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[Reply #15 on:]

I got some questions on the similar topics.

I want to drive a 12V PC Fan speed using an Arduino PWM pin.

I read a few examples on the net and got the following components from the local electronic stops to control the 12V PC Fan...

Darlington :TIP-122, NPN, 100V, 5A, Base current 0.12A

IRFZ44, N channel MOSFET, 60V, 50A  ( i think this is overkill, also the most expensive )

IRF630, N Channel MOSFET, 200V, 9A


I got it working with the TIP122 to control the fan speed using PWM... hv not tried the two MOSFET yet..

My questions, is it the "correct" component to use a Darlington TIP122 just as a switch for 12V fan, should I use something like
TIP31C, NPN, 100V, 5V Base current 1A instead ?

Is the Base current the max it can take, for my TIP122, I had a 1K resistor from the PWM to the Base.

Thanks

[Reply #16 on:]

If it works then it's probably the right device.  Typically for a transistor used as a switch your main concern is it's on/ off frequency response and then if it will withstand the operating voltages and currents.   If your concerned about power dissipation and not so much about frequency (switching a motor is low frequency) i.e. battery operation I would think about using the Mosfets.

Ideal conditions you want are as follows minimum transistor turn-on current and voltage (base to emitter current for NPN).  Near zero voltage drop across the transistor (Coll to Emitter volt.) when transistor is saturated.  Zero current flow when transistor is OFF.  All within your frequency (Bandwidth) of your application.

Datasheet TP122
http://www.fairchildsemi.com/ds/TI/TIP121.pdf

I can go on and on.