I use my Sencore PR-57.
The only way that light bulb lit up at 8:28 is if there is a direct connection on the secondary between the neutral and ground. Current path is from secondary "hot" to bulb to scope ground to service panel to secondary ground and then through secondary coil to "hot" to bulb.
I (and probably lots of other people) would love to see you demonstrate the USE of a variac. People often talk about powering up vintage gear with a variac, bringing up the AC power to the vintage gear slowly (how slowly?) I'd like to see a demonstration of this, along with an explanation of why it is being done (allowing capacitors to reform?) I have been told that doing this helps to keep from damaging the vintage gear. Could you demonstrate this process and shed some light on what precisely is going on?
So my kinda layman's understanding of it is the following from what I remember from videos on the subject. Basically, the reason people care to supply lower AC voltage is because failed old capacitors in Tube gear have the potential to basically short the power supply to some degree and cause excessive current through the rectification section, including of course the rectifier tube(s). With bad enough capacitors and enough current/time powered on, you can potentially damage/destroy the rectifier tube(s) and transformers of old, broken gear in need of Cap replacement. As far as I can tell as an amateur, the ONLY reason to bother with a Variac is for 'testing' old tube electronics with presumably bad Caps that will overload the supply components. This is IMO stupid and unnecessary as the caps should be presumed bad (shorted, kinda) on anything old enough for Tubes and this seems like a complete waste of time given the only people who'd be doing this would also be replacing the certain failure-prone Caps (paper, Electrolytic especially, etc) afterwards anyway. It seems to me like you could skip this entirely by checking resistance and/or continuity from the input to the power supply section to see if it'll be partially shorted instead of just running it at lower AC voltage to mitigate damage versus full 120v. Anyway, the theory/practice behind the Variac is that lower AC voltage will proportionately lower current through the device with faulty Caps and thus lower heat/damage to sensitive components. I don't see why you couldn't instead just limit current instead with full 120v line with an inline fuse or something (like lots of devices have at their input, what a coincidence!) if you're gonna be 'testing' equipment that you suspect is faulty and liable to be damaged plugged in. So yeah, basically it's that simple. Lower line voltage equals lower fault current which is very common as old Caps are notorious for failing internally (basically turning into resistors/jumpers) and is by far the biggest problem in Tube gear not working or working poorly. Any vintage tube equipment you intend on using needs replacement caps (again, certain types only 99% of the time) or they're liable to burn out the rectification and transformers. Not exactly catastrophic for the vast majority of cases but maybe a bigger deal because Tubes are scarce.
@@Demoralized88 Thanks for spending all that time and energy to reply. (I'm surprised that this guy hasn't bothered to answer my comment.) But I've been an analog circuit designer for decades. I know how Variacs work. My question had to do only with the apparently common practice of slowly (how slowly?) bringing up line voltage to (perhaps long-stored) vintage equipment to reform caps, etc. I'd like to see this actually being done by someone who knows why it's done and who will provide commentary during the process. All of my experience (50 years) as an electronics guy have been with new equipment or things that have been in daily use since they were new. I want to restore some cool vintage audio gear and vacuum tube based test equipment that I've bought on ebay, at ham fest's, etc. So, I welcome input from people who do this slow warm up thing. It makes sense to me in theory. A piece of gear could have some extreme fault(s) that could cause damage when power is applied. I imagine that in addition to the capacitor-related issues, there could be other things. I think that using a Variac would allow a guy to initially trace through circuitry without having it blow up in his face.
So far loving the Tech tips Tuesdays. The Tektronix manuals discuss this now in the warnings. I had a run in once with thinking I was isolated and being told I was isolated but actually wasn't due to this same condition. My experiment didn't match your nice light bulb example. No, I had the blue flash once I got the alligator near the DUT. Lucky for me I wasn't hurt and I was holding the rubber shell. I now double check with a voltmeter first for any potential difference. The probe and scope were saved by a semiconductor going into escape velocity.
I bought a 1000 watts hospital grade Isolation transformer and opened it up to find it was grounded !! I removed the ground wire inside . works awesome ! its parked up and under my bench .
Hey Paul, maybe it's worth mentioning, that also many variacs exist, that ARE insulation transformers. Toroidal transformers with primary winding and a secondary winding, that comes with the brush... I do have one of those transformers in use in my lab since about 10 years now, and it works great. Also, beside the risk of accidently connecting ground level to your DUT via a scope, it could also easily happen via other units, e.g. sine generators etc... For testing my guitar amps, I selected all my necessary units to be NOT ground connected - which is an oscilloscope, a sine wave generator, a loop station with an instrument signal, a dummyload that includes a wattmeter and that has an insulated output that is connected to my recording studio (for recording the result of 1 - 1,5 hrs test-runs of amps that I repaired, to confirm that they are working 100% correctly).
Hi Lieselotte. In all my time, I have never come across a "Variac" that has any isolation. I encourage you to test your isolation with an OHM meter to verify any resistance. If you get any resistance reading from the input to output, your device is not isolated. If there is isolation, it's not a "Variac."
@@MrCarlsonsLab Hey Paul! My Transformer is an insulated variac without any doubt! The schematic next to the terminals shows a primary winding plus a separated insulated secondary, that has got the variable tap. Also I confirmed this with an ohm-meter and, on top, with a specific device tester (it shows >20MOhm between primary and any connection in the secondary in the mode for "protective insulation", measured under 500V). So, there is definitely absolutely no connection between primary and secondary... If you are interested, I can send you some photos of this device. Also, at the tube-amp workshop I used to work years ago, we had such devices (insulated Variacs) at every workbench, though I never ever opened one of these units, so there might have been 2 separate transformers (one for insulation plus an extra variac without insulation) in them...
I enjoy your videos and you are very knowledgeable. I was very surprised however to see this video on safety and then your demonstration using the light bulb and the exposed line voltage of 120 volts. I only hope new people (hobbyist) to the field don't follow that example. Maybe at least a warning like: "Don't try this at home kids". Again I do enjoy most of your videos.
I break that primary ground loose and then I install a Heinneman breaker and a GFCI on the secondary side. If I want to get really fancy, I will isolate the whole transformer from the housing. I then tie the primary ground into the housing and I hook the secondary ground up to the isolated transformer frame, and bond the neutral to the core & frame but NOT the housing. I will have to use an orange isolated ground receptacle so the grounding plug prong is NOT strapped into the strap frame. I will also fuse the primary side.
@@MrCarlsonsLab That is why there is an isolated ground hospital outlet on a Heinemann breaker outlet and a GFCI outlet running out through a Schurter line filter. I have a sailboat inverter charger too. THAT SUCKER IS SUPER NOISY, unless it is running off the batteries, but at the dock on dock AC power it's beyond horrible for noise. Your radios are worthless on dock power through those.
Mr. Carlson, I have a power filter for my equipment, an isolation Transformer ( fixed after watching your video👍) A variac. I also have a current limited lightbulb when needed . My question is ,...Where is the best location to place my isolation Transformer? At the beginning of it all. In the middle or at the very end???
After viewing this video, I do have to say that Paul is on the money here! Plugging the DUT into the iso tranny is the smart thing to do. If the DUT has the circuit ground independent from the chassis ground, for the most part, you're good to go. Most older transformer-based equipment, TV's, Radios, amplifiers are much safer to do testing without fear of creating a deadly referenced path. The equipment's power transformer is providing the isolation. But, as he stated, working on AA5's or any series heater string set puts up a challenge. Isolated/Differential scope probes are nice, but expensive...and start at around $300+ American dollars. So far, I've never blown up a scope (yet)...then again, those are famous last words LOL :). Merry Christmas! :) Frank Ferraro--Audio Craft Electronics
I suggest you check whether your isolation transformer and\or variac (my "variac" is isolating) have fuses. One of mine did not have a fuse, so I had to install one. I blew it often since. The other even had two fuses, one on each primary lead, but they were 3.15A good to keep the office powered but too much to limit the damage to the DUT.
Thanks for the reminder! Safety is #1. As a 45+ year HVAC/R service tech, I’ve been “poked” by 24v, 120v, 575v, and even lightning (100’ underground behind Niagara Falls)-and I wouldn’t wish the experience on my worst enemy. Is the methodology of using “balanced input” (where you use 2 probes, with their ground leads clipped together and not connected to anything else, and their signal probes connected to the AC line) a safe alternative?
Hi Paul, wow I don't understand why that transformer is officially out in the market as certified isolation transformer. In Europe this would never ever get the declaration of confirmatory to be sold as isolation transformer. Following VDE/ EN norm the secondary side never is attached to protective ground/ earth. But anyway what ever norms are telling at all it doesn't make any sense...., or is there is something so much substantially different in Canada''s power distribution compared to Europa I don't know?? Anyway very interesting video ! Thank you. 73 Peter
+TRXBench There seems to be a lot of confusion about this even in europe. Mostly an "isolation transformer" has a earth connection on the secondary and a "safety transformer" has not, but has this clearly marked with a warning next to the outlet (and warns you not to hook up more than one device). But I have seen a "safety isolation transformer" (made by peaktech) that has a earth connection on the secondary, completely defeating its purpose (even false advertising imho). But I'd be very interested, if there are codes in Canada, that prohibit safety transformers without mains earth reference. After all these things are only for full galvanic separation (thus preventing any current flow out of the DUT into any earth around) during repairs under power. Nobody in their right mind would use them for anything else. But a well made and very important video nontheless. Thank you for it.
+Róbert Valdimarsson Hi Robert, yes very important issue and you are right a lot of confusion all over. I'm talking only about a galvanic protective separation transformer (Schutztrennung) according VDE 0570 Part 2-4, DIN EN 61558-2-4, EN 61558-2-4, IEC 61558-2-4
+TRXBench I agree fully, but politics and bureaucracy gets in the way of sense once again. If they want to include that "Earth" they need to re-label the transformer, and remove the word "Isolation" Merry Christmas to you and yours Peter!
+TRXBench Yes, you pretty much have to rely on the norm numbers here. I have a propper one here for up to 1500VA. I got it very cheaply second hand and quickly found out why: It does not have a inrush current limiter. So every time i switch it on, I have to make a trip to the switch box and reset the breaker :D But a limiter module is not too expensive. Still the best money I have spent so far, since it might very well save my life some day. Mery christmas and a happy new year to all of you too.
Great presentation as usual. Beginner here. I'm trying to get my bench up and running and really want to understand this concept fully. This video clarifies. But like so many others I'm left wondering what do I do with my oscilloscope? I have to attach my probe ground to the chassis constantly. What should I do? float my scope as well? I haven't seen a single explanation of how to proceed from this point. I use an isolation transformer. I make sure the object under test is floating and there's no ground to neutral. Then what? I have to buy a differential probe? What about a battery powered oscilloscope? Pretty sure we're not far from that being commonplace, outside of automotive scopes. Will that solve some issues? Please explain to me how to proceed and get my bench up to speed
Paul, I want to first thank you, for all of your informative videos. Thank you, so much , for explaining about modern day ISOs. I own a VIZ isotap model WP-26A 400va. This was a gift, from an older tech. I seem to be unable to find any literature or manual, for the ISO. If by chance, you or your subscribers have any information, I would be rather grateful. If you have one of these older ISOs, maybe you could do a tutorial... Thank you, so very much and Happy New Years.. Cheers!
I just received a Jameco isolation transformer this morning. It has 2 isolated outlets and 2 non isolated outlets and the standard 1 to 1 transformer. The primary is earth grounded to the ground lug on the chassis along with the power cord ground. However the 4 outlets are grounded to the the chassis lug and they are all tied together at the ground terminals with a non insulated .026" fuse wire. I cut the fuse wire between the isolated and non isolated outlets so the non isolated outlets are still connected to earth ground. This is what I have always had with older isolation transformers. I connected the non isolated outlet grounds with 16 gauge insulated wire. The only danger is that now when using 2 scope probes they must always be connected to the same point since placing the reference lead on any point in the DUT is causing it to be grounded. So it is still possible to short through the scope, making smoke and sparks. You must always be aware when probing in a powered up device. I am not an engineer so I do not recommend anyone to do this.
And that is why seasoned service people install ground rods and ground halo, and separate neutral and earth in their mains drop. It also adheres to electrical code, which is good. Well maybe not in California, but, hey..... seasons greetings to you mr. Carlson!
In Australia it is normal for the output socket on an isolation transformer to be NOT connected to earth, if you need an earth on what your working on, then that has to be provided separately. Anyway I would NOT use one to float a scope, or any other test gear. If you do, and connect the probes earth clip to any substantial voltage, then that will appear on the outer metalwork of your scope, as in the WHOLE chassis will be LIVE, very dangerous indeed. If you then go to connect a second lead to another input, you will be touching a live socket, and you will get a shock. If you want to look at the mains using a 'normal' earthed scope, then tie back the probes earth clip and just connect the probe tip, using the scope's earth as the return path. Also before doing so check to make sure you are not exceeding the voltage specs. In the US you only have 120V, in Australia it's 240 on the active/live pin, so often an X100 probe is a better choice. Some early digital scope's can only take 5V directly into their BNC, check. Another method for a mains look-see is to use an AC output power pack (wall wart in the US) with say a 12V output, this can be handled with pretty much in total safety, and give you an X10 display, same as an X10 probe. Toroidal transformers seem the best, some small & cheap EI types, especially the ones that run warm, can be running a bit close to saturation and can round off the tops of mains crud your trying to observe. Another problem with floating equipment, is that the chassis will probably be sitting at anything other than earth, this is usually due to stray capacitance in the power transformer, and is usually exacerbated by mains filters that can easily put the chassis at half mains, whilst there is little to no current seen here, the static charge contained in a largish chassis, and the stray capacitance, can cause a current surge/voltage spike when you hook the earth clip, or an X1 probe doing damage to your gear. I once connected a floating video camera into a video input on a computer capture card, the spike destroyed a few hundred dollars of equipment - it does happen, floating gear is bad.
I used to work with an old guy who always came to work with one rubber glove........he used to put the DUT on the Variac and always wore one rubber glove and stand one the rubber floor mat under his bench.........he lived to a ripe old age.
Many years ago we did some testing on the Variac (or Powerstat) ramp-up / power-up method. The sample DUTs were 5U4 and 5Y3 and 80 rectifiers. What we determined is that continued "ramp-up" turn-ons can lead to cathode striping. Apparently what's happening is that as the filaments are gradually being brought up to temperature, while the B+ is also being applied. With the high potential of the plate voltage and the Cathodes not at proper operating temperature, that cathode damage eventually occurs. This was from many repeated cycles to prove a theory. Obviously, if this condition is only done once (or a few times) during the repair process, then the damage would probably be non-existent. Anyhow, I just thought I'd share that info.
That looks like a medical grade isolation transformer Paul. That is typical wiring for these I believe. I use a Sencore PR-57 isolation/variac for my bench work. I bought it on ebay years ago and had to open it up to repair the switch bank for the different display modes. I honestly didn't even look to see how the thing was electrically wired for the output. I think I'll do that now. But I don't see why Sencore would wire their unit as your example is. But who knows... Thanks for the heads up and Merry Christmas to you and your family! Love your videos. Tom
Thanks for the informative video. Here's a thought for you: You have an oscilloscope and a function generator plugged into a power strip and you routinely check your wave form by directly plugging one into the other with a bnc. we all do it, no problem... Now you change out your fancy new generator for an older tube based one that has a non polarized plug, the transformer is center tapped to chassis ground.You are about to plug it into the power strip... You''ve got one question to ask yourself punk. Are you feeling lucky? Well, are you? The greatest piece of safety equipment in the shop is resting on top of your shoulders. Be safe you all.
My neutral rail is earthed with a screw into the concrete wall in my fusebox. When my house was built only the cooker and the fridge were even attatched to that earth and the wall sockets had no earth panel in them.
@si james GFCI - 'ground fault circuit interrupter', a.k.a. RCCB, - 'residual current circuit breaker' a.k.a. ELCB - 'earth leakage circuit breaker'. Often known simply as a 'safety switch'. It monitors the current flowing in the live and neutral conductors and trips if they differ. It won't work if connected after an isolation transformer.
This a great subject and I can relate to it. I was working on a project in the military and was using a variac and a scope. I blow up the ground and I should have know better.
paul, I completely agree with commentator TRX Bench. A transformer like the one you are showing is a deathtrap. Here (not often) this sort of transformer is used for the lighting and plugs in bathrooms and such. Our electric code specifically states that NO EARTH connections can be in the secondary circuit (for obvious reasons) Of course if your country uses that 'split phase' system, this adds to the confusion, so much so, that one ends up with polarised leads on AC! The mind boggles! Cuddles!! Paddy
I believe your drawing of the Auto Trans was over simplified. Per the scale on the trans the output is variable from 0 to 140% of the input. Therefore the AC main should be someplace lower than the auto trans tap.
Thank-you for another great lesson. I have a concern regarding your presentation of the autotransformer variac. Autotransformer by definition has a primary and a secondary except that the primary and secondary are connected together in an electrically common manner. That is what prevents isolation. Many autotransformer variacs allow for a secondary voltage above the line voltage 140 to 150 volts. That being accomplished by additional turns ratio on the secondary. Regarding the illustration you showed I believe I may have some 40 volt variable transformers like that. I haven't given much thought to this until your video. I'll have to inspect them again.
Thank you so much for this and for all your videos. I understand that, when you restore old test gears, you usually prefer to avoid connecting a cable with ground pin. I understand the concern expressed in this video, but I wonder if there are other situations in which the ground connection could be counterproductive (or very dangerous). Thank you for your attention.
11:14 Paul: "This little adapter could save your life, but whatever you do don't use one, because legal niceties are so much more important than actual safety". Another issue Paul didn't mention is having two isolated outlets on the same transformer. This is dangerous,. For example imagine two hot-chassis radios , one in each outlet, and the phase of either one was reversed, there would be full mains voltage across the two metal chassis. So if that transformer was mine, I would open it up and remove the ground connection from the outlet, and disconnect the second outlet completely. I'd rather be safe than compliant.
I have a Really Old "Hammond" isolation transformer, model 171B60 that solves this problem. It has just a 2-wire male plug on the AC Line input, and a 2-wire female socket on the transformer output. Got this transformer from my uncle back in 1974, and I'm not sure how many years old it was then (I see the 'newer' Hammond 171B are 3-wire input, and 3-wire output)
I made my own box with an isolation transformer, some bulbs which can be individually engaged and disengaged and a volt/current meter. However, I have a question. The upside of the isolation transformer is that if you touch the live coming out of it, you don't get electrocuted. However if you touch both live and neutral, you do and the RCD won't cut power as it's located before the transformer. At least this is my understanding. Do we feel that the main advantage of having an isolation transformer is worth giving up the RCD protection? I am honestly asking, not questioning.
Even though it's a floating output, still exercise caution. Never touch the two output wires of the secondary at the same time, as your body will complete the circuit!
Good tip with the isolation transformer, I've never seen one with the ground connected through!** I also don't like seeing a double outlet on there. One ground on one piece of equipment, and the isolation to the other outlet is removed. **well not with test type isolation transformers. Construction site ones are different
So dumb question then. What is the point of an isolation transformer if it does not solve the problem we are trying to solve in the first place? I just bought an oscilloscope and was buying an isolation transformer and then ran across this video. Wow am I confused now. What if I just plugged the oscilloscope into the wall with the three pronged adapter not having the ground connected? Total confusing now. Can you post a video on best practices?
Nice Video! I've a B&K 1655 isolation supply. It also shows running a ground from chassis of the device under test back to the Isolation Supply. The manual of this unit also gives a lot of info on the dos & don'ts and specifically warns of many of the concerns shown here and not to run the test equipment thru the isolation supply. You can d/l the manual is one wishes to print a hard copy. If you go to the B&K precision site you can download the manual for the 16655A. For some reason I am not able to paste the link here. -Bob
some houses have the ground and neutral hooked up in the electrical panel. It's against code to do so. the ground is supposed to be isolated to a 8 ft copper grounding rod going into the dirt outside the house. you can check how the house is wired by checking the ground to the neutral, and then ground to hot lead. when wired correctly, you see half the voltage on both circuits. when wired with neutral and grounds together, you see voltage drop on the ground, and main line voltage on the hot side.
