i'm trying to use a 12 v dc fan with TIP122(npn).i've biased it correctly when i turn on my 12 v supply the fan does not spin instead makes a light humming noise.
Does the fan work if you connect it directly to the power supply?
Is the transistor turning on?
It may be you need PWM and a MOSFET.
yes the fan works perfectly fine
on leftmost pin (base) arduino+1k resistor. middele pin ( collector) wire to - terminal of fan. right most pin(emitter)the - ve of supply + supply to the positive of fan
Is this the way you have things?
You may have a bad transistor.
What is the voltage pin 2 to 3 on the transistor when you think the transistor is ON?
I/most would use a logic MOSFET.
What DC fan? Is it an inductive load? If its an inductive load you've missed the most essential component, the free-wheel diode.
Getting information is as rare as hens teeth.
Assumption, the ground to the Arduino was missing. .