[sdiy] Diode Matching

Didier Leplae didierleplae at yahoo.com
Fri Apr 10 22:44:28 CEST 2020

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 (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.  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. 

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