Re: Looking for a better in circuit ESR meter


Richard Knoppow
 

With a lot of editing of the thread. I think I am missing something. I don't understand why ESR is so difficult to measure, it is measured by any impedance bridge although usually as Dissipation Factor. The difference is that ESR is independent of frequency while DF is a measure of the ratio of reactance to resistance at a given frequency. It is the reciprocal of Q. It is used because it is mathematically more convenient for capacitance but is exactly the same thing.
   Capacitors have both series and parallel resistance. Parallel resistance is often called Leakage. It is a measure of the direct current passed by a capacitor. A theoretically ideal capacitor should pass no DC but all practical ones do. Of course, the parallel resistance can also pass AC, lowering the effective reactance of the capacitor. So, one wants the lowest possible series resistance and the highest possible parallel resistance. Series resistance is strictly an AC measurement but parallel resistance can be either DC or AC. It may be difficult to distinguish series from parallel resistance for AC but for measuring the quality of a capacitor the DC measurement is sufficient for "leakage".
   An ideal capacitor has zero ESR and infinite parallel resistance. In fact, while ESR is a general AC measurement it  can be affected by losses at various frequencies so Dissipation Factor may be more appropriate to measuring the quality of a cap at specific frequencies.
    Of course, its well known that electrolytic caps have inherently high leakage because of the nature of the dielectric. They are usually used where the bulk value of the capacitance is more important than the quality, such as in power supply filters. The tolerance for the amount of DC passed, i.e., the leakage, depends on the application. It can also vary with applied voltage.
   It is possible to find capacitors which have reasonable ESR but excessive leakage. This is common in paper capacitors used as coupling caps where the capacitance will measure OK on a bridge but the DC allowed to pass will upset the bias on the following stage. Its also a problem for by-pass and decoupling caps in AVC circuits. Some capacitor checkers will measure both ESR or DF and leakage, typically impedance bridges will not measure leakage.

On 5/16/2020 11:51 AM, Bert Haskins wrote:

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Richard Knoppow
dickburk@...
WB6KBL

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