Transistor as Diode burn question

Hello everyone,

For a school assignment, where I had to control a DC Motor, were no diode’s available for the flowback circuit. I looked up online how to use a transistor as diode, which then subsequently burned out.

Some details.

  • PSU is 12V
  • While the transistor burned, the motor was running
  • The circuit was functional in the tinkercad circuit simulator
  • The transistors are both of type BC337
  • Please see diagram below for the circuit

If any one could help me in troubleshooting this issue, it would be greatly appreciated.

Thank you:)

maybe a small load for the flyback? I've never heard of using a BJT like this. A mosfet might be better too.

Transistor as Flyback Diode.png

OK, the lesson on using a transistor as a diode - don't!

Just go get a pack of 1N4004s (or equivalent number).

Why does it not work? Well, the base-emitter junction has a breakdown voltage of around 6 to 8 V. Minimum specified as 5.0 V for a BC337. And - it is breaking down if you connect it to 12 V.

Hahaha that's probably why I've never seen this. Thanks Paul

Transistors may be used as a diode in such configuration. For example it is (used to be) used in temperature sensing circuits. I am not sure what are advantages over normal diode, maybe only easier implementation inside an IC.

Maybe you could use collector base junction as the flyback diode, leaving the emitter open. But ofc real diode is better solution.

If you know how a diode is made, you will know that it is 2/3 of a BJT transistor. It is a PN junction, where a BJT adds a third doping. That is why they are called bi-polar (think two diodes that share a common doping). I think that is what your instructor may be trying to reinforce, but I wouldn't do this outside of a classroom exercise. At any rate, your NPN transistor should be placed in the circuit with the P-junction tied to the positive, i.e. the base, and the N-junction tied to the negative, i.e. the emitter. The collector is left open.

Transistors can be used as a diode, but the junctions usually have a low break down voltage.

I have seen where a reversed emitter base junction can be used as a 6 or 7v zerner, a TO3 2N3055 can be used as a high power unit.

However, use a 1N4001 thru 7 diode !

The base-emitter junction has a very low breakdown voltage reverse biased, a few volts - this is the easiest
way to fry a transistor - basically never reverse bias the base-emitter junction.

The base-collector junction can handle voltage, but is very low current, so don't use that either.

Don't use a transistor as a free-wheel diode, use a diode as a free-wheel diode...