Help selecting a NPN transistor

Question

Looking to use a relay with ardiuno but having a hard time trying to figure out what transistor I need to use, all the tutorials I can find always use an example transistor (usually 2N2222) to figure out what resistor to use, but not how to select a proper transistor in the first place. Once I have a transistor that is properly suited then I should be able to use those previous guides to figure out what resistor I need for arduino.

Can anyone enlighten me on how I should go about selecting the proper one for my needs? You're more than welcome to tell me what would work (properly suited not overkill) but I'd also like to know how to figure this out for future projects.

Note: The relay will be powered by a separate power source and not from arduino. Relay is not going to be located on or near the circuit board (remotely mounted) hence choosing a relay with a built in flyback diode.

Relay: 301-1A-S-D1-12VDC

Coil resistance: 123 Ohms, Coil Current: 97.5 mA https://www.mouser.com/datasheet/2/378/301-2555.pdf

Answer 1

If you drive a relay (with a flyback diode) you basically need to look at two parameters (or analogous for MOSFETs):

1. Continuous collector current (this must be larger than your relay coil current)

2. Maximum collector-emitter voltage (this must be larger than your supply voltage to the relay)

If the first one is not met, then the current will burn out the transistor. If the second one is not met then the transistor will be blown by excessive voltage.

After some years in the business you will just have your favourite parts (in this case my would be: IRLZ44N, BUZ11). If you need to drive many relays: ULN2003, ULN2803 (watch out not to exceed the max total current).

If you are buying from a small shop, then go into the right category, sort by price and keep going through datasheets until you find one that is good enough (and has some headroom).

If you are buying from a large distributor, then filter by the required parameters.

Answer 2

You can get special relay driver chips. These have darlington pair transistors (i.e. high gain ON or OFF essentially) and also incorporate fly-back protection diodes. They also drive high currents, if necessary for the relay. So a low power processor pin can easily drive very large relays if required.

Look at products like ULQ2004A