Driving relays with MCU GPIOs

If you use a multilayer board with a solid ground plane layer, and ground/bypass things well, there should be no problems. Bolt the board to your enclosure with metal spacers, with the mounting holes hard connected to the pcb ground plane.

Sure.

John

The +12 V could go directly to the relay board, but the real issue is the return current through the darlingtons to ground, in order to avoid the large currents flowing through the MCU ground plane.

Putting the darlingtons close to the power feed point on the MCU board will eliminate most of the problems, when the actual MCU ground can bounce around (which on the other hand can cause problem, if there are grounded inputs to the MCU).

Have you considered putting the darlingtons on the relay board ?

The +12 V side should not be an issue, but you should look at the ground side and look if there are common current paths in the ground (which always has some resistance).

Paul

A little decoupling and a good *solid* ground plane and you'll be fine. Make sure you put the free-wheeling diodes at the drivers.

I don't see any problems, assuming the regulator is stiff enough to supply the relays.

Thanks all. This will help.

I tend to prefer optoisolators in such situations. That way, you know you're not going to have problems. Certainly you can make it work with drive transistors, but you may need snubbing diodes, bypass caps, and good ground planes to make it all work.

John Nagle

I would put the ULN2xxx on the relay board if at all possible.

Do the relays have internal back EMF diodes? If not, you MUST put diodes on the relay board, otherwise the kick-back from those coils down 5-10 feet of cable could crash your CPU!

Actually, you need to buffer the GPIO outputs too! You don't want wimpy GPIOs driving 10 foot cables.

I suppose you're trying to avoid buffering the GPIOs.