Re: Looking for a better in circuit ESR meter

Chuck Harris

Lower the frequency, and you can easily measure the DC
resistance. If the waveform clips at the the clamp diode's
voltage, your DC parallel resistance is greater than 500 ohms.

Why should the diode conduct? It won't conduct until the
sum of the ESR and Xc is greater than 500 ohms.

If the diode conducts at 100KHz, your capacitor is bad.

-Chuck Harris

Reginald Beardsley via wrote:

The DC resistance in parallel with the capacitor must be measured as it affects the AC measurement. If the diode conducts during either measurement, the results are not accurate.

This is what the B&K 881 and EDS-88A II do that. Nothing else seems to take into account. One of them uses a 25 mV signal for testing to avoid turning on a diode junction.

An inductor in parallel with the cap makes it rather messy. Fortunately, those don't appear in power supplies that I know of. They are usually series tuned to short the undesired frequencies to ground.

Have fun!

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