transistor + relais

Hi folks,

I'm currently working on an arduino based Laminator-control, and the relais ist driving me crazy!

My setup is as follows: I use the analog 0 Pin set to pinmode output, and as a test I turn it on/off every second. Attached is a 2n3904 to base with a 19k resistor. Across C/E I feed 12V with a 220R resistor in line, to reduce current for the relais. Grounds are connected.

I orignally thought that 19k is to high to fully open the transistor, but this is obviously not the case, since even when the resistor is omitted, the relay won't trigger!

Appreciate your help, this is driving me crazy since I used the very same resistor in my ambilight project to switch 12V LEDs.

cheers!

I use the analog 0 Pin set to pinmode output,

But your drawing says something different.

Your relay should be connected to the collector, not the emitter, and should have a diode across it to protect your transistor.

But your drawing says something different.

would this be a painto? ;-)

Your relay should be connected to the collector, not the emitter, and should have a diode across it to protect your transistor.

Yeah! Thanks a lot! Can you explain me why? I always imagined a transistor to act as some sort of switch, which turns current on/off so it shouldn't matter on which side I connect it!

cheers!

I always imagined a transistor to act as some sort of switch

Think of it as a current amplifier instead. Only when it is in saturation is it a switch.

You had the transistor in a common collector configuration. The voltage on the emitter can never be higher than the voltage on the base, which in this case is 5v. So the relay never gets any more than that and so will not trigger. You need a common emitter circuit.

Thanks :-)