Looking for a transistor tester

Phil hodson

Active member
I just built one of these Rully boards. Not yet used it but I ordered a couple of extras. I think I have three left. If anyone want one Pm me. Happy to help out.
They are just the PCB (unpopulated)
 
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Bricksnbeatles

Well-known member
I’ve got the Taobao, and aside from the ZIF socket being kinda pointless with how it wiggles around, it seems to be fine for what it is.

I was just testing some diodes today and noticed that if you measure a diode in test pins 1 and 2 or 1 and 3 it gives an “Ir” reading in addition to the Uf (forward voltage). I previously always tested diodes in pins 2 and 3 of the socket (just by chance, not a deliberate effort) and it only gives you the forward voltage and the capacitance of the diode in that case.

As I is of course current (derived from the German word for current I think) I’m assuming Ir of a diode is reverse current or something to that effect. It’s been quite a while since I took an EE class, so I can’t remember if that’s a common measurement or what purpose it serves to know— Google was of little help.

As the Ir tends to be around 2nA for most diodes, and that’s 2/1,000,000 mA, it seems to be pretty insignificant, but some diodes were as much as 0.2mA so I’m curious what these readings signify and what applications they may be taken into consideration in. Paging Dr. @Chuck D. Bones to the Tool Box!
 

benny_profane

Well-known member
Ir is the reverse current. When a diode is reverse biased with a voltage not in excess of the breakdown voltage, a small amount of current is able to flow. So, even though this is the ‘off’ state, there’s a non-zero current. Check out the I-V curve of a diode’s data sheet. In the reverse side, you’ll see a small jump from the voltage axis (x). That small bump is the Ir reading you’re seeing. As reverse voltage approaches the breakdown voltage (Vbr), the current begins to increase until it is essentially an open circuit.

EDIT: The following graph is taken from the onsemi 1n914 data sheet:

1n914 Ir graph.png

Silicon diodes are typically in the nA range you cited. Germanium devices can exhibit quite a bit higher Ir values.

This phenomenon is what is talked about with 'leakage' and is exhibited with transistors as well.
 
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Bricksnbeatles

Well-known member
Ir is the reverse current. When a diode is reverse biased with a voltage not in excess of the breakdown voltage, a small amount of current is able to flow. So, even though this is the ‘off’ state, there’s a non-zero current. Check out the I-V curve of a diode’s data sheet. In the reverse side, you’ll see a small jump from the voltage axis (x). That small bump is the Ir reading you’re seeing. As reverse voltage approaches the breakdown voltage (Vbr), the current begins to increase until it is essentially an open circuit.
Thanks! Yeah, I understand what the reverse current of a diode means— just couldn’t confirm that that’s what Ir was. What I’m unsure of though is what applications the reverse current is relevant information for. I’m guessing it’s not too significant in any audio applications, but out of sheer curiosity I’m wondering where, even outside of audio applications, a higher or lower Ir is preferable.
 

benny_profane

Well-known member
Thanks! Yeah, I understand what the reverse current of a diode means— just couldn’t confirm that that’s what Ir was. What I’m unsure of though is what applications the reverse current is relevant information for. I’m guessing it’s not too significant in any audio applications, but out of sheer curiosity I’m wondering where, even outside of audio applications, a higher or lower Ir is preferable.
One practical reason in re pedal building is if a diode is used in a feedback loop for clipping. If the Ir is too large, that can affect the design of the gain stage. This spec is the main issue folks have with the G2 and low output.

Edit: An ideal diode has no Ir. I don't know of a situation where you'd want a specific Ir since the closer to zero, the better. There are special cases where that reverse behavior is an asset, though. Zener diodes are designed to have specific breakdown voltages when reverse-biased. The voltage assigned to a Zener is its breakdown voltage and they can operate in that region. They are typically used to prevent over-voltage conditions in a circuit.
 
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