Author Topic: Installing NPN Transistor Backwards  (Read 4525 times)

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Offline Syntax_Error

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Installing NPN Transistor Backwards
« on: September 01, 2016, 09:37:03 am »
Hi, quick question:

Installing an NPN BJT "backwards" with the collector at a lower potential, such as circuit ground, and the emitter at higher potential, such as to circuit supply through a resistor, would permanently damage the transistor, right?

My thinking is since the emitter is specified to pass current (or rather the whole device is constructed to) in one direction only, connecting it to a circuit "backwards" would damage it. Is this correct?
It's perfectly acceptable to not know something in the short term. To continue to not know over the long term is just laziness.
 

Offline Audioguru

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Re: Installing NPN Transistor Backwards
« Reply #1 on: September 01, 2016, 10:48:17 am »
The collector-base junction of a transistor is usually reverse biased and has a high breakdown voltage. The base-emitter junction is usually forward biased. When the emitter-base junction is reverse biased then it has avalanche breakdown at a low voltage (5V or 6V is the max allowed voltage for most silicon transistors).
But when the collector and emitter are connected backwards then the emitter-base junction usually has avalanche breakdown which creates hot spots that reduce the hFE of the transistor.

Years ago I used a reverse biased emitter-base junction of a transistor as a 7V zener diode and it worked well. But later I never tried to use the damaged transistor as a transistor.
 
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Offline MK14

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Re: Installing NPN Transistor Backwards
« Reply #2 on: September 01, 2016, 10:50:16 am »
Hi, quick question:

Installing an NPN BJT "backwards" with the collector at a lower potential, such as circuit ground, and the emitter at higher potential, such as to circuit supply through a resistor, would permanently damage the transistor, right?

My thinking is since the emitter is specified to pass current (or rather the whole device is constructed to) in one direction only, connecting it to a circuit "backwards" would damage it. Is this correct?

There is a risk of partially or fully damaging it. But they have a degree of robustness, and are often not so easily destroyed. But it could be. It also depends on the exact circuit configuration the voltages, currents and what the datasheet says.

E.g. Transistors usually don't like the Emitter/Base voltage to be the wrong way round, and greater than a datasheet value, of around 6V/7V, typically. As above that limit, it can conduct and break itself.
 

Offline Ian.M

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Re: Installing NPN Transistor Backwards
« Reply #3 on: September 01, 2016, 11:49:50 am »
As long as you don't Zener the B-E junction, (which tends to degrade the transistor's characteristics, even if the current is limited), a transistor with C and E swapped will actually amplify.  Usually the gain will be absolutely lousy, more than an order of magnitude less than in the forward direction, but many years ago, before FETs, there were a few bipolar transistors specifically designed for reverse operation in bidirectional signal switching applications.

Also see: http://www.eevblog.com/forum/beginners/why-a-bipolar-transistor-is-not-symmetricalreversible/
 

Offline David Hess

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Re: Installing NPN Transistor Backwards
« Reply #4 on: September 01, 2016, 02:59:33 pm »
Before JFETs became available, they used to make "chopper" bipolar transistors where the emitter and collector were symmetrical.  They had low gain but could be used in reverse and had a much lower saturation voltage than normal transistors.

It is not seen often but sometimes a standard bipolar transistor is deliberately used in reverse to take advantage of its extra low reverse saturation voltage.
 

Offline Ian.M

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Re: Installing NPN Transistor Backwards
« Reply #5 on: September 01, 2016, 03:57:52 pm »
The 2SC2878 is an example of this specialised type of transistor, intended for bidirectional switching and muting applications and has a reverse hFE of 150 (typ) and a VEBO of 25V.  You can still source it from specialised distributors.
I don't think its as symmetrical as its predecessors for this sort of application.
 

Online Alex Nikitin

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Re: Installing NPN Transistor Backwards
« Reply #6 on: September 01, 2016, 07:31:38 pm »
The 2SC2878 is an example of this specialised type of transistor, intended for bidirectional switching and muting applications and has a reverse hFE of 150 (typ) and a VEBO of 25V.  You can still source it from specialised distributors.
I don't think its as symmetrical as its predecessors for this sort of application.

One note though: there are plenty of fake 2SC2878 on eBay, which are just re-labeled some other TO-92 devices. It is easy to detect the fakes by measuring the reverse hFE. Another clue is that the fake devices usually have straight legs, and the real ones usually have pre-formed legs.

Cheers

Alex
 

Offline Syntax_Error

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Re: Installing NPN Transistor Backwards
« Reply #7 on: September 02, 2016, 12:40:00 am »
Thanks everyone. This is similar to what I thought.

The transistor in question is a BC337 NPN BJT. It's connected as a base biased switch with emitter grounded and an IR LED fiber optic driver on the collector. Upon inspection of actual units that allegedly worked, I see the BJT is installed backwards with collector at ground and emitter connected to the LED cathode. I cannot for the life of me see how this could operate, but whatever.
It's perfectly acceptable to not know something in the short term. To continue to not know over the long term is just laziness.
 

Offline danadak

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Re: Installing NPN Transistor Backwards
« Reply #8 on: September 02, 2016, 12:48:08 am »
Another potential positive characteristic of reversing C-E is lower
saturation voltage when used as a switch. Depends on transistor.

http://electronics.stackexchange.com/questions/29756/bjt-in-reverse-active-mode-of-operation



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