Electronics > Projects, Designs, and Technical Stuff
Capacitor Leakage tester issues
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Tac Eht Xilef:

--- Quote from: splin on December 03, 2018, 11:11:24 pm ---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 …
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Because sometimes a fairly high current is necessary. Remember, it's not designed for modern low-leakage caps - it's designed for testing & reforming old 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 pm ---… 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.
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Which, 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 pm ---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. :scared:

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).
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Agreed, 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 the :scared: :scared: :scared: s…
001:

--- Quote from: Tac Eht Xilef on December 04, 2018, 02:08:54 am ---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

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WOW
It is interesting to see it  :-+
Calambres:

--- Quote from: Tac Eht Xilef on December 04, 2018, 02:08:54 am ---
--- Quote from: splin on December 03, 2018, 11:11:24 pm ---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 …
--- End quote ---
Because sometimes a fairly high current is necessary. Remember, it's not designed for modern low-leakage caps - it's designed for testing & reforming old 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).
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The 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.
AngraMelo:

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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 the :scared: :scared: :scared: s…

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Pictures or it didnt happen!!! Please!
mk_:

--- Quote from: AngraMelo on December 04, 2018, 04:32:13 pm ---
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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 the :scared: :scared: :scared: s…

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Pictures or it didnt happen!!! Please!

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Buy a IXTX8N150L Linear FET and do the usual stuff you do if you want design a current source....
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