Build This Dim Bulb Current Limiter 

The parts list is missing. Perhaps something changed?

Can you give me some advice? I've been looking for a true power isolation device for years -- not just a transformer and surge combination. Something more like a 120VAC motor connected to a 120VAC alternator with an insulated axle so the output voltage is truly isolated electrically from the mains. Does something like this even exist? I haven't been able to find anything like it despite countless searches.

Why is it necessary to remove the break off tab from the switch? and what if the switch does not have a break off tab on it? then what do i do? Thank you, very interesting video and also informative👍

@FluxCondenser Would you consider building & uplosding a bucking transformer? (For the purpose of knocking down AU & UK voltages from 250v to a China friendly 220v and/or from 120v to 110v.)

Huh? No soldering was shown or advised in this video.

@@FluxCondenser 4:00

@@FluxCondenser I think he means where you tinned the leads

Thanks, Stephen.

What is it called???

Thanks Irwin, and thanks for sharing those memories. Great idea! If anyone wants to send a pic of their build let me know and I’ll be sure to publish it.

Glad to hear!

Fantastic, Randall! Gald to hear that. My pleasure.

Fantastic! Well done.

Thanks, Crimson!

Glad to hear! Well done.

My pleasure, Marty.

That’s about the best testimonial I could ever hope for. Thanks so much, Armstrong. And welcome!

Thanks for sharing your build, Andy. There are a lot of ways to go about it depending on what the user is looking for.

If you build one that way using only one duplex outlet, then you also have to break the tab on the other side of the outlet also and then use a jumper wire from the neutral terminal of the first outlet to the hot terminal of the second outlet before finally going back to neutral from the second outlet, otherwise you end up with reversed polarity on the second outlet, which is of course a dangerous shock hazard when testing certain equipment that has the neutral connected to the chassis, because with reversed polarity on the DUT outlet then you end up with hot being sent to the chassis of some older equipment!

@@JoeJ-8282 True, but you shouldn't be working in hot chassis equipment without an isolation transformer. I do have one and put it in front of the dim bulb tester if working on hot chassis gear. That would include modern switching power supplies. And safety issues aside, even with the chassis on the neutral you still have a problem with the neutral wire impedance back to the main panel where it ties to the service ground. This is more than enough to cause significant measurement errors with grounded test gear..

@@andydelle4509 I don't think you understand what I mean when I refer to "hot" going to the chassis of the equipment. There is NO piece of manufacturer built equipment that deliberately wires the hot lead to the chassis, (that would be just plain stupid and a major safety and even fire hazard!), but when I said you may eventually get shocked by having the hot on the chassis, due to the NEUTRAL lead of the power cord of some older equipment being connected to the chassis in certain circuit designs, and then your outlet box of your dim bulb tester, (with the way you have it wired), sending hot onto the neutral lead of the equipment being tested, because it backfeeds THROUGH the bulb itself on the bulb's neutral side, and then, (unless you have BOTH metal connecting tabs on your duplex test outlet broken off AND you wire it as I mentioned earlier, above), then you have the neutral side of the bulb outlet, (which essentially has the hot lead of incoming power on it because it feeds through the bulb first), you have that then going directly to the neutral side of the equipment to be tested outlet, which is not good, because then the hot can get back onto the chassis of the equipment under test on the neutral side of the equipment's power cord! That is why it is essential for best safety to wire the test box duplex outlet as I said in my first comment to prevent the polarity reversal on the second, (DUT), outlet... Now an isolation transformer will help with the danger of that and you getting majorly shocked as a result of having the test outlet wired backwards, but it won't be as safe as it could and would be if your test outlet was wired properly in the first place. In order for any electrical power sources to be totally connected in series, you must wire hot to first device, (bulb), then neutral of that to the HOT of the 2nd device, (DUT outlet), and then the neutral of the 2nd device/outlet back to the power source neutral, but that is ONLY possible, (i.e. with the correct polarity on BOTH outlets), IF BOTH metal connecting tabs on both sides of the duplex outlet of your test box are broken off/separated, and then you wire the neutral screw of the 1st outlet to the hot screw of the 2nd outlet with a jumper wire instead... Think of it kind of like a flashlight with 2 batteries in it. In order for a proper series connection to happen you must have the 2 batteries connected + to - in the middle, otherwise the circuit won't work. (Maybe that's not the best analogy, but hopefully you can see the resemblance, even though the voltages are totally different, along with it being DC in batteries, and AC with this tester we're talking about), but even AC has a polarity to it in relation to ground, as you prob. already know, and the hot side can be quite dangerous to touch in most cases. In the case of you always using an isolation transformer, that alone has saved you from getting majorly shocked with your test device as it is wired, but you wouldn't even NEED an isolation transformer in the first place to be safe if your test outlet box was wired as I suggested in the first comment. Anyway, I hope you can understand WHY I'm suggesting that you should change the wiring around on your tester's second outlet, because the way you have it now is ultimately not totally safe to use, and if you or anyone else was to ever use it as it is without the isolation transformer then you could potentially get injured, but if you fix the wiring to be like I described in my first comment then it wouldn't really matter if you used an isolation transformer or not, because it would then be safe either way, at least provided that the device you're testing also has a polarized plug, which most do. Of course you still CAN use an isolation transformer in either case, for the ultimate in safety and isolation, but if the wiring in your test box was properly fixed then it wouldn't be absolutely necessary for prevention of shocks, it would just then be more for the equipment's sake. I'm not trying to diss on your overall design at all, I'm only trying to help you make it a safer design for your sake!

@@JoeJ-8282 I am an EE of 40 years. I fully understand what you are proposing and may even do it. It's a simple fix. My career has also involved work on broadcast transmitters and large UPS systems. I am fully versed on deadly voltages and current. But no audio gear above an old table radio uses a hot chassis any more. "Hot chassis" is an industry term for tying one side of AC line to the chassis or signal ground. It does not necessarily mean to tie the hot side to the chassis. Some pre-1980s TVs still used hot chassis as they needed a cheap source of over 100 volts anyway for some of the circuits. But in any transformer or switch mode power supply fed device, the chassis ground is isolated from BOTH sides of the AC line. But once inside any gear, you should use an isolation transformer. Especially today in that many switch mode power supplies are build on the same PC board as the rest of the electronics to keep costs down. You have many exposed internal components that may be direct to the AC line, both sides. You also can't assume the wall outlet is wired properly. It's quite common to find them reversed as well. Most videos here show dim bulb testers with open terminal pony sockets mounted on plywood. Talk about safety hazards!

Thanks, Ritchie.

And this channel loves you, man! Thanks again for your continued support, Dave.

Thanks, Connor. Good luck with the build. It’s a fun, simple and rewarding little project.

@@FluxCondenser just finished the dim bulb tester today and it worked like a charm! I’ll be sure to purchase the volt meter through your Amazon store tomorrow. Thanks for the hours of entertainment and helpful videos. It really helps beginners like me out! Much appreciated :)

@@PrimeHiFi Thanks for the support and kind words, Connor. Glad the dim bulb tester came out well. Take care!

Thanks!

Thanks! The Tripp Lite isolation transformers seem quite good. There are three models available which are similar but have the following different wattage ratings: 250W, 500W, 1000. The higher wattage units are naturally more expensive. For most radios and basic stereo equipment, the 250 watt unit may be fine. If you’re dealing with larger amps, etc., you’ll need to go higher depending on your needs. Figure out the wattage of the most demanding device you’ll have on your bench and go from there. A little extra cushion wouldn’t hurt. Remember that these units don’t isolate the ground, so if you need that, use a simple cheater plug. Tripp Lite isolation transformers are available on Amazon, and you can find more info here: www.tripplite.com/products/isolation-transformers-standard~25-112

Thank you so much for the prompt and courteous reply. I feel I have learned so much from you good teaching.

If it doesn’t state that it’s an isolated unit, it’s not going to be isolated. Most Variacs are not. If you do find one that claims to be isolated, though, it’s a good idea to double check. You won’t really need to disassemble it. A simple continuity check will do the trick. Isolated Variacs are hard to come by and I believe the one I own has been discontinued.

Thanks, Ron. Glad you liked the video. Yeah, I find that little voltmeter indispensable in this setup.

It’s not a traditional schematic, but I do show a diagram of current flow which amounts to the same thing. Further, if one can’t understand the wiring of such a simple device, I’d have to wonder at not only their need for current limiter, but also their ability to be working with high voltages at all. Truly, if someone has a problem building this thing from the instructions I provide, they have no business building this thing in the first place.

That’s a good question, Richard. In this setup, the bulb serves two roles: 1. Provide a low-resistance pathway of current for a device plugged into the current limiter that is operating normally. 2. Provide a safe place for current to go when a device plugged into the current limiter is drawing too much current due to a short, etc. Under the first scenario (when a non-shorted device is plugged into the current limiter) the bulb never receives enough current for the bulb to draw its maximum wattage (it glows dimly). This is because the circuit to the lightbulb has the resistance of the device plugged into the current limiter reducing the current to the light bulb. It’s like wiring a light bulb normally, but introducing a resistor to the circuit. In the second scenario (when a shorted device is plugged into the current limiter, which is basically what the current limiter is designed to provide protection for), the bulb does receive enough current for it to draw its maximum wattage (it glows brighter). That’s because a shorted device plugged into the current limiter is basically the same as wiring a bulb normally with no resistance. I still haven’t explained why the lower wattage bulb reduces more current in normal operation. That’s simply because a lower wattage bulb has a thinner filament than a higher wattage bulb. The thinner the filament, the more the resistance. The thicker the filament, the less the resistance. So using a lower wattage bulb is like adding a bigger (more resistance) resistor to the circuit. Hope this helps.

@@FluxCondenser Great! Got it now. Wasn't thinking of it in terms of a resistor. Makes perfect sense. Thanks. Keep the vids coming.

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@@FluxCondenser??? "gang sockets" means "recepticles" or "the holes the plugs go into" and the "gang" means more than one socket in the same housing since many sockets only come with 1 set of holes in Europe, UK, most switched of course since things are safer over here...???

My question is not what the simplistic terms “gang” and “receptacle” mean. But rather, how your comment pertains to this project. If you’re just pointing out some differences in availability of products and codes in the US vs UK, that’s fine. However, some readers might infer from your comment that using such products would negate the need for “such complex wiring” in this project.

I’m not certain where things went wrong, but the issue, evidently, is that the switch isn’t acting as a switch (enabling and disabling current. So use this to help your diagnosis. Perhaps do a simple schematic illustrating how the circuit should function and compare that to your wiring. Good luck.

And also, you may want to temporarily disconnect your wiring to do simple continuity tests on your switches and outlets. Put your multimeter in “beep” continuity mode and confirm which terminals open and close continuity (no beep/beep) when you activate the switch.

@@FluxCondenser Thank you! I went back and discovered there was a tiny sliver of metal left over from the break-away tab.

To be fully isolated, yes a cheater plug can be used to eliminate the ground.

That will isolate the ground, but your hot and common won’t be isolated that way with a typical variac. It takes more than an adaptor to isolate a Variac.

That Variac no longer seems to be available. A company called PHC enterprise once had that isolated Variac on the market and available through Amazon, but their web site and variac seem to be gone. If you want a new setup, I’d recommend getting a typical Variac (non isolated) and also purchasing a Tripp Lite isolation transformer to use with it. You could also take your chance with a used Sencore Powerite variac/isolation transformer on ebay. Those are good units, but you’ll want to verify the safe operating condition of anything you buy from ebay. Good luck!

Check to see that the model number on your box is E9802E. That box is designed to be used with what’s called a ¾” connector, but the actual hole in the box is much larger. If your box has a ¾” marking on it, it will work with the ProConnex 44741 NM/SE Plastic Cable Connectors which are 3/4-Inch.

That's what I bought from Amazon and it's the same number with 1" hole. Anyway thank you for video construction and advice.

Yes.

I don’t recommend conditioning capacitors and a dim bulb current limiter is not the device you would use for that anyway.

The short test just sees if there’s continuity. By continuity, we mean there’s a closed circuit and electricity can flow through the path under test. Of course nothing should be plugged into the wall. The continuity is tested using the battery in your meter. It simply sends a small voltage out of one probe and checks to see if the voltage is returned at the other probe. If so, there’s continuity and the meter will beep.

Absolutely! The voltmeter is completely optional. Useful, but optional. Just make sure you’ve got some means of double-checking the voltage coming out of your Variac/Isolation transformer. I’ve got all the parts to build this listed on my Amazon Affiliate page if you want to start there. Anything you purchase there helps out the channel. Check it out, if interested: www.amazon.com/shop/fluxcondenservintageaudioelectronics

@@FluxCondenser TY. After reading your reply and giving some thought, apparently the the digital voltmeter outlet can be used as a standard outlet (with the same voltage as applied by the variac to the other outlet). And, AAMOF, the voltmeter could be removed after finding the actual voltage being applied by the circuit and even another receiver being tested could be plugged in? (Not sure why I would want to, but just trying to understand how the circuit is working).

Yes, that’s correct. Both outlets on the dim bulb tester are the same and either one or two can be used. I can’t think of a use where I’d want two components plugged into it at a time, either, but if they were within the limits of the bulb and Variac, yes, that would be possible. I wouldn’t recommend it though, as if there were a short in one component it will put full current into the other. The real intention of the extra outlet is just to make it simple to add an inexpensive, yet reasonably accurate voltmeter.

Yes, that would be a useful addition. Simply plug the ammeter between the dim bulb and DUT.

@@FluxCondenser I was thinking of a way to add banana plugs on the hot line to measure with probes but then it would leave the line open if the probes are not used or just add an ammeter gauge somehow

I’d consider getting one of those plug-in amp/watt meters. They’re handy to have around the house and good for work on vintage electronics. You can use it with the dim bulb and without.

Richard, sorry to hear that. Fundamentally, though, if it’s similar switch it can be wired in the same way I show. It’s just going to be a matter of locating which terminals on your switch are intended for the hot connection, and switch connections. Once that’s established, you should be able to use my guide to make the proper connections. If you’re at all unsure, though, you might want to try to locate a switch that more closely matches the one I show in the video. Good luck.

@@FluxCondenser well it didn’t work so I guess the power coming in to side of double outlet was wrong side of socket. I got zippo out of anything no matter where switch was. Went to home depot and all they had was outlets with 2 screws on each side as well.

@@richardadelberg8961 OK, just use a multimeter to check for continuity while activating the switch. That should help you figure things out.

@@FluxCondenser shouldn’t I run hot wire to switch first then instead of double outlet and then from switch outlet to double outlets?

Yes, if the bulb was removed or the filament blows, the circuit would become open and power would be removed from the device under test.

Thanks, Charles, that’s a good suggestion. I’ll try to put a list together today and will either put it in the description or as a pinned comment.

Also, there is a special glue-in adapter available at Lowes or Home Depot, (or most other electrical supply stores), that you can use with that PVC box that has a rubber insert in the end that has a hole in it that's the same diameter as the round power cord, along with a threaded ring that tightens down onto it so the cord doesn't pull out. It is specifically designed for cords of this round type, and it is an even neater, more professional looking way to do the cord strain relief, and it works just as well as (or even better than), the black push in cable clamp that was used here in this video, except that with the glue-in one you don't have to modify the box hole at all first, you just glue it in with some conduit or PVC glue. The black cable clamp that was used in this video is made specifically for Romex wires going into a metal enclosure such as a breaker panel box.

@@JoeJ-8282 Joe , do you have a model number or picture for this part? I was unable to find it Lowes or Home Depot. Thanks

No, a short won’t blow the bulb, it’ll simply make the bulb glow to it’s maximum wattage. A light switch is nothing more than a short in the wiring. So, a short in the radio is nothing more than a light switch. That said, while a short in the radio won’t blow the bulb in a dim-bulb current limiter, the bulb will absorb a great deal of current which can potentially greatly protect the radio or other device from severe damage. A fuse, on the other hand, IS designed to blow in the event of a short, provided enough current runs through it that goes over its rated amperage.

If the winner is from outside the US, I will rewire it. The kit has options for different voltages and the UK will not be an issue.

@@FluxCondenser Understood. Thank you very much.

If the outlet is hot, then the limiter can provide voltage to the device under test. If you mean by saying, “the thing is dead” that the bulb doesn’t light, remember, whether the bulb illuminates is dependent on how much current is passing through it. In a circumstance where you’re using a high wattage bulb and there’s little resistance at the outlet, you won’t necessarily see the bulb illuminate. And, if there’s no load at the outlet (nothing plugged in or switched on) the bulb would not illuminate at all as there’s nothing to close the circuit.

@@FluxCondenser thanks for getting back to me. I meant that the volt meter isn’t working when plugged into limiter. It works fine when plugged straight into the wall, but I get nothing when plugged into limiter. I guess I must have wired something incorrectly, but it’s a little surprising to me that every continuity test worked perfectly…yet no outlet on the limiter is getting any juice. That make more sense? Thanks

Absurd comment.

@@FluxCondenser double ground wires on a single outlet screw terminal is against code in just about all 50 states. 4 more inches of spare wire (you had it) and a wire nut and it would have been legal, cleaner, and properly done.

Ha! You don’t “screw” around!

Judge: "You are charged with the purchase and use of an incandescent light bulb. How do you plead?" I have noticed though that my Lowe's is now stocking more incandescent bulbs. I guess they got tired of all the returns where the "60watt equivalent" has the light output of a nightlight.

@@andydelle4509 Yeah, that "watt equivalent" nomenclature should be banned. Just give the Lumen output - works for incandescent, compact florescent, LED, and the pants-on-fire of the advertisers. The wattage draw is a separate matter, and of interest only to those who pay the electric bill. 😉

Ha! I hear ya. I’m actually all for the transition to more efficient bulbs and with many types in my home I’ve had good luck. But, even now, I’m finding a lot of LEDs just don’t mimic incandescents as claimed. Especially in light output, color and dimming-ability.

Absolutely right, but I think we’ll be stuck with the soon-to-be outdated watt system for quite a while. Getting Americans to change their thinking on that will be like getting them to switch to metric!

Nah.

@@FluxCondenser Soldered lugs would look neater and give you scope to manouver the positioning to make it look awfully tidy ...

Uh, obviously.

Hm. I’ve had no other complaints and many successful builds of the device. Some people need things slowed down quite a bit to process though and you may be among that camp. Consider watching at a reduced speed which RU-vid provides an option.