[sdiy] Diode Matching

Harry hbissell at wowway.com
Sat May 9 11:06:27 CEST 2020


The transistors will work well, but you only get a matched pair. Watch out for the base-emitter breakdown voltage rating, it may be as low as 5v and not much higher than that. It could be fine depending on your circuit. Another trick, if you need really low reverse leakage (at low power) use the base-collector junction instead.

HarryB

> On Apr 10, 2020, at 5:42 PM, Schwarz Raphael <raphschwarz at gmail.com> wrote:
> 
> Seems to me like you have a crappy multimeter, 
> temperature only plays out when when you want to match diodes up to a few mV. On a 1n4148 there is about 150mV variation between 0°C and 90°C . At room temperature lets say 15°C to 30°C there should be less than 5mV . If you need 2mV matching I would tape them on a sheet of paper and let them rest a bit before measuring
> 
> Recently I started to test replacing hand matched diodes by matched transistor pairs in a single SMD package to avoid having to match SMD diodes. It works well in simulation but I need to test that
> 
>> Le 10 avr. 2020 à 20:19, ColinMuirDorward <colindorward at gmail.com> a écrit :
>> 
>> Others will have more informed feedback, but did you rule out temperature changes as the cause of your measurement drifts?
>> 
>> Colin
>> 
>> 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 (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. 
>>> 
>>> Thanks,
>>> 
>>> Didier
>>> 
>>> 
>>> 
>>> 
>>> 
>>>  
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>> 
>> 
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