[sdiy] Diode Matching

ColinMuirDorward colindorward at gmail.com
Fri Apr 10 20:19:38 CEST 2020

Others will have more informed feedback, but did you rule out temperature
changes as the cause of your measurement drifts?


On Fri, Apr 10, 2020 at 4:48 PM Didier Leplae via Synth-diy <
synth-diy at synth-diy.org> wrote:

> Does anyone have thoughts on the best way to do diode matching?
> I am building some ring modulators and need matched 4148 diodes, or
> similar.
> So far I have tried three different methods of testing and find the
> results to be confusing and unreliable.
>     The first method I tried was using a DROK transistor tester
> <https://www.amazon.com/Transistor-DROK-Capacitor-Capacitance-Automatic/dp/B01MS1FOYM> (which
> also tests diodes). I taped a strip of new 4148 diodes to a sheet of paper
> in a notebook and one by one wrote down the results. I noticed that most of
> them had pretty similar results, but there were a few outliers that had
> wildly higher readings (forward voltage drop, I think). So, out of
> curiosity I went and tested those again and found that the readings the
> second time were much more similar to the others.
>     Then I started rechecking all of them and found that I was getting a
> completely different set of readings. The differences between readings on
> any one diode seemed to vary as much as differences between diodes. And as
> I rechecked diodes I kept getting different results.
>     So, I concluded that my transistor tester must not be very good.
>     I then tried a the second method which is the diode testing setting on my
> Etekcity multimeter <https://www.etekcity.com/product/100111>.  This time
> I found that all the diodes read pretty much the same forward voltage drop.
> (They seem to increase slightly the more diodes I tested but if I go back
> to the first diode, for example, the reading will be at the same increased
> value). I came back later and tested again, this time the results were
> similar in that they all read the same value, except the value was 100mV or
> so lower. And again if I came back the next day similar results. So, I
> concluded that my multimeter's diode testing mode might be unreliable.
>     I then tried the third method of using the voltmeter, in mV setting,
> to test forward voltage drop. Since my voltmeter's mV setting only goes to
> 2000mV, I used a 1.5V AA battery and a 10k resistor (to give about 0.15mA)
> in series with my diode. The results were quite similar to the readings I
> was getting using the diode testing mode on the multimeter. The values in
> one set of tests would be virtually identical for all of the diodes, but if
> I come back and test again they are quite different from the previous
> value, but the same for all diodes.
> I'm pretty confused by all of this. Do I just have a crappy multimeter? Or
> are my diodes all miraculously matched?
> Any thoughts would be helpful.
> Thanks,
> Didier
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