How to isolate 5V and 12V circuit

The most important points are:

The 12 volt supply and the 5 volt supply commons connect at one point, only, at the relay driver

The relay supply be bypassed to this common point with a capacitor, before the power heads for the relay coil.

The relay coils have reverse voltage suppression connected across them. This can be a diode, a diode in series with a zener or resistor, or a series RC (or some other choices). The point is that if a driver turns off the relay coil current and there is no place for the inductive current to go, the coil produces a very high voltage pulse.

These 3 solve most relay next to logic problems.

If I could add one more to Mr. Popelish's list:

• Do what you can to minimize arcing at the relay contacts, especially if you're switching an inductive load.. The EMI can cause upsets on logic level lines.

One good way to do tis is to put an R-C snubber across the inductive load. What this does is reduce the rate of rise in voltage across the load so the relay contacts can get far enough apart to avoid arcing.

Good luck Chris

I have always totally isolated, using split planes and creepage distances, any area of my boards that drive relay coils. Yes, snubbers on the load and diodes (TVS) on the coil help. I would still supply an isolated 12V to the relay-driving side of my circuit and communicate through optocouplers to it.

As an alternative, you might consider solid state relays. They might be a bit more expensive than the mechanical one you are using, but they won't kill the rest of your circuit with +/- spikes. They come in big and small sizes (I use them to drive 6A motors!).