Using Kelvin's strategy, the current is supplied and measured using I+ and I- leads and the voltage is sensed using separate V+ and V- leads. The voltmeter has high internal resistance so very little current flows in the sense leads and the voltage across the sense leads, V=(0)r, approaches zero... the voltmeter measures the voltage actually across the DUT. The current leads do develop an additional voltage, V=Ir, but this is NOT the voltage used to compute the device's resistance, R=(V+ -- V-)/I. As long as the sense leads are connected directly to the test resistance, R is rather exactly equal to that resistance. If you must do even better, you can add a guard shell around the current leads to eliminate leakage current and you can compensate for sense lead resistance by applying the voltage divider to the lead resistance and the voltmeter's resistance.

Thank you mr Dranfield for all your hard work getting these videos out there for us to enjoy and learn.Your wealth of knowledge and experience is invaluable.

Thank you for taking the time to provide the details and background info on this. I am definitely going to build one and will follow much of the layout you did, perhaps with some minor changes but mains powered and an LED panel meter will be part of my unit.

I want to thank you again Michael for alerting me to this circuit (and thanks to Alan Wilcox for the original design!). I have now constructed my own unit. I already had a suitable enclosure and the majority of components, so the whole thing cost me about £28 (kelvin leads & panel meter accounting for £21). I am truly astonished by the accuracy! I've powered it with 3 x 18650 batteries (including separate supply for the meter) and included recharging sockets.

Ha! What a gift! Thank you! I have already downloaded the July 2005 PDF and I plan to source my components soon. Top stuff! You are a star!

Thank you for this video and the fantastic source of radio and TV information! I was hoping you'd give us a schematic and here you've given us the whole magazine and a ton more stuff on top of that! I'm recently retired and am just getting started in radio restorations. I'll repair anything that comes my way and may do some TVs if I find some worth restoring. The links you provided should be extremely helpful in both my learning and my restoration work!

Thank you for sharing this, a great added bonus is the way you made it to run off mains, I look forward to having a go at it

Thanks Michael for the great video and very helpful instructions. I made one, works great. As yours, mine uses a small spare mains transformer with 2 secondary windings. This is very important as you mention because most digital displays use a common ground with the measurement circuit and supply. My meter is 12V supply but I see others using 5V or similar. A 7806 regulator gives a nice stable supply for the measuring part as per the schematic. The kelvin probes are readily available so all in all a great addition to the test bench, thanks again.

Great stuff! Your series is so practical. Thanks.

Thanks for sharing. Lots of awesome articles just love the old magazines.

World Radio History is a fantastic resource, I’ve been using it for some time. Considering it’s an American site, he has managed to scan an amazing number of British magazines and books. Thanks for a very useful video and especially for the advice about avoiding fake components!

Oh man, how complicated is that! It's just a constant current source and the good old LM317T precision regulator with just a resistor and just about any power supply found in the junk box would have done the same job. But I have to say: he did it very neatly and beautifully! A real gem of DIY!

I watched your video with great interest, where you found a SMD Capacitor shorted-out on that TV Time Control Circuit. You used your milliohm meter in order to pinpoint the short! I had never realised that you could do this. I have always removed many transistors, capacitors to check if the current flow had gone down in the circuit. Also, I have used cooling & heating various items on the PCB - to effect change (hopefully). Short circuits aren't real easy to locate, well, not for me anyway. Thanks for your contribution in teaching those that are well-under your level of knowledge! Greetings from Sydney.

Thanks for sharing - nice to see your embellishments too!

Thank You! I didn't know how usefull it is until I watched your videos, I've been repairing electronics most of my life, fifty at less( I'm 62)...

Thank you for tracking down the info Michael - much appreciated.

Very nice indeed. I will build this, in the form you describe as it is nicely thought out.

Pleasant and entertaining while very instructive. Thanks, Sir!

Excellent, Michael. Many thanks!

You can also find a self-designed from-scratch milliohmmeter from another great british electronics youtuber, Scullcom Hobby Electronics (video #31 and #43). I appreciate seeing your own design and it's source. That that website is preserving all those magazines digitally (for free!) is a great resource. I bet there are some really neat simple hobby designs scattered throughout the 1000s of pages of magazines you could peruse there. I'm pretty sure it doesn't matter if you connect the voltmeter's ground to the current sink's ground, as long as the voltmeter is doing a proper differential voltage measurement on its +/- wires. Completely isolating it probably doesn't hurt with stability though. Funny seeing a lot of comments exclaiming minutiae improvements. There's probably 100 different designs you could do for this (it's kinda fun seeing them), if yours works conveniently with enough resolution for your purpose then it's clearly doing a good job!

Thank you so much for this video. I just subscribed because of this. You really help people solve problems, and also, revive some forgotten knowledge.

Super video and good info.Keep up the good work and nice to see an old practical television mag on show, ahh Good days....Cheers...

Thank you for sharing that Michael. I am now turning on my computer and printer. 👍

Well done finding it. Thanks for sharing.

Thanks Michael for taking the trouble to find the printed article and directing us to getting a print of the article. Very useful video and I'm glad you've got your comments on - even if it means picking up some strange foreign comments (that need translating) along the way. PS Have now Subscribed to your channel.

Would be great to see other home made projects you built!

Thank you very much for this video, especially the links to practical television which I never knew existed I thought they were long gone. I will be doing a lot of nostalgic reading. I recently built a pocket voltmeter using an esp8266 in a plastic box powered by a mobile phone battery so that I could read voltage on my phone over WiFi. I may be able to adapt this project to work with it.

Thank you very much for this. Much appreciated.

Great inspirational video thanks Michael

Thank You Michael! Great video!

I’ve got one of the RS ones (it’s branded Rhopoint though). I rescued it from a skip at work, I had no idea they were that expensive!

Very big Thank You Michael. Always simple when you know. Thank You So Much cheers. 73

Dear Michael, I would love to see your meter in action. That is, measuring some low value resistors, for example. Thank you for making and posting this video.

Thank you sir ive been looking forward to this.

The leads do not compensate for each other. If you like, you can make your probes with 4 wires of different lengths and gauges. It works by generating a constant current which is passed through the resistor being tested. The voltage produced is measured by the panel meter. Since the panel meter has a very high input impedance, it does not noticeably perturb the current. There is no compensation anywhere.

Many thanks 🙏🏻 you truly are a gentleman 👍🏻🇬🇧

Great idea! Some time ago I wanted to build a simple frequency counter and function generator but I found that there is no point in building one yourself when a Chinese device perfectly suitable for hobby use costs less than what I'd need to spend on parts. With a milliohm meter at least I stand a chance of breaking even! ;--)

I have become accustomed to using the finger test to find out which is the component absorbing all of the power, but this is so much more refined an idea. First - this works even if there are poly fuses in the way that shut the power off before components get appreciably warm. Second, I find it difficult sometimes to work out exactly which of the components caused the painful burn (have been known to get the calipers out to try to measure the blister on my finger, to work out which component got hot).

Very nice project! Thanks for sharing.

Awesome, with important details.

new fan here! loved the video❤.

Thanks Michael from Italy

Refreshingly simple - it makes me wonder how much more is inside the RS unit!

That’s great I may have ago at developing a PCB for this. I have been thinking of buying one, the cheapest I have found is a Chinese one for about £130. They get good reviews but simple to make. Note some of the Chinese Kelvin clips are garbage but the version you have shown are ok.

This is a Kelvin Probe. The two wires should be connected to each side of the spring clamping device as long as there is no resistance from each side of the clamp. . You have to take into account the there is mili Ohms of difference going through the clamp.

As an old TV engineer myself I was wondering if Practical Television is still going these days? Great video by the way!

No I did not ask for this but you read my mind and thank you for this my bench is Shirley needing one of these

Hi, thanks for a great video, i built an adapter to go on my multimeter with an LM317 as a constant current source, but it seems a bit flaky. I use it to test the ground connection on mobile radio car aerials. I think i'll build your one as its an all in one device. With mobile radio, a decent earth is fairly easy to achieve at VHF and UHF, but under 70mhz it becomes a lot more critical

I designed a digital milliohmmeter and wrote an article and sold it to 2 different magazines in the 80’s and 90’s. One was Electronics and Wireless World Dec 1990 and Modern Electronics March 1989

The TLE2425 is pretty nifty--I'll have to use it in an upcoming project. But meanwhile, what would you think of, instead of using a 7806 on the 2nd power supply, rather use a constant-current regulator, like a LM317 with a 12.5 ohm resistor? (That, I definitely could cobble from my junk box...)

Great video Michael. Found your site by accident and loving the style and quality of advice - even if I don't fix TVs! Your link to the World Radio site is so useful and I have spent a few hours now browsing old editions of Wireless World which I purchased as a schoolboy and later a student in the 1970s. Where abouts in the UK are you based in case my LG OLED fails?

For short circuit fault finding it is better to have the output in the acoustic form (e.g. by the tone pitch,...) and with a relative reading mode (so you zero it on a value and it then indicate the lower resistances than that zero). In the 90's I remember such single purpose tester was published in our ham magazine ("A-radio" or the previous "Amaterske radio" named "Vyhledavac zkratu" aka "short circuit finder" or similar, it was published multiple times as various designs around the same principle). It used straight 2-pin connection, but the "zero memory" function took care of the probe wire resistance, so it did not need the 4 wire connection. It had just the beeper tone pitch as the main indication output and all the zeroing was by "remembering" the reference resistance as a voltage on a capacitor. It used a quad JFET opamp, one as the preaplifier, second as the "zero memory" sample and hold buffer (this was the part actually requiring the JFET inputs), 3'rd as a differential amplifier (amplifying the actual minus the stored value) and the 4th was the tone generator VCO. The sound output made the high current (30..40's mA test current from an 78L05) flowing for just a second, so the whole contraption could be made in the form of a probe supplied by a 9V battery. Components below 10Euro (my guess, prices according to the electronic component retail stores here). Today I would approach this using some microcontroller (an attiny45 or so, featuring a preamplifier for the ADC and working down to 1.8V, so it could be run from two AAA's), but keep the "user interface" essentially the same (a pushbutton for "zeroing", piezo for tone output, maybe just add a few LEDs as an additional bargraph style indication output), just implement the circuit HW way simpler and the "zero value" memory in a more stable digital form.

Awesome! Thank you so much!

Thank you!

It was a joy to watch this video ! And yes, us old farts like to build our own equipment 👍🏻🤝🏻🇳🇱

Creative video, thanks for sharing, like it :)

Good stuff. Thank you.

Thank you Michael :) I’ll be making one of those.

gracias señor

Brilliant

Nice Video!

Many thanks

When I was on the bench in early 70's for MOD I used a Marconi bridge to detect shorts from 5V to 0V which crowbarred the PSU. Usually had 100 DTL chips on a board and the short was always under a chip.

A small tip. When you have the entire magazine as a PDF in Acrobat Reader you can extract the three pages of interest as follows. Choose Print, select as your printer 'Microsoft Print to PDF'. In Pages to Print choose 'Pages' and '24 - 26'. Then click the print button and enter a file name.

Sometimes things that may look trivial like using a double cable for probes is not considered when designing electronic circuits.

If i want to run it on an external power supply (6volt). How do i separate the power if i want to build your version with a display? Do i need two power supplies?

I cant find the Television Magazine on that site now. Has it been removed? I still have some of the original magazines but have a lot of issues missing as my pocket money went missing too!

Great project 🙂 I have send you a email regarding a public link to the magazine you mentioned. Grtz

Thanks for the video, You can also get cheap Chinese PCB ones for £40.

Hello Michael! Where can I get a schematic to assemble one of these milliohmmeters? Or do you sell this model in the video?

Of all the lucky videos to stumble across!!!!! Excellent tip about fake components, becoming more and more common from China, sadly.

Hi Michael. Thank you for creating this informative video, this is going to be my holiday project. If you have time, I was wondering if you knew what the difference was between TLE2425CLP and TLE2425ILP. I'm not sure if they're interchangeable for the purposes of this project. The data sheet says the only difference is the device marking (2425C and 2425I respectively). I'm guessing the C or I before the LP is the version number? (LP refers to PLASTIC TO-226AA). Thanks in advance.

Thank you

Sir so nice and kind, sir what are the values of ur addition like bridge rectifiers, capacitors and linear regulators etc and how they were connected, 2ndly are the 2 mobile phone chargers of 5v not used? 1 for miliohm circuit and 2nd for digital meter, please guide as m a fresher. Also any substitute for TLE2425 ic..,. , Be Blessed and live happily ever

What voltage are the 2 voltage regulators? And how much voltage does this circuit inject in the circuit you are testing?

Thanks!

Hi Michael I enjoyed watching this video however your description of how a kelvin clip works is wrong one wire of the pair forces a current into your device under test and the other lead of the two wire set measures the voltage and because the voltmeter is of high impedence this doesnt disturb reading too much not sure of the impedence of the panel meter but your fluke handheld should be of the order of 10 Megohm impedence on the volts range hope that helps

Yeah I have to have one

While I do not plan on building this, I think a worthwhile upgrade would be to use 18650 batteries. They are rechargeable, commonly available,have good capacity and you do not have to mess with having to plug it in to use it.

It would have been nice if you credited Alan Wilcox as he designed the unit. It was his intention to make it as simple as possible and not require a panel meter as at the time of publication cheap digital volt meters cost as little as £2.50. The circuit only pulls 100ma when the probes are attached to a short, no need to worry about batteries, you can leave it on forever and the batteries will be fine. (Alan Wilcox was a personal friend of mine,)

Can I use two lithium-ion batteries and just run it on 8.4V instead of 6V? Maybe put a 10ohm resistor in series with the batteries? In the magazine they refer to using audio coax cable soldering the shield to the core at the tip. I could just get test lead ends and a stereo RCA patch cord cutting off one end, soldering them onto the test leads. Then install a female RCA connector into the case and be sure to run the 100ma high current through the outer shields. At that point one could run the audio cable core out with banana plugs to use with an external DVM or install in the case one of those cheap E-bay 200mv panel voltage meters into the case.

Any substitute for TLE2425CLP ic as unable to find in the local market.,., thx

If anyone downloads the PDF of the Magazine & you read, open it up in a reader, its pages 24-25-26 / 68 RS have the components. Chip - OP-Amp in stock 6/6/2022

I prefer to use NE5534 instead of TL071, less noise in the signal and therefore probably more accurate.

Hi can you show us a drawing of the power supply and list the components used please

Will a DMS-20PC-0-RS fit as a voltmeter?

hello Michael, a happy 2023! If possible, I would like you to send me some links to electronic circuit assembly sites and test equipment. Here in Brazil, we have little information on these matters!!! Hug happiness.

Are those the original fluks test leads they look like cheap ones?

Wow! That magazine started in 1950.

Does TL431 replace tl2425?

One can put a link in the video description tho :)

Where can i find tle2425 or can you tell me replacement for it?

I do not have TLE2425. Could I use TL431?

Didn't you write articles for Television mag?

Your Kelvin clips aren't exactly right. The two leads should each occupy a separate side of the clips, not come into the same side as you have shown. I love the proto build, very clean!

Actually, the current leads and the voltage leads can’t touch each other at the device being measured. They should be close together, but not touching.

Why two 51ohm resistors? That's 25.5ohm. Why not two 50ohm resistors?

That's not really what Kelvin probes are or how they work. Nifty instrument though!

If just finding relative resistance, wouldn't a constant current source (cc of bench psu) and dmm be sufficient? For absolute readings, use a precision resistance calibration sample.

Help me understand something... if a cheapo meter costs $40, why in the world would a meter with 4 terminals to cancel out the resistance cost hundreds? I was blown away by the simplicity of the circuit and I just don't understand the price jump....