[sdiy] Diode Matching

Ingo Debus igg.debus at gmail.com
Sat May 9 13:09:58 CEST 2020

> Am 08.05.2020 um 20:57 schrieb Ingo Debus <igg.debus at gmail.com>:
> While we’re on the subject, there’s a funny thing I observed on my DMM measuring resistors. When I connect a 560 kOhms resistor the DMM shows something like 0.565 MOhms. It drifts a little, but not much. Now, when I grab one of the test lead, just the isolation, without making contact to the wire, the value changes drastically. When I grab the other lead, the values changes into the other direction. Seems like a mains hum probrem to me.
> On the other hand, with a 470 kOhms resistor the displayed value is rock stable. 

Sorry, this is getting OT, but I really got curious. So I hooked my scope across the resistor, in parallel with the DMM. With the 560 k resistor, the scope showed some heavy 50 Hz hum. With the 470 k resistor there’s absolutely no hum. Then I grabbed some other 470 k resistors of a different type: voila, the hum as well as the instable reading of the DMM is back again! When I use 470 k resistors of the same type than the first one, from the same belt (is belt the correct term here?), there’s no hum, and the display is stable within one digit.
I also tried resistors from the first type with a different value (301 k) and they do hum.

This is really getting mysterious.

The good news is, diode testing with that DMM isn’t affected. No hum, stable readings.


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