Repairing a Broken Multimeter Part 2

• June 2, 2022
• electronics  
• fighting-entropy  

In Repairing a Broken Multimeter, I identified a faulty capacitor C18 and desoldered it from the board. I wasn’t sure what its purpose was and what to replace it with.

Recently one reader, Marcin, who’s interested in how multimeters work emailed me telling me that C18 is used in capacitance measurement, that the correct value is 1nF and that it helps with linearity for values lower than 4nF.

I opened my multimeter and looked at the datasheet again.

Sure enough, in section 14.17 Capacitance Measurement there’s a passage saying:

(In actual application, if the linearity is worse when doing 5.120nF measurement, you can take a consideration of paralleling a proximate 1000pF capacitance in input end of capacitance measurement during design. When doing the measurement, to press REL key and make the value be zero before measuring. The linearity of the smaller measurement in capacitance mode will be better.)

To test this, I’ve checked capacitance of capacitors in the μF range, and it agreed between my working multimeter and the multimeter without C18. For a 1nF capacitor, however, my working multimeter correctly identified it as 1nF in the REL mode, but the multimeter without C18 showed 1.25nF in the REL mode.

I should note that when no capacitor is under test the working multimeter shows 1.25nF and the one without C18 was showing 0nF. That suggests that the working multimeter has a 1nF capacitor with the function similar to C18.

I soldered a 1nF capacitor in place of C18 and that made it produce correct measurement for 1nF in REL mode.

While trying to understand how the measurement is performed and how C18 helps, I’ve found Measuring capacitance & ESR. I’m still hazy on this, but from what I understand, passing a waveform through a capacitor will make its period longer. Measuring that period makes it possible to infer the capacitance.

As to how C18 in REL mode makes that measurement more accurate, I’m still unsure. I tried to follow the line of reasoning about the possible sampling rates on a 4Mhz clock, but I looked at the generated waveform on my oscilloscope and the frequency of the wave was about 65khz with C18 in place. It seems slow enough to accurately measure with a 4Mhz clock.