How to control 12V DC motor with a NPN transistor, a GPIO pin and a 12 V DC power supply

Question

I wish to control a 12V dc motor through the GPIO pins of a Raspberry Pi. I am currently referring to this tutorial.

I have a 2N222 NPN-transistor which will used as a Switch, a 12V-2A power supply, a resistor of 1K ohms and a diode.

This is the circuit I am using:

1. motor in the collector of the transistor
2. Jumper from 3.3V GPIO pin through 1K to transistor base
3. Emitter to ground
4. Battery negative to ground
5. Battery +12v through motor to collector
6. ground GPIO pin to ground.
7. a reverse biased diode in parallel

I fail to get a working circuit - every time the transistor ends up frying. Please, how I can fix this?

Answer 1

The 2N2222 is rated for a continous collector current of 600 mA only (that could be issue 1). To drive this current however a higher base current is required than is possible in this setup with the 1k base resistor - and the limitation of the Pi's GPIO pins (issue 2). For switching applications the goal is to drive the transistor into the saturation region (keyword to look out for when reading data sheets). For a collector current of 500 mA a base current of at least 30 mA or even more is required. If the base current is limited - in this case driven by 3.3 V and the 1k base resistor to less than 3.3 mA - saturation is not achieved and the collector-emitter dropout voltage increases. That in turn means that thermal dissipation in the transistor increases and might exceed its design limits.

Solution:

Select a transistor with appropriate ratings (e.g. collector current, Ic) and sufficient driving. Darlington type transistors or FETs come to mind.

Answer 2

If you only need on-off control of the motor you may wish to replace the motor in the above circuit with the coil of a 12V relay and use THAT to switch the supply to the motor. That is much more likely to be within the capability of the transistor! You will want to leave the reverse diode in the circuit of the relay's coil as it too will generate a reverse voltage when power is removed (though you will no longer have it to protect the transistor if someone should grab the shaft of the motor and give it a spin "manually" to see what happens - which is of course something that a project designer sometimes forgets to cover their project against!)