Thank you for the posting on this CIRCUIT (despite what one poster said here) and article. It inspired me to plagiarize the original 2 transistor circuit, but adding a third transistor, then, selecting the level of sensitivity by moving the test lead to the base of either the first, second or third transistor to control the gain. I found it will light an LED with about 5 Gigaohms of resistance between the 2 test leads, and can be used to indicate leakage in foil, mica or ceramic caps using a low voltage. It's debatable weather a full "rated voltage" test for leakage may breakdown an insulation fault that a low voltage tester possibly won't find, but finding a fault of that value will definitely indicate that the capacitor will have internal/parallel resistance that it should not have.
Tom - a very nice modification! Jim Newman, one of my subscribers leveraged a Darlington pair using 2N3904s as well. I didn’t see his final design, but I appreciate both of your contributions to keep the design simple and make it practical for the end user. All the best. Don
Hi Tom, Don from RestoreOldRadios referred me to your wonderful site, I took Dons leakage circuit and replaced his single transistor with a Darlington pair and it works very well, and very sensitive. I have checked over fifty paper caps and only one passed.
Good to hear from you on your tests. I haven't tried many old paper caps on the new (Darlington) tester but I have tested several hundred on the old PopCom design and they all had some leakage. Only the good Sprague capacitors (like those used in better Hallicrafters receivers) passed with no leakage. I first built the PopCom design in 1959 to find bad capacitors in vertical circuits of TV sets. Later I found it also worked to find leaky I.F. transformers that caused the same bias up set in the Video IF stages. Admiral sets seemed to have the most leaky I.F. transformers.
This looks like a very handy device to have available on the bench, to quick-check new caps before installing into repair/refurb work. (I agree with your position that you should check new items before installing. Failure rates are low, with items sourced from quality vendors, but they are there.) If a quick method of checking is available, it makes no sense to risk inserting problems into your work. Now I hope you’ll turn your attention to developing a similarly simple method to identify the outside foil end of the capacitor. It irritates me to no end that manufacturers do not mark the outside foil, at least on the higher voltage rated capacitors. How difficult would that be, and how costly, really? It is an important consideration in some areas of tube design and construction. Thanks for your continuing excellent videos. I’ve learned a lot from them.
I think that most manufacturers assume that everyone uses caps in low impedance circuits where the pickup through the outside foil is less of a problem. I did build a small switch box that makes it easier to test them using an oscilloscope and 60 Hz pickup. It appears in the Frugal Repairman series (around Part 12 - TTT281) if you haven't seen it.
Well done I like this idea. At 6:56 you meant: "a shorted capacitor" not transistor. I think this circuit needs further modification to distinguish between capacitors and electrolytic ones. This could be done by making a switchable circuit where there is the 390ohm calibrator resistor. With your low voltage circuit at least you've decreased the risk of the danger in everyday practical use. What's the point if a capacitor fails under the low voltage tester to test it with the higher voltage one? Should a capacitor not fail under the low voltage tester then one might choose to take that risk and test it with the higher voltage one. Thank you from 🇲🇹 Malta.
Thanks again for the videos. I have watched Mr Carlson's capacitor tester and it works the same way, detecting very small currents. There are several iterations of his. The question I have had in the back of my mind is that what if you get a capacitor that only conducts at high voltage. I know it's probably not common, but a simple spark gap will close suddenly
Thanks for the comment and excellent question. This new design only tests whether a capacitor has leakage, it does not stress it. To do that requires that you actually apply high voltage to the capacitor. Of course, when you use high voltage safety becomes a concern.
@@tomtektest8042 I understand the safety part. Nothing about tube circuits are safe by today's standards. No way we would allow cars on 2 lane roads if invented today either.
Hi Tom, I put together this circuit and got ready to calibrate but you never gave the value of the resistor string to which you calibrated to. I did go back the the video that you used the chain and the meter measured 147 M Ohm (150M ohm) I just wanted to confirm with you the calibration ohms. Also, what resisters is this ment for? Electrolytic and film? I actually put a 500 Ohm 20 turn trim pot at the 390 ohm resistor and put electrolytic in the test spot and tuned the pot to get the LED to dim. At 390 ohm, the LED goes out, at 400 ohm the LED will glow ever so slightly. (to let met know its working) I used the electrolytic to calibrate as I do not have a 150M Ohm resistor or enough 10's to solder together to get that high. I will order a 150M Ohm soon. Oops sorry, I just caught the part where you said you set it to set it so it will sense about 250M Ohm. (I missed that the first 2-3 times of watching the video.) You must have used a 250M vs the 150M. Thank you, I enjoy watching your videos, I have learned a lot.
Interesting idea. I test cable leakage of transducers (load cells) using a megohm meter and the debate has always been what voltage should be applied. Some of them use 50vdc, some 500vdc and some 1000vdc. I have always had good luck with the 50vdc version. I wonder how the applied voltage would affect the test results. Could a "good" cap tested with 9dvc fail in circuit @ 500vdc?
To test leakage I use my iso tfmr & variac to power a doubler ,with a neon "leakage" indicator like your first schematic, but I put an ammeter before the cap under test. The neon shows excessive leakage, but also as a normal healthy caps dialetric should have a constant resistance, therefore the relationship between the voltage it's tested at and leakage current should be linear yes? So if I test a cap at 450v, the leakage if healthy, the ammeter reading should be 45 something, with electrolytics this is usually 45uA, poly types 4.5uA. I use this and the neon to test for leakage, but I'm still testing at full working voltage + 10%. I can't see how a cap can be tested properly at a lower than wkg V, it will behave differently. Yes high voltage is potentially lethal, but if you need to test a HV cap and don't feel comfortable doing so at full wkg voltage, then you shouldn't be working on that sort of gear. I don't mean you "Tom", I mean in general. That said if a healthy caps V and dialetric R should be linear, maybe you could use a comparator to compare V and leakage I?....DA.
I always worry that someone with little experience or safety knowledge will think that if they see it on RU-vid, it must be safe. On the other hand, even experienced people make mistakes (I certainly have) and touch something that they should know better. You are right that the only way I know to fully test a capacitor is to stress it to its full working voltage while measuring the leakage. But this new idea from RestoreOldRadios (with the Darlington addition) should find most of the bad caps in a batch before you install them. I showed a 630 volt film cap in TTT338 that failed badly with just 330 VDC applied even though it had never been used before. But you are right, most modern capacitors are pretty reliable (if you stay away from the E-Bay bargains.)
Yeah I was wondering the same thing, I have seen videos on Mr Carlson's Lab where he shows how a leaky capacitor can still test good at a lower voltage other than what it's going to be working with or rated for. Carlson has his own tester but it doesn't use high voltage to test which kinda makes me skeptical about it's operation, it appears to work well and he's very intelligent but there's something about it that I just can't put my finger on why I'm not jumping on it, maybe it's the indicator setup? Maybe I'm used to seeing someone turn a voltage knob and seeing where the capacitor begins to struggle? Whereas with his low voltage design, the led meter lights up and you have to just sit and wait for something to happen? So the problem I have with this circuit is that it operates at 9V which is great for 10V capacitors, unless it's so sensitive that it's able to pick up current leakage in the micro amp range which might give a clue to what's in store higher up in the voltage range? I don't know but I think that it's an interesting idea, maybe the old Heathkit and EICO capacitor testers can't show leakage at lower voltages on high voltage capacitors due to not being sensitive enough? As in the leakage problem might be a linear thing where if it's Leakey at it's rated voltage then it's also Leakey at a lower voltage but you would need more sensitivity at the lower end of the voltage scale because the leakage will have a sort of reverse snowball effect, the lower the voltage, the higher the sensitivity needed, not sure if any of this makes any sense but in my head it sorta does. Or you can always run the capacitor to ground instead of going to the transistor, then use a comparator across both sides of the capacitor to test for leakage, and possibly a second or third stage to bring up any leakage that's hiding below the circuit's threshold? I would love to be able to sit down and give this a go, I know what outcome I want to see and if it can work, I know that I can get it to work but my anxiety has crippled me and I just don't have the patience at this time to attempt it. This is about as helpful as I can be right now.
Sir but that's not your circuit sir..that is nothing but Darlington transistor config.. no other components then pure darlington.you may as well avoid the 2 transistors and but a Darlington and capacitor leakage tester done all you need according to you sir is cap and one Darlington..which I do not think so. Sir. Try buying a single Darlington to make a leakage tester sir not a circuit. Adding voltage decider and led still makes it only one single Darlington as there is no current limiting circuit is plane Darlington and nothing else. We could make what Darlington can detect ion Geiger counter them leakage current or shall be say what's the sensitivity of bias. Sir. That's what I think. Not a circuit at all sir.
