Capacitor leakage tester design & build - Part 3

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The design and build of a capacitor leakage tester to check capacitor leakage at their rated (or working) voltages in tube radio and other tube era equipment.
The linearly variable voltage implemented in the design also serves as a means of reforming old electrolytic capacitors.
I could have bought an old one and restored it, but this seemed like a better idea - designing and building a more accurate and modern one, as well as making the applied voltage linearly variable so that I can get a true reading of the leakage as the operating voltage rises.
Obviously, another requirement was safety. High voltage zaps you, and I like living.
This video is the second of the design stages of the circuit.
Have fun, and be careful - this thing has high voltages that can kill you.
The other parts of this series are available in the playlist:
• Capacitor leakage test...
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3 авг 2016

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Комментарии : 46

@joeteejoetee Год назад

Your penmanship is excellent, and your pop-up text of the calculations is very welcomed !

@electronicsoldandnew Год назад

Thanks

@mbaker335 7 лет назад

Very, very clear explanation. I do hope you work as a lecturer as you have a skill in putting across concepts in a way that agrees with my aging brain. Impressive.

@electronicsoldandnew 7 лет назад

+mbaker335 : not a lecturer, only a businessman. I'm an (also aging) electronics engineer and enjoy this hobby as it takes me back to the reason I studied engineering in the first place.

@zsoltm2119 4 года назад

Very thorough explanation! One thing to note is that putting an LED across the 6.3VAC winding will damage the LED over time. LEDs typically have a 5V reverse peak voltage, and the 6.3VAC * sqrt(2) is higher than 5V. An easy fix is to put a standard diode in parallel with the LED, such as a 1N400x.

@electronicsoldandnew 4 года назад

Yes. Corrected that. Thanks

@washerdryeradvicechannel8041 2 года назад

Excellent

@ianide2480 6 лет назад

My most expensive component was the front panel meters. Since I am in the US my wall voltage is 110v. I purchased a cheap step up transformer (110 to 220) from ebay which was like $5.... Like I mentioned on another of the videos, I added a self made ESR meter as well. I yanked a 7v transformer out of an old wall wart for charging an old drill, that I don't own anymore; this powers the ESR circuitry. I would have liked adding some winds to the other transformer to power this but I didn't have magnet wire at the time. I am watching these videos again because I like to see if I can learn anything new. I've been thinking of making a "digital" version of this, which is much more complicated but it would be fun =)

@electronicsoldandnew 6 лет назад

Ian Ide : yes, a digital one would be fun.

@extoprak 4 года назад

what I do (takes more space but cheap or almost free) I take 2 pcs of 230-12v transformer and connect back to back. you will have voltage drop to 200-210vac. if you need more voltage, connect the 12vac to a 9v-230v transformer then you can go up in dc voltage. these transformers can be found in old adapters etc that goes to trash.

@electronicsoldandnew 4 года назад

Yes, that’s a good trick to save on an expensive high voltage transformer.

@stanyoung4974 7 лет назад

very good video. thank you. if the voltage pot fails , what will happen to the cap being tested?

@electronicsoldandnew 7 лет назад

Stan Young : one of two things- if the fault leaves the wiper connected to the lower end, nothing happens. If the wiper is left at the high end, then the full supply voltage is passed to the capacitor. If the cap is high voltage, no problem. If it is a low voltage cap, then .... boom.

@toddanonymous5295 4 года назад

great circuit. You may want to put another 1n4007 in series with the output to prevent a charged cap from feeding back through the circuit.

@electronicsoldandnew 4 года назад

That’s a great suggestion. Thanks

@toddanonymous5295 4 года назад

I just watched part 4 which has the diode. You have a very interesting site. Thank you for your efforts and sharing with us.

@electronicsoldandnew 4 года назад

I actually forgot that I’d put the diode in there. It’s been a while :)

@donaldfilbert4832 6 лет назад

Thanks for the excellent series !! I have been looking for something like this for a long while !! I do have one question. Have you found the the 2 Watt 12K ohm limiting resistor to work OK? Because it seems that the possible short circuit wattage thru it could be much higher than 2 W ! I.E. 0.03 x 0.03 x 12000 = 10.8 Watts !! Am I missing something ?

@electronicsoldandnew 6 лет назад

Donald Filbert : you’re right in your calculation, but because the actual currents are very small, the resistor wattage has been fine so far. The 30mA is a worst case short circuit condition that you would immediately turn off. I’ve used this constantly and have not had any problems. If you want, put in a 10W, it can only help.

@donaldfilbert4832 6 лет назад

Thanks for your quick and comprehensive reply !! I understand. After I posted this Q - I also saw in the next video after this that you changed the wattage on the updated schematic and addressed the situation. Thanks again !!

@drtidrow 6 лет назад

Damn, looks like I'm going to be building one of these after all, current going rate on ebay for Heathkit IT-11/IT-28 testers is at least $300. :-( One of the IT-28s I was bidding on finally sold for $355, too rich for my blood... besides, I rather like the idea of making my own.

@electronicsoldandnew 6 лет назад

RayDT : and it works great. I’ve been using mine a lot more than expected, and it hasn’t disappointed me yet.

@hernancoronel 4 года назад

Hi Mr Caldeira, how would you modify this circuit to accommodate for a higher current transformer? You mention yours is just 30 mA but us mere mortals here don’t have that kind of thing lying around ;-)! Thank you for the great content and excellent explanations. Keep up the great content please!

@electronicsoldandnew 4 года назад

A higher current transformer will work as is. The only difference us that if you create a short circuit and there is no limiting resistor (the 12k that I use), then you could blow the mosfet.

@sficlassic 7 лет назад

I know this might be a bit trivial, shouldn't the fuse be placed between the mains and the power switch ?

@electronicsoldandnew 7 лет назад

+Sficlassic : strictly speaking, you're right. Thanks.

@artsandclouds 6 лет назад

Great desing and excellente explanation. I do however have a question (please take in consideration that Im a novice in electronics), isnt that zener also dropping a large voltage? If the potentiomenter is feeding the full 338Vdc and the zener is dropping 12V, in my head that zener would need to dissipate a lot of power as well. Am I mistaken?

@electronicsoldandnew 6 лет назад

artsandclouds : the Zener is there as a protection device to prevent the gate to source voltage from exceeding a safe level. The mosfet turns on very fast and what the Zener will see is the gate to source voltage, effectively never reaching its breakdown voltage of 12V.

@artsandclouds 6 лет назад

Thank you very much for taking the time to answer me. Now I understood it!

@helmutheller1538 Год назад

Wouldn’t it be better to combine the two current limiting resistors of 10 ohms and 12 kohms into one single resistor of, say 220 ohms in the position of the 10 ohms resistor? It will have a much lower heat dicipation of only around 200 Milli Watts as the mosfet is doing all the heavy lifting, but that device sits on the heat sink so there is no problem there.

@electronicsoldandnew Год назад

You’re probably right. One could do current limiting at the source. Good idea.

@darrylgodfrey9604 3 года назад

Probably too late to get any answer to this, but let's give it a try ... many thinks for this video - I enjoyed it. But can I say why the 240V/240V transformer? Obviously for someone living in a 115/120V country, then the two 120V windings would be used in parallel to give the required high output voltage. But for folks living in a 220/240V country, the most the transformer would do is give isolation. Is that its only function in such countries? Thanks again for an informative video.

@electronicsoldandnew 3 года назад

Absolutely. You always want isolation, or you get a hot chassis risk. also, this system had to be completely floating, so if you used a transformer-less circuit, you would have fireworks.

@darrylgodfrey9604 3 года назад

@@electronicsoldandnew Ok - makes sense - thanks for your reply.

@jordiguimera2398 6 лет назад

I am working on that beatiful project. The referenced switch 2P5T (Mouser Part# 105-SR2511F-25S) is rated at 125v DC, and there is no way to find one suitable for 400v. Any help will be greatly welcomed. Thanks in advance!

@electronicsoldandnew 6 лет назад

Jordi Guimera : that rating is way too low. The current is very low so you can get away with a slightly lower rated switch, but not that much lower. I found a 300v one in Rsa components (RS Stock No. 0352250) which would probably be just fine. You can actually use a 2p6T as well and the last position can be left unused.

@jordiguimera2398 6 лет назад

Excellent !! It is exactly what I was looking for and I was not able to find. Thank you so much indeed!

@mohare134 4 года назад

Great Video, but what I don't understand is why use the MOSFET at all? Since you're just charging-up caps, why can't you just go right from the variable resister out thereby eliminating the MOSFET circuitry?

@electronicsoldandnew 4 года назад

mohare134 : the idea is to get a steady voltage output which is variable. The a lot will not do that. The actual voltage will depend on the current drawn through the top part of the pot through the capacitor.

@mohare134 4 года назад

@@electronicsoldandnew I see. Thanks for the clarification.

@thehifiworkshop1452 2 года назад

Hi there, I actually just rebulit this circuit and it kind of works but I have one problem: I cant get more than 3mA (at full voltage) out of this circuit. I used all the same components as you did. The transformer is out of an old tube radio...I think it should be able to supply more. What could possibly be wrong with mine?

@electronicsoldandnew 2 года назад

It should. You’ll have to test voltages before and after the circuitry, and make sure the mosfet is actually turning on.

@thehifiworkshop1452 2 года назад

You were right! I must have been pretty tired when I soldered this. 😄 The zener was shorted out. Now it works like it should...I only need to get a new amp-meter...mine is open...dont know why. Maybe it came broken from the factory...

@electronicsoldandnew 2 года назад

👍

@washerdryeradvicechannel8041 2 года назад

Excellent

@electronicsoldandnew 2 года назад

Thanks