Thread: 5v to 24 volt relay driver

The output transistor of the 4N25 will handle up to 70V at 50mA, so depending on the relay current, you may not need the external BC547s and associated resistors.

Originally Posted by sjm

The output transistor of the 4N25 will handle up to 70V at 50mA, so depending on the relay current, you may not need the external BC547s and associated resistors.

Ah, yeah.... that's what I was hoping with version 1 of the design and drove the relays direct from the opto's. I used NY2 and NY4 relays but didn't check the coil currents. I presumed (ooops) that they would be OK, but I think the pull in current was a bit much for the 4N25's.
I just bit the bullet and put the 547's as drivers to stop the mucking about. From memory, there's not a great deal more currnt rating on them, but it was enough to do the trick (that's what I had in my junk box).
The resistors are probably overkill, just trying to stop any floating that may be in the circuit.

Fair enough. I'm assuming you've fitted back-EMF diodes directly across the relays, since they don't seem to be on your board?

I checked the specs on the relays this morning (Finder relays) and the coil currents are between 30mA and 70mA depending on it being a 2 pole or 4 pole. That's probably the operating current, so it would be a little more for the pull in amps.
I didn't use any diodes because the spike from drop out is minimal into the 24V supply. I know it's good habit, so I'll revamp the schematic and repost.
I think I'd also probably use some higher current drivers too, because I'm now thinking of direct driving a coolant solenoid as a future project.