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Simple question about LED driving by transistor

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Mac7978

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Dear all

If uses a single NPN transistor to drive LEDs, can anyone tell me what is the advantage and disadvantage to put those LEDs at collector (graph A) or at emitter terminals (graph B)?

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btbass

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The turn on voltage for the collector connected LEDS can be supplied by a logic level output from something like a micro.
 

BradtheRad

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Introducing a load in the emitter leg requires you give more attention to the bias. You must raise your bias V so as to turn on the transistor sufficiently.

Therefore your B circuit will require a higher bias than your A circuit. The amount will need to be 2.0 to 2.5 V higher. (I'm assuming 1.8v for red LEDs, plus a few tenths V drop across your current-equalizing resistors.)

This can be done by reducing bias resistor (R80) to 100 ohms. A 3V control pulse will deliver a milliamp or two of bias current.

In comparison the A circuit has a more sensitive bias input which will operate on a lower control voltage. This is an advantage of putting the load in the collector leg.

By the way your equalizing resistors can be as low as 100 ohms per LED (for both A and B circuits). Any oddball LED will still be within a milliamp or two of the others.
 

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