Looking for OptoIsolators, NPN & PNP, >200 ma.

Using Arduino Motors, Mechanics, Power and CNC
#1

I’m redesigning two different Arduino circuits, into one. Both drive external relays. In re-designing, I’m looking for some Opto-Isolators that will drive the relays directly (no external transistors) so I need optos that can source and sink about 200 ma. (Actual requirements are less, but I want some safety margin.)

I’m replacing:

  1. First is currently a NANO, drives an 8 Channel 5V Relay Shield Module Board for Arduino. I use them to provide a switched 12v to the external DC antenna relays. (About 60 ma each).

  2. The second is currrently a MEGA (using 2 RS232 comm ports) and drives a series of 4N35 OptoIsolators to provide the necessary logic to drive relays (2 in this case, each about 60 ma, 120 ma total per circuit). These require a pull down to gnd to activate the relays. Because of the higher current, I added an NPN transistor to each output.

Because one set of relays requires 12v to activate, I need a PNP output opto, and the other set of relays requires a pull down to activate, I need an NPN output opto also.

How do I go about finding an pair of OptoIsolators to meet my needs?
Thanks in advance!

Sir Michael

#2

Why do (think) you need opto-isolators rather than simple mosfets for driving relays?

#3

Solid State DC relays are optoisolated. Examples

#4

200mA is a lot for an optoisolator. All optos have a ratio of input current to output current. Many times that is quoted as 100%. So 200mA out requires 200mA in. That ratio also degrades over the life of the device. After a few years it could be 50% of its starting value. So you need to design enough headroom to ensure that it gives the required output after that ageing.

So you need to find a 400mA (or maybe more) optoisolator. That's going to be rare. Everyone uses smaller ones with transistors, since transistors are much cheaper than optoisolators. Plus you would have needed a transistor on the input side to achieve 400mA input current and total current consumption for in plus out is now 800mA.

800mA drives a lot of relays directly. 8)

#5

Thanks for the replies, guess I’ll keep looking for something else. I wanted something like the opto to keep the circuit footprint small.

Sir Michael

#6