Re: Looking for a better in circuit ESR meter


Here's a video by Alan Wolke explaining how this works: You want a square wave out of the
function generator, and the faster rise the better. With zero rise time,
the peak-to-peak voltage you measure is only a function of the capacitor's

Chuck proposes to add a series resistor with the function generator, and a
back-to-back diode clamp across the capacitor under test to make sure
nothing gets fried.

On Sat, May 16, 2020 at 10:39 AM Ernesto <ebordon@...> wrote:

Hi David,

Let me see if I can make a simple recount of what Chuck is explaining:

If you have an oscilloscope and a function generator able to output a sine
wave of about 100 kHz, you have everything you need to make simple, quick
ESR measurements in situ.

- Connect the ground and tip of the oscilloscope probe on both ends of the
- Inject as a current the 100 kHz AC signal into the capacitor side that
has the oscilloscope tip.
- Observe and estimate on the oscilloscope the voltage this AC current
creates on the capacitor.

I set the function generator to 10 V, then connect this output to a 1 K
resistor, and touch the capacitor with the other end of the resistor. This
will create an AC current of 10 mA.
If the ERS of the cap is 1 ohm, the measured voltage will be 10 mV,
something that can be recognized on the oscilloscope set to its high
sensitivity of direct input.

It is not necessary to measure the value of ERS precisely, but only to
get an idea whether the capacitor is good or not.
"Acceptable ESR" is also a function of the capacitance. In a large cap, a
fraction of an ohm can be fine. In a small one, one ohm or larger can be

Electrolytics are polarized. If you are a meticulous person, you will put
the ground of the scope on the negative of the cap, and will see that the
AC signal has a positive offset.
If you are a practical person, you realize that a small negative AC does
not affect the response of the electrolytic cap. They are not like
schottky diodes.

For this measurement, the less noise and less bandwidth your oscilloscope
has, the better. And use a direct probe instead of a x10 probe. If you can
trigger the scope on the AC signal (at the generator) like Chuck
recommends, it is easier to evaluate the AC signal in the noise.
But the use of protective diodes may not necessary, since a few tens of
millivolts with good ESR on the cap will not damage it, and several volts
on a bad cap, won't make it worse. If the diodes are used to protect other
components in the circuit, they won't interfere in the measurement because
the signal on a good capacitor is so low.


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