Home-made Darlington problems

Frequently asked question on transistors: types, substitutions, how to test, use and misuse them.
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commathe
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Post by commathe »

So I was playing around with slapping two transistors together to make a darlington and I have come across a really weird issue. For all transistors that I use, the darlington only behaves as it should when one or both transistors are put in "upside down".

For instance, if I am using two NPN transistors: When both their collectors are facing "upwards" and connected to the load/resitor that is between them and the power rail - they don't conduct. Nothing gets through. However, when I flip one of them round so the emitter is "upwards" it works. It doesn't matter which one. Actually it can be both! Why is this?

I thought I had labeled my transistors wrong, but I hadn't. I am running into this with all of my BJT transistors. I have double and triple checked datasheets to make sure I don't have the pinout mixed up. When I use a transistor individually though, it behaves like expected.

I should note that I came across this making a bazz fuss, so I am wondering if it has something to do with the collector-base diode in that design?

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Post by Rumbler »

I just finished my the 7-minutes fuzz (http://www.diyguitarpedals.com.au/shop/ ... ematic.pdf), which I think is based on the bazz fuss.

I did not have a Darlington and I combined two 2N2222A connecting the pinouts as you see here: https://imageshack.com/i/jwjwm6j. The fuzz works just fine.
I just double-checked the pinouts of my transistors, and I am sure I did not invert collector and emitter :lol:

I would control the connections to the other components. Maybe the mistake is there.

hope it helps.

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Post by DrNomis »

commathe wrote:So I was playing around with slapping two transistors together to make a darlington and I have come across a really weird issue. For all transistors that I use, the darlington only behaves as it should when one or both transistors are put in "upside down".

For instance, if I am using two NPN transistors: When both their collectors are facing "upwards" and connected to the load/resitor that is between them and the power rail - they don't conduct. Nothing gets through. However, when I flip one of them round so the emitter is "upwards" it works. It doesn't matter which one. Actually it can be both! Why is this?

I thought I had labeled my transistors wrong, but I hadn't. I am running into this with all of my BJT transistors. I have double and triple checked datasheets to make sure I don't have the pinout mixed up. When I use a transistor individually though, it behaves like expected.

I should note that I came across this making a bazz fuss, so I am wondering if it has something to do with the collector-base diode in that design?

That is weird, those NPN transistors should operate correctly when the emitters are pointing to circuit ground, two possibilities I can think of, could be a biasing issue, or, one or both of the transistors may be B-E short, but it's probably more likely a biasing issue, remember the Darlington Transistor configuration tends to produce a huge amount of current-gain if you're using two fairly high-gain transistors....... :hmmm:
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