Transistor drives LED both ways round. How?

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Joined: Mon Mar 31, 2008 5:09 pm

Transistor drives LED both ways round. How?

Post by wal » Mon Mar 31, 2008 8:35 pm

Hello to everyone. This is my first post and I'd like to thank in advance all those who help dispel some of my substantial ignorance.

I'm evaluating someone else's circuit and I've found what I think is an error. A BC337 is being driven from a D to A converter to power an LED in an optical sensor. The Emitter is connected to 5V, the base to a pin on the D to A, the Collector to a 470R resistor then the LED. I think this is wrong but it seems to work.

I have mimicked the circuit on a plug board using a 2N3904 and a 220K pot connected to the Base. The LED lights with the transistor connected both ways round as long as the base is connected to the pot but behaves differently each way round.

When the Emitter is connected to the LED the pot has to be turned down to around 50K before the LED lights then it gets progressively brighter. With the Collector connected to the LED it kicks in much sooner but still brightens.

Can someone enlighten me as to why this happens? Is it to do with the breakdown voltage of semiconductor junctions.



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