Inside a mechanic's test probe - with schematic 

As an ex mechanic of 25 years that specialised in auto electrical I love these teardowns of stuff I just took for granted. Thanks Clive!

The brand name version of this came out in the early 90's. It was called a Power Probe and everyone loved them because you could quickly provide power or ground to relays without diagnosing the circuit fault. Basically you quickly know if the fault is before or after the relay, fuse, or whatever. Still to this day it is a must have tool that scores a 10 for most used tools and value. Now I want to take mine apart to see if it is the same.

Thanks Sr. Clive. The Switch is the weak point of these Power probes, Especially with high Current. The Switch is not designed to 'Switch" High loads. To save the switch Press the button BEFORE touching the tip to the Load under test. The contact points in the Switch will hold High current once connected together before the load is applied. The Arcing of the contacts is what kills them. They are a light duty Switch.. Mike M.

You did it again, Clive. You put me to sleep! I don’t mean to say your video was boring, not at all. It’s that damn, soothing voice of yours! I’ve had a really long day and sat down to watch your video after work. It’s only 1900 here and I had no intention of drifting off. I managed to regain consciousness and backed up the video. Yet another good one!

we had a brand name version of that at the shop i worked at during my apprenticeship. one very neat additional feature that one had, compared to what you showed here, was that in addition to lighting different colors depending on the polarity it would also emit one of two distinct buzzer patterns. very handy when poking around in cramped spaces or when you have already stretched your arm an additional 3cm because the thing to be poked is at the very back of some cavity and checking the LED is not really an option. as others have noted: despite its simplicity its an incredibly useful tool for quickly zoning in on the source of an electrical issue

I used to work with vintage and historic vehicles and I was always the workshop "go-to guy" for auto electrical stuff. I had one of these from a branded supplier, can't think of the name now but it wasn't cheap. I ended up being off work because of a spinal cord injury and loads of my tools went walkabouts, this being one of them. I might buy one of these cheapies to replace it as I still do work on my own cars at home as and when I can. Keeps me marginally sane... ish.

The most intriguing part for me was the rectifier. I never ever thought of putting dc through the ac input of a rectifier. How utterly ingenious. I suppose it just shows that I never paid attention in college.

As far as the positive- negative detection, I learned about 35 yrs ago how to make one that is super simple and works great. The leads from the battery are connected to 2 - 360 ohm resistors, the other leads of the resistors are connected together and connected to one leg of a bi-color led the other leg of the led goes to the probe. Connect the battery lead’s, current will flow through the resistors connected in series and when you touch the probe to ground/negative the led will light green, touch positive the led will light red, if the led is in circuit properly. Has been an extremely handy bit of test equipment over the many years. I first saw one offered to mechanics by Mazda in the mid 80’s, then with in a year Volkswagen instructor gave us the plans for building they out of multimeter test leads. Hope I explained it well enough. Thank you BigClive for all teaching skills you have been of great service to all of us.

Thank you, Clive. I was just working on electrical on my 6v vehicle yesterday and lamenting that I couldn't use my Power Probe. I started watching the video with an eye for what it would take to make this run in 6v. Then I read your description and searched for the product and sure enough, no changes required! I think I'll pick one up and maybe do some conversions to use the PP accessories, which are quite nice with banana jacks.

I like the one that Eric O (South Maine Auto) uses as it beeps different tones depending on whether it's a positive or negative voltage, so you don't need to take your eyes off what you are testing to look at the probe LED!

I’ve sold hundreds of the name brand Power Probe versions of this. Never even knew there was a generic version or tore one apart. Very interesting!

I love this simple test tool! I started as a auto electrician. Did some armature winding. Then became a electrician doing industrial switch gear panels. And later electronics. But my love was always electronics.

I wonder if maybe the extra diodes and resistors are there from another incarnation of this tester where filament bulbs were used in place of LEDs as the indicators for positive and negative.

I always wanted to take mine apart to do this. It is an awesome tool. Thanks for the video Clive! I have been a mechanic since 1992 and my forte is electrical diagnostic. When these came out I was very excited. I had built myself one that helped but it wasn’t near as nice as the one I bought. Mine has a 20 amp breaker in it and I have shorted it many times and you just push the button back in. Also mine came with a cig lighter adapter and a long extension for it.

Basically a logic probe for 12V systems. I made one of these years ago and it's proven itsely to be amazingly useful. Every car owner should have one of these

These have a drawback: they light up with very little current, so won't show you a bad connection (typically a bad ground)

Exceedingly useful. I wouldn't really do much on 12v systems without one of these to hand. Although unusually for me I have never put much thought into the innards of one other than the occasional reattachment of power/ground leads Where the leads are too short to reach the other end of a vehicle (BMW with rear mounted batteries spring to mind) I just take power from elsewhere, eg switch a tail light on and find a way of attaching the power lead to it. Least I used to. I have since cut off the clips, added a connector and created all sorts of adapters, bulb, lighter plug, smaller croc clips, even simple spade connectors to plug into existing harness connectors as well as creating another 4m extentsion cable.

I've been using 1 of these for over a year in my workshop , great tool , no problems with it .

Having this tool would have come in real handy for me last week. Wish I had known about it sooner. Very convenient to have a quick read for polarity or to supply current locally when trying to diagnose electrical issues. Might have to get me one.

Got one of these in my tool box, very handy for diagnosing electrical faults in vehicles. Quickly solves many common problems.

Thanks as always for a great Video. I try to make sure I have one of these type of PowerProbe type devices in any Vehicle I own, certainly one I'm doing a long trip in. I actually own 3, an orginal powerprobe 1 with no buzzer (black), the red ones had beepers on them. I got a orange Chinese one with volts, ohms display plus other functions They have got me home many times from being broken down by hot wiring your own car, they are so useful. Certainly the ones with a parallel body benifit from a small cable tie, so its slippable along the main body to hold the rocker switch down to free your hands. I can attest that a genuine power probe has supplied power to my 12v coil and driven 20miles home after total ign switch failure on a vehicle, with the probe jammed unto the +VE contact of the coil and the positive rocker switch held down for 20miles with that slidable cable tie ( can slide to hold down negative if needed too) They are the best thing to find bad earth's on vehicles...... Like you said just don't go poking around Vehicle ECUs with them or Can Bus wiring unless you know what you are doing..... An indispensable tool for overland trips or get you home from deepest greenlaning in the hills! Thanks again for the education of circuits over the years, better than electrical college, cheers

I just got this same one. It's not awful. Nice to know exactly how it works. Thanks, Clive.

You got me again Clive. Just woke up, with coffee in hand . Read the thumbnail title and my dyslexia got the better of me. I read 'Vieicle test probe' as 'Testicle Probe'. Nearly spat me coffee out!

Would have come in handy many years ago. Love the simplicity.

I use to work on old cars (re wiring and repairing electrical) and I have a unit called a Power Probe (kind of expensive) but it's a bit like this, it does have a display though to show voltage and it has a physical breaker that pops and you reset to fix. They also have a 25' extension for it and yes I have it and used it as I worked on lots of motorhomes and this allows me to use it anywhere outside, inside and on top of them, also they make some of the best wire probes I have seen (similar to Fluke but made for 3 sizes of wire common in automotive) and they hole they make is so small it's like it self seals. Very handy devices indeed.

Many people dont realize that a full-bridge rectifier has usefulness with pure D.C., and not just A.C. My favorite use for a full-bridge rectifier with D.C. is for use with a reversible D.C. motor. With the bridge connected up to the 2 the motor connections, you will get the same polarity output from the bridge no matter what the polarity to the motor is.

Nice vid! It is a so simple and powerful tool for electrical testing. Just a bi colour led, 2 resistors , a two way pushbutton and some wire

This is definitely something that Eric O. would use over on SMA. So simple and no mystery chips on board...very kewl!

I loved mine when I was wiring up my restoration project car from scratch. Although mine was slightly more complicated with a digital display/voltmeter. For some reason it didn't live very long though.

I like these teardowns! I didn't see it mentioned but for the probe tip, I think it slides out because on the name brand PowerProbe, you can either replace the tip or plug in accessories like an alligator clip cord or a 12v-to-5v reducer. I wonder how this clone would act with those accessories.

I've got that exact tool. Comes in super handy doing electrical diagnostic work.

You can buy these branded as 'Power Probe' as used by Eric O and his ilk ... cost about US$165 :)

ive had a power probe III for around 3 years and a couple of months ago i finally managed to kill her just because the solder joints for the tip where weakened and i put too much of a load with a 24v circuit i was testing for a bus... this was single handedly one of the most valuable tools you can have for electric diagostics if used consiously, it allowed for quick and dirty testing while also doubling for a voltage tester since it did have a screen... this chinese one seems neat for the price but ill still gotta get me another power probe...

Reminds me of the test lamps we had in telecomms, which was 2 50v lamps in series across the supply cnetre tapped for the probe, both lamps glowed dimly when connected (useful to know you had a supply) Having 2 led's in series would be a problem with matching current.

This exact one is availible under the british Sealey Brand to. Very very useful tool.

Clive.. I believe the diodes and resistors are to ensure the LED's dont give a false positive if accidentally touched onto a 5v circuit. Do you believe the volt drop across the Diode combined with the 200 Ohm in series could be enough to stop ghosting of the LED in such 5v cases?

Good afternoon Clive, sending you love from the other side of the planet. 😊

0:10 - It's not handy, Clive, it's BRILLIANT.

I have one exactly like that, it is very useful for probing/testing stuff on bikes and cars 😁

Inductive loads wouldn't be an issue since you only get a high voltage spike across the mechanical switch, and there is nothing vulnerable across it. And yeah I dont see any point in having the extra diodes and 200 ohm resistors either. Some people mentioned reverse voltage across the LEDs but the forward-biassed LED will always clamp the voltage below the reverse conduction voltage of the other LED - especially if the 200 ohm resistors weren't there. But even if there was enough reverse voltage, the current would still be limited by the 1k resistors and probably not damage the LEDs.

Nice review video

7:57 - "Be careful what you're probing." Good advice that applies in many circumstances!

I really appreciate your content thanks

There IS current limiting on that flying black clip... The wire will turn into a fuse in short order if you connect it to positive ;)

I already know what is inside my probe, Clive. Thanks.

In automotive videos here on RU-vid I've often heard these things referred to as a Power Probe I'm not sure if this is the correct name for it but I've heard it enough that I think it's the generic name for this particular tool or so it seems! I'm like to be able to come up with a design for someone like this that's not specifically for 12 volts though for troubleshooting low-voltage systems even with maybe integrated continuity tester and more and also just the capability of having a single test lead connection to allowed to determine whether the probe would be positive or negative possibly to LEDs on for AC for trouble shooting things more for electronic devices then Automotive. I envisioned something that would look almost like a logic probe except have a similar function to this type of tool that featured in the video! Also I have a lot of those Harbor bright test light kits I've modified for various things it makes a good housing for a one-off😮 piece of toast gear. One essentially is just a red and green LED and a suitable value resistor just to indicate polarity others I've got lower voltage bulbs is that original just so can use this for testing Marshall are voltages even than 6 volts. I should found out the continuity tester of theirs is also great for verifying do you miniature low-voltage bulbs that I use quite frequently even though it's only one double A battery is enough to like them and if there's a short that leads going to them or any of the wiring the continuity tester we'll come on fully and indicated there's an issue. I build all kinds of models and also help others with model railroad stuff as well not to mention do a lot of stuff custom when it comes to being an adult fan of Lego and no not always a purist but at times do strive to be it depends on what I'm going for and the project at hand technically even in some of the stuff at the Waco parks and more not all this is hundred percent Lego for example there is Metal Frames non Lego electrical in some cases even pneumatics so kind of throws the entirely like nothing out the window anyways so it just depends on how you feel about doing things when you do something outside the box anything's fair game. Then again everything has been or will be open-source eventually for this as well heck they even have a connector that's designed for other then their own accessories on one of the sensors for the powered up system if you know about it It's actually an industry-standard connector for sensors and so on which is totally awesome and if you look online you can actually find connectors that fit all there systems even newest as well as possible to modify other connectors to fit so it's just easier nowadays it's like there are more encouraging people to go outside the box and they used to it's kind of opposite of what it used to be however only reason they did not you stared Toco connectors on the Mindstorm systems that use the telephone like connection essentially it's the dec plug but the tab is on the opposite side it was a toy safety thing so basically you cannot plug something into somebody that might have higher voltages present and yes or why those clones and third-party things especially from China actually use standard Telephone Connections

i love my brand version of this! mine came with an extension lead that'd reach the back of semi trailers. nice to know i can get a MUCH cheaper replacement. BUT the name brand has a lcd display for voltage + or-- and the beeper can be used for sensed power for having to not look at the display

Love these things. Really good for electronics repair business. They should design them a to be a bit safer with lower current settings and a lower 5V output setting.

The reason being, the person that designed this circuit did not know what they were doing. If you like to use a different resistor for each LED, for instance to compensate for equal brightness, then both LED's should be in parallel. The diode - resistor combination can load the "bipolar LED" with a different resistance, depending on polarity. But both resistors are 200 ohm making the whole thing completely redundant. Knock it off the board.

A knock off of a tool I have used for two decades as an automobile technician, the "Power Probe". A very handy tool.

Useful video, thanks

Clive did you check at what current its internal protection trips? I hope is way less than the usuals automobile type fuse.

Would be nice if they had included a high load switch so you could pull the 12Vs down with 1A or so. I stopped using a dmm to fault find on my cars and now use a lamp on flying leads. I find the most common faults are connectors going high impedance, so will still show up as a good connection with a low current like a dmm or led but can't light a lamp. Old 5w tail section from a stop/tail bulb gives it a second lease of life if the stop filament has gone.

Clive, would it be possible to modify to a potentiometer so you can adjust the output power for testing lower voltage systems/items. Would be interesting to know if it could be done as it would be very useful. Thanks

I've made something similar for industrial sensor testing and setting for 24v. Ive put 15v zener diode to make leds off. So you get a string of led resistor zener zener led resistor. Ends to power and sens to center of string

I have the brand name Power Probe of this with a voltage display, very very handy tool!

The issue measuring device used by all Exchange engineers on selector switches was a test lamp. It plugged into the rack battery jack and had a three spring switch to reverse polarity of the probe. They were useless. Most of us would fix a lamp holder to either side of a sharpened piece of power bus. Used as the probe and the outer legs of the lamps were connected to +&- of a 50v power cable that plugged into the rack. We had some eddison style carbon loop filaments which glowed gently so you could stare at them while you tested connectîns.. So you had a common probe with 50v on the outside. So the two lamps glowed at the same intensity as you probed round the switch if you touched earth one lamp glowed. 50v and the other lamp glowed brighter. With a bit of practice you could measure the resistance of relay coils and resistors and know if it was right or not. Noisy contacts would flicker. They were far more use than a meter or the single bulb testlamps. Now they have a diagnostic rig that tells someone 50 miles away what is faulty and a taxi driver will drop the part in the doorway ready for the poor bugger who drives 100 miles to plug the card in..

Professional test probes use a buzzer to indicate power and ground. They use a different tone for Ground or Power. This is very useful

Got the same one had it for years to me a ex auto-electrician used to power up the distribution boards I used to design and build for ambulances and police cars for the blues siren interior lighting and video equipment. I could power up the separate circuits and check the out as my boards also covered the batteries management system but you stayed away from sensitive circuits

Considering the size of your hands Clive, that probe is huge. Think I may still have my old 12v lamp in the handle, ever shortening earth lead tester. ;)

Since the 13.2VDC is divided in half by the two 1k resistors, the probe's voltage to negative is 6.6V minus the 0.7V diode drop, minus the small voltage across the 200 ohm, minus the forward voltage of the LED. From probe to positive the polarity would be reversed and slightly different due to the color of the LED. So it looks like the voltage to anything you test would be less than 5V as long as you *never* press the switch. It seems to me that the two 'current steering' diodes and two 200 ohm resistors add some voltage drop to prevent the probe voltage from going above 5V. The switch is what could do the damage. I think I would put a 22 ohm 10 Watt power resistor in series with the probe tip so the current could never go above 0.63 amp, which should be enough current to light up almost all lights in a car. That would also make the PTC, etc. unnecessary. That small black wire and clip look like they may get in the way and be troublesome. Tape it out of the way, to the big cord with some black electrical tape. Thanks, Clive for the teardown of what seems to be a unique torture instrument. 🤣🤣

Clamp to mass Clamp to battery Poking the probe into the test object "BEEEEEP" "Yes, it's a vehicle."

I will guess that the resistors and diodes are insurance against overvolting the LEDs in normal use with 13.6V supply when the alternator is running, charging the battery. The inductor case you describe would probably kill one or both LEDs with or without the resistors and diodes.

Hi Clive, what role does the transistor play in this circuit? I didn't see it in your schematic but may have missed it. Great video as always.

I actually own one of these! Recently used it for testing trailer lights and connectors.

Looks good, but I would prefer to see some fuses on the main power leads. As you pointed out, there is the possibility of probing a positive supply with the negative terminal so I would certainly want one on the negative side, and for completeness one on the positive side too.

6:47 - If Clive ever becomes a character in Mortal Kombat, his fatality one-liner should be: "Get wrecktified!"

This video left me rectified, +1

Thanks Clive 👍🇮🇪

Awww...come on Clive, read the instructions. That's the best part.😀

12:35 I always cut the battery clamps off about 10 cm from the end and crimp spade terminals onto them. Makes it mega easy to lenghten/shorten the wires. And makes it easy to hook up more stuff too, using daisy chain spade terminals. I love spade terminals. I just always need to pay attention to polarity, and risk assess what’s going to happen if I yank on it and pull it out. You don’t want a male spade terminal hanging out with unfused positive voltage, for example. Also, they don’t handle high current over time if they’re being unplugged repeadetly. I’ve learned this the hard way😂

Seems to me that the reverse voltage limit of many LED's is quite low. I would have added the regular diodes as well although I'd gave to think about the need for the dropping resistors. 🤔

Eager to see at which point we're going to get a video that is both about carbonated beverages AND electronics. Perhaps see if carbonated beverages can undergo electrolysis. Now that would be interesting. What kind of gas/chemicals can be obtained by doing such experiments.

Very good timing as this warm spell has us mechanic wannabes out in force.

Another nice video. Thanks. Just a sidenote for our American viewers. 4 meters length of the test leads corresponds to roughly 18.4 bananas.

I've never been a fan of the LED probes for diagnostics on vehicles. Either i would use a multimeter or some kinda load (usually a 5w bulb) for testing, depending on the expected outcome. Diode probes tend to show false positives, when testing a high Q output.

I've been thinking about this. If the 200 ohm resistors and diodes were not there, as suggested, then it should work fine, as each LED would be protected against high reverse voltage by the other one. But with the resistors in place, reverse voltage would be 6v across the LED that was currently off (since no current down that path = no voltage drop across the resistor). So then you need the diodes. Which is just silly. Or have I gone wrong somewhere?

Temu has a similar device that has an LED voltage display on the side and a red or green led to show polarity. It has a simple wire piercing point and a negative earth clamp. It sold for around $4.50 USD for it.

Clive, I enjoy your analysis of reverse engineering. Can you show us how you take pictures and what software you use? Thanks!

Do you have the Ebay link for this tester?

Power Probe and knockoffs of it can cause problems when used wrong, used on 5v circuit, and more. Starter motors draws 120+ amps and Solenoids on the starter draws 30 to 40 amps and likely won't spin/activate with that "Breaker" or just burn up the tool. Other Motors & Coils can see a "dead short" to ground tested this way too. Many car relays have EMF diodes on the coils but not all so you need to watch polarity when test out of circuit or the Diode smokes.

Oh, the reason those leads are so long, is that us Mechanics need the length to chase down the electrical gremlins! LOL! Have a great day, Sir! o7

I have one of these with built in dmm, interesting how high voltage it can be used on, cause locos run 32v.

What I’ve been looking for is a cheap clamp meter that has, like, long leads or something so that I can sit in the car seat and read off signals whilst flicking switches/ignition on and off. And being a clamp meter I wouldn’t have to cut wires to test anything. Does this type of product even exist? Or should I build it myself using a current transformer and some frankenstein electronics?

Hi Clive, stumbled across your channel years ago when you had lithium battery bits igniting all over the place lol have followed you since that day, I was on ebay recently and bought a 6000 watt inverter for 35 quid, what a bargain!!! Needless to say, you get what you pay for! I the paid five times as much for a proper 1100 watt one, I've a suspicion the ebay one may not be what it says it is, is this something you would be able to review? If so, your welcome to check it's true capabilities lol, in other words, I wouldn't want it back !

I suspect the extra diodes are because the designer doesn’t trust the longevity of the LED’s when reverse biased. Nice to see them drop a cent on belt and braces rather than go extreme minimalist

I have a more advanced model that has a screen to show the voltage, and can also function as a multimeter. (Autel ps100) Very useful for troubleshooting basic vehicle electrical issues.

It would be interesting to see what the difference between the chinese clone and the original power probe that it's a knockoff of. I've got a sealey version and there are a few differences in using it. the LED blinks for +ve and comes on solid for -ve, and the beeper beeps in the same way. It does one slightly confusing thing though. If you probe one end of, say, a lightbulb when the other side of the bulb is connected to -ve, it will beep to indicate that you have probed -ve even with the resistance of the bulb. not sure if yours didn't do that purely due to it being LED? I'll pop the cover off and send you a pic to compare if you would like?

These are also a fantastic way for apprentice level mechanics (or those who just start poking at circuits without any care) to blow up expensive electronics on cars. My previous job had me working as the shop f-up fixer, fixing the issues that other mechanics installed in customer’s cars. I can’t even remember how many times other mechanics took their fancy PowerProbes ( or knockoffs), applied power to a low current ground circuit (or vice versa) and released the magic smoke from an expensive magic box. Regarding the weird diode/resistor/LED setup…this is outside of my pay grade, but a lot of automotive stuff is designed to work across a wide range of voltages. Typically from 9V to 24V (most heavy duty equipment or OTR trucks are 24V). Is it possible the added complexity is due to the wide voltage operating range?

I don't think those extra diodes will make things worse. The other half of the bridge will still clamp to something that's probably below 5V (typical reverse voltage rating for LEDs). Will it make things better, though? Probably, yes. Compared to 50/60 Hz line voltage, those spikes you're worried about might have a fast rise time. If you assume an inductive spike would rise to however many volts before the forward facing LED has a chance to conduct, the reverse facing LED might experience excessive reverse voltage. The non-LE diode then has a much better reverse voltage tolerance. Even if it might have a few uA of leakage, it would still have a big voltage drop in the reverse direction. If I were to muntz anything from the circuit it would first be the the 200 ohm resistors (and switch the 1k's to 1200 ohm). And second those diodes, if something else had to go.

I am sorry I missed the livestream, one of the first i've missed, mum was in the emergency dept, spent 7 hrs waiting in a corridor (also, screw the newspapers who say 'nhs keeps people in corridors'.. they have to, and the care is above and beyond even when queueing patients for triage. Mum's mostly ok, just very confused due to stuff... and re: the video, i was expecting something like 'if you press the wrong switch, the fuse in that circuit will blow'

Did they put those diodes and resistors in to reduce the turn on threshold??

I have a different version of something similar to this that is faulty has injector test 5volt supply and a scope, relay tester and several other functions.... can I send it to you for a tear down I would love to see how it works and what happened to it...let me know

Extra diodes: protection from overvoltage, e g. if the probe was touched to a wire from the spark coil secondary?

I own this very probe, so I'm very interested to see what you have to say about this one, Clive.

The extra diodes and resistors may be to protect against accidental powering of CAN networks without damaging anytbing?

The extra diodes and resisters on the LEDs might be to protect the LEDs, thus extending the life of the device.

Clive, thank you so much for your videos. I purchased this exact probe under the pretense that it had a buzzer. I have used power probes in the past and when you touch the probe to 12v + it would make a beep sound. This is very handy when you have to check a fuse box it cramped places. I was disappointed to find that this probe only has the buzzer for short circuits. Is there any way to "Hack" this probe to make the buzzer sound while touching the probe to 12v+? When i bought this probe it was $13.00 shipped and that is a far cry form the $200.00 they want for the actual Power Probe brand. I use this for one thing only and that is quickly checking fuse boxes in vehicles. Thank you again.

I have one that has a little tiny lcd display and can be a small oscilloscope for reading low voltage sensor output

I just have the standard test lamp probe nothing fancy like that lol thanks Clive