Hi there
I have a 24V 5 pin automotive relay and i would like to power it's coil via a transistor connected to my arduino and 24V.
What would be the best transistor for this application?
Relay coil 24V DC, 200mA
I would use a logic level N channel MOSFET but many NPN transistors would work plus a base resistor of some 220 - 470 Ohm.
It's hard to find one that can not be used therefore.
What's your "best" criterion? Price? Color?
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I need it to be very reliable ie not fail within a year.
I am putting a flyback diode across the relay so it should be protected from reverse voltages.
I read that the 2N2222A is pretty good.
Almost all "correctly used" transistors don't fail in just one year.
I use the 2N3404 and will until I die or run out, looking like 50/50 on which comes first. 
Def gonna leave some 2N3906s behind.
a7
2N2222A should work well with 390 or 470 Ω base resistor and a 5 volt Arduino, just be sure to get collector and emitter leads right.
https://media.digikey.com/pdf/Data%20Sheets/NXP%20PDFs/2N3904.pdf
That's not what a flyback diode does.
It protects the transistor from a higher voltage when the relay turns off.
Leo..
I would suggest you post a schematic.
For FETs you would need a logic level device.
Check the data sheet for maximum current.
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