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Topic: Should my transistor heat up THAT much with this circuit (Read 1 time) previous topic - next topic

MarkT

The key phrases are "emitter follower" (which is the circuit you started with and only put 4V into the motor) and "common emitter" which is the one that can actually have voltage gain as well as current gain and thus switch 12V from a 5V microcontroller.
[ I won't respond to messages, use the forum please ]

AndrewStone

@eddiea6987:  As a rule of thumb, to use a transistor as a switch an NPN transistor should be on the "low side".  That is, connect it directly to ground and put your "load" (motor) "above" it (on the V+ side).  The reason is that to turn it on the voltage of the base needs to be raised a bit above (.7 volts BJT rule of thumb) the voltage at the emitter.  If the emitter is connected to ground that is really easy to do!  But if the load is below the emitter (i.e. between it and ground) it opens the question what is the exact voltage at the emitter?  This voltage will depend on the current running through the load, and what the load actually is, i.e. resistor, motor, led.  So it is a lot harder to guarantee that the base voltage is raised above that of the emitter.  You end up running your transistor in the "active" region (i.e. the switch is not fully on) so it gets nice and hot.

If you need a switch on the "high side", use a PNP transistor.  Connect its collector directly to V+.  Now to turn on, the base needs to be pulled below that of V+.  Again, that is pretty easy to do.   But note that in your particular case, since you are using 5v arduino and 12v motor, in fact you would not be able to turn the transistor off.  In fact what would happen is the Arduino pin would get exposed to 12v which would be very unhealthy for your chip!  So a PNP transistor/high side switch would not work for you without some addtl stuff between the arduino and the PNP's base...

Cheers!
Andrew
http://www.toastedcircuits.com Lightuino LED driver: 16 sources, 70 sinks, remote controlled.  Also high powered LED drivers.

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