I'm looking for a recomendation for a part No. for a diode to put across the relays in my panel. They are 24 VDC DPDT P & B relays. I thought a 1N4002 should work. What have you used?
The coil when switched off can spike up to 6x or more of supply so voltage is important but the most important aspect is this happens fairly quick so I think high speed diodes are just as important.
1N4002 reverse rating is 120 volts. ON semi datasheet.
1N4004 and 1N4007 are prefered devices (mass-produced).
There isn't a speed rating, because these aren't designed for fast applications. In most cases, they're fast enough.
I prefer and use 1N4937. Datasheet. These are rated for freewheeling applications. Again, 1N4935 and 1N4937 are prefered devices. That means they are mass-produced so price should be lower.
The link posted by Ron above is very good, but I have one caution. They say '(eg 1N4148)'. That diode is fast, but only rated for 200 mA. You could only get away with that for the tiny relays they are discussing.
Use a 1N4937 as a universal device. 1 amp, 600 volt.