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(01-26-2020 10:34 AM)AndiGer Wrote: [ -> ]From time to time I read on the forum that tantalum caps degrade, often measure right values but high ESR.

What do you suspect to be "high ESR"?

I bought new 2.2µF/16V tantalum caps and they measure values between 3.6 and 5.6 Ohms ESR (Vloss between 0.3 and 0.6%). Measured by LCR-T4 transistor tester.

So 15 Ohms I measure in an old one is high ESR or where is the border you call it high ESR?

Imho there is no absolute reference esr value to be used as a maximum limit.
It all depends on several factors, starting with the cap type and the requirements for each application that are linked to the running frequency and ripple current.
For switching power supplies like these ones used in calculators the esr requirements are modest but must be able to filter pulse currents with periods around 10uS, and that is why the original designers are using regular electrolityc caps and maybe a tantalum as well.
My reference is the cap manufacturer specifications. The lower esr the better for the running frequency.
So I basically take a practical approach, by visually checking the cap for electrolits leakage, then check for shortage and current leakage using a regular meter, then reading the current capacitance value with a cap meter, and if all looks good so far, then them for esr and compare it with a recent manufactured cap (new old stock is nice only for collectors, like myself, but new in box old caps are not trustable).
That said, I expect to see an esr below 1 ohm on modern recent manufactured caps running at 100 to 200 kHz, but again, check the manufacturer specs. 15 ohm esr is too high to be used in such dc-dc converter imho.
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