Stupid question about using transistors as diodes

[perfectlyfineusername]

So I know you can use transistors as diodes if you bridge two legs, but I was wondering if you could just only use two legs without bridging the third to anything. I'm only asking because I got some transistors with a different pinout (I've learned since to check that before I buy any) and because of the pinout I could cut the middle leg and use the outer two as a diode, would this be fine?

[Dormammu]

Since there are a lot of diodes — the idea always seemed stupid to me. But yes, if you haven't cut off the base — you can.
I did this a couple of times with powerful transistors, with 1 burnt junction, because it was a pity to throw it away.
I assemble a couple of "garbage" rectifiers for the charger.   

[amptramp]

If you connect the base to the collector, you have an amplified diode where a small amount of current going from base to emitter turns on current in the collector which gives you a base-emitter diode with the current handling capability of the collector.  There may be restrictions on Vbe that will cause degradation of the device if the reverse voltage from base to emitter exceeds about 5 volts.

Base to emitter and base to collector have a diode junction between them, so they can be used as diodes.  Emitter to collector does not work as it sees two diodes connected back to back and there is nothing other than leakage current to turn anything on.  Keep in mind, emitter to base voltage still has limits.

[perfectlyfineusername]

Since there are a lot of diodes — the idea always seemed stupid to me. But yes, if you haven't cut off the base — you can.
I did this a couple of times with powerful transistors, with 1 burnt junction, because it was a pity to throw it away.
I assemble a couple of "garbage" rectifiers for the charger.   

Yeah base on the ones I've got is the outside leg and emitter is on the other outside leg. I have found a layout for a Sam ash fuzzstainer on effects layouts that uses a similar pinout so ill try some in one of those but honestly I bought way too many so throwing them in something as diodes seems like a good way to use up a few lol. I suppose I could also use this as a way to come up with layouts myself so I could use them in a fuzz face or something

If you connect the base to the collector, you have an amplified diode where a small amount of current going from base to emitter turns on current in the collector which gives you a base-emitter diode with the current handling capability of the collector.  There may be restrictions on Vbe that will cause degradation of the device if the reverse voltage from base to emitter exceeds about 5 volts.

Base to emitter and base to collector have a diode junction between them, so they can be used as diodes.  Emitter to collector does not work as it sees two diodes connected back to back and there is nothing other than leakage current to turn anything on.  Keep in mind, emitter to base voltage still has limits.

Fair enough, I did try them in a harmonic percolator when I was putting one together on a pcb so I could tell they were doing something but I was wondering if there was a reason you'd bridge them. Like would it blow up if you didn't or something lol. At least I know it's safe now, thanks

[antonis]

As Ron already - partially - explained, a diode and a transistor wired as diode are two totally different devices..

[pacealot]

There's also that little thing called bending the transistor's legs and insulating one with shrink tubing if necessary (often with skilled bending it is not). Personally, I find transistors to generally be much more valuable and useful as transistors rather than diodes, but YMMV.

(I also have more regular call for BJTs with BCE/ECB pinouts than most due to the choices of circuits I tend to work with, so perhaps I'm not the best one to judge... )

[perfectlyfineusername]

Maybe I'll put them in an opamp big muff, 6 transistors used as diodes. Muahahahahaha

[antonis]

If you search for someting like "Fire Red Fuzz", Muahahahahaha should sound untimely, at least..

[Rob Strand]

Both types of connection work.  You can even use the BC junction as a diode.  However all three connections are not the same. The configuration with the b-c connection will have a lower voltage drop.

[amptramp]

The b-c connection usually has a higher reverse voltage rating.  The b-e connection is usually restricted to the 5 to 7 volt range.

[Rob Strand]

The b-c connection usually has a higher reverse voltage rating.  The b-e connection is usually restricted to the 5 to 7 volt range.

It depends on the circuit if that's an issue.   The b to c connection is probably the most common diode connected transistor and that does have the VBE breakdown limit.

[johngreene]

So I know you can use transistors as diodes if you bridge two legs, but I was wondering if you could just only use two legs without bridging the third to anything. I'm only asking because I got some transistors with a different pinout (I've learned since to check that before I buy any) and because of the pinout I could cut the middle leg and use the outer two as a diode, would this be fine?

You can also wire the transistor as a "programmable diode" by wiring the base to the emitter and collector via a voltage divider. With this you can "program" the transistor to have a forward drop like a diode with any voltage you want, greater than 0.6v of course.

[antonis]

  Rubber diode or V_BE multiplier..

[johngreene]

Rubber diode or V_BE multiplier..

ah, I've always remembered it as programmable but that is what I meant. Thanks

[amptramp]

The Vbe multiplier connection is often used to set the bias on power transistors in an audio output stage.  You make a small change in the pot setting to make a change in the current through the NPN and PNP output stages:



Since this is a Darlington output, the Q5/R8/R7 Vbe multiplier must simulate four series diodes.

[antonis]

You make a small change in the pot setting to make a change in the current through the NPN and PNP output stages

And to compensate for that (always neglected) %$#@ Q5 Base current..

[Frank_NH]

Here's an application of the "amplified diode" idea as a soft clipper for a distortion effect.  From way back in 1983! 



https://worldradiohistory.com/UK/Hobby-Electronics/Hobby-Electronics-1983-07-S-OCR.pdf

BTW, I think R2 should be 470K (not 470 ohms).