Relay driving? transistor, fet or ULN2803

[William At MyBlueRoom]

There are many ways to drive a small relay. When I've only got one or two relays to drive I usually use a 2N7000 fet. It has it's own clamp diode and doesn't require a resistor like a transistor would; although the transistor costs less it does take up more board space.
For lots of relays I'll use a ULN2803 octal driver, it's a popluar and easy way to drive upto eight relays and fairly high current.
Question 1, what is the minimum number of relays to justify a ULN2803 driver in your opinion?
Question 2, what parts do you use most often to drive a relay?

 

[philba]

Question 1, what is the minimum number of relays to justify a ULN2803 driver in your opinion?
Question 2, what parts do you use most often to drive a relay?

based on what? parts cost? board space? both are fairly straightforward to calculate. building 1, 1000, 1000000? assembly cost could be an issue as well.

Also, what kind of a relay are you driving? Coil current can make a difference.

For coils less than 100 mA, I typically use a small signal NPN (2n2222, 2n3904, ...). at a few cents per transistor, it's hard to beat, cost wise. You do, of course, have to consider duty cycle.

The ULN2803 has base resistors and diodes so you can save a fair amount of board space. It's a bit overkill for the typical relay current needs but is relatively cheap. I've used one to drive 4 relays. Mainly because I had one and it was less assembly work...