Capacitor Leakage tester issues - Page 1
Forum section: electronics / projects

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 03, 2018, 01:50:04 am
content: Hey guys,I built this tester from Mr. Caldera.I had to make some modifications given that I didnt have exact the same components.He uses the mosfet IRFP27N67K I used the IRFP460He had a 1:1 transformer I used 2 back to back transformer. Both identical 24V 800mA.My Ac mains is around 230V and Im getting the variable feature of it.I used another small transformer in parallel (12V 100mA) just to power the LED indicator and the Volt meter. The grounds coming from both rectifiers are tied together.My issue is whenever I try to measure anything that would draw say around 1mA or even less, the voltage goes to 0.Even measuring the output with my Fluke87V loads the circuit enough to bring down the voltage around 40V.I have no idea what is going on.could you help me?my Schematic attachedoriginal video

author: Calambres
author_rank: Frequent Contributor
timestamp: December 03, 2018, 08:55:55 am
content: I also made that project with IRFP460 and back to back transformers like you did and it works like a charm:It is a clever and simple design. I even made anarticle in my own web pagedecribing how I did it. It is in spanish but you maytranslate it with google.

author: 001
author_rank: Super Contributor
timestamp: December 03, 2018, 12:23:24 pm
content: awesome work guysOfftop: tell me why this DIY unit popular in latin countries?

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 03, 2018, 06:24:24 pm
content: 001 - You wont find those old style capacitor tester here. Like you find on ebay. Importing is a enormous hassle and extremely expensive (customs charges a insane percentage).

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 03, 2018, 06:26:11 pm
content: Calambres, did you have to use another transformer like me?Perhaps the fact that I tied both grounds together is messing up the project?

author: Kalvin
author_rank: Super Contributor
timestamp: December 03, 2018, 08:53:46 pm
content: Check the polarity of the 12 Volt Zener diode connected to the MOSFET gate.

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 03, 2018, 09:45:38 pm
content: Bingo! Zener was not reverse biased.Also, it took my mosfet to the grave with that mistake.Thank you so much! Unit is working!

author: splin
author_rank: Frequent Contributor
timestamp: December 03, 2018, 11:11:24 pm
content: Looks too dangerous to me - why do you need a 100mA range? Ì'd change the limiting resistor to allow 1mA absolute max and 100uA preferably. Any capacitor with more than 100uA leakage is probably u/s, with the exception of very large electrolytics for which you could limit the maximum voltage to say 50V or 70V if you want a higher current range.And very large capacitors charged to 350V are potentially very dangerous if they were to be accidentally shorted. Flying molten metal ain't very healthy.If you really want a high voltage, high current range I'd only enable it with a press-to-test switch (or two even, seperated by more than hand's width). Personally I would include it for all voltages over 70V, whatever current range. I'd arrange the switch, with both NO and NC contacts, so the capacitor would be discharged when the button is released.

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 04, 2018, 01:52:35 am
content: Splin,I agree that the push to test would be safer for higher voltages. I will look into the desing of such feature. I am a beginner and although Im familiar with most of the safety procedures while testing capacitors it takes me a while to figure out a design change as you proposed.Thanks!

author: splin
author_rank: Frequent Contributor
timestamp: December 04, 2018, 02:05:36 am
content: Very sensible. High voltage AC is nasty; high voltage DC is *very* nasty... It seems the above is mostly an urban myth. AC is generally more dangerous than DC in that:a) it takes 4 to 5 times more DC than AC to affect the human body (ie. feeling it, to it stopping the heart) andb) AC is more likely to induce fibrilation of the heart.The apparently common idea that a DC shock is more likely to 'cause a death grip' than AC, because the latter has zero crossings which allow the muscles to relax enough to escape the source, is simply not true.

author: Unknown
author_rank: Unknown
timestamp: December 04, 2018, 02:08:54 am
content: Quote from: splin on December 03, 2018, 11:11:24 pmBecause sometimes a fairly high current is necessary. Remember, it's not designed for modern low-leakage caps - it's designed for testing& reformingold electrolytics, which in some cases may have had ~1mA or more leakage even when they were new & good. In those cases, you need 10's of mA for reforming (at least at the start).Quote from: splin on December 03, 2018, 11:11:24 pmWhich, if you go down the road to include that as an automatic function, adds considerable complexity.In practice, a semi-regulated / high source impedance design like this limits the voltage across the cap during reforming to a few 10's of V when current is high anyway. In use, you set up the target voltage, connect the cap, and if the leakage current is high, the voltage drops to a few 10's of V anyway. If the initial current is too high, you wind down the target voltage until it's acceptable. Over time, if the capacitor is reforming, you'll see the V creep up and I decrease. If not, it's toast.Quote from: splin on December 03, 2018, 11:11:24 pmAgreed, it can be dangerous. But it's test equipment, not a toy, and should be treated as such - on only when using it, don't leave the leads lying around the bench, use properly insulated and rated leads & clips, appropriate safety gear & fingers away from the bitey bits, etc, etc.A press-to-test / safety switch precludes use as a reformer, unless you're going to stand there with your finger on the button for hours…Is it perfectly safe? No. But it's as safe as most of the other vintage capacitor testers/reformers that have been legitimate test equipment in the past, and people seek out now.I'd tell you about my 700V / 20mA current-limited version, which not only tests and reforms caps but doubles nicely as a B+ supply for testing/repairing small vintage radios, but it might cause some people to have a bad case of thes…

author: 001
author_rank: Super Contributor
timestamp: December 04, 2018, 03:40:24 am
content: Quote from: Tac Eht Xilef on December 04, 2018, 02:08:54 amWOWIt is interesting to see it

author: Calambres
author_rank: Frequent Contributor
timestamp: December 04, 2018, 06:08:04 am
content: Quote from: Tac Eht Xilef on December 04, 2018, 02:08:54 amThe project I made and showed in the second post in this thread is, as stated in the article, limited to 30mA for the very same reasons. The 100mA scale in one of the various shunt selections is for ease of dial reading only. No more than 30mA available in that range.

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 04, 2018, 04:32:13 pm
content: QuotePictures or it didnt happen!!! Please!

author: mk_
author_rank: Regular Contributor
timestamp: December 04, 2018, 08:37:34 pm
content: Quote from: AngraMelo on December 04, 2018, 04:32:13 pmBuy a IXTX8N150L Linear FET and do the usual stuff you do if you want design a current source....

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 04, 2018, 08:50:03 pm
content: I don't have a really good photo of the GR 1617A capacitance bridge, but it does a great job with leakage measurements and reforming. It has a 20 mA maximum current range and minimum of 60 uA, which can be read down to 2 uA or so. Just to give an idea of practical values. The bias switch is a DPDT switch so you can switch it on for reforming. When switched off it puts a resistor across the cap to discharge it. This doesn't seem to affect the value and loss measurements that can be done under bias (or not), so they probably have some diode drops or a current sink rather than a simple shunt resistor. It also has a warning light to show when more than a few volts is across the cap. The front panel has various warnings about possible death if the unit isn't used correctly. The instructions suggest insulated test clips, rubber gloves and a chair insulated from ground!