[sdiy] Diode Matching
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.
More information about the Synth-diy