Looking at driving two LED strings with a DC/DC constant current LED driver module. The constant current side can't be referenced to ground of the control circuit, so I need to have an isolated switching to choose which of the two strings I would like to drive.
I'm "only" driving 300mA current, so I was thinking of using two optocouplers to select the string I want. I am having trouble finding a optocoupler that can take 300mA continuous current. How can I change my circuit so the optocouplers can drive 300mA? Can I add another transistor?
Schematic of what I had in mind:
(note - the driver I'm using cannot handle no-load situations, so I can't just use a SPDT relay, as I will have no-load conditions during switching causing voltage spikes and god knows what other issues)
Answer
The opto-coupler outputs don't need to switch the 300 mA current directly. Use a transistor or two to amplify the few mA thru the opto to the 300 mA thru the LED string.
In this case, I'd probably use the opto to switch the gate of a FET. Something like this:
You didn't say what the LED supply voltage is, so it's up to you to make sure that the opto output can withstand it when off. You also have to adjust R2 allow enough current thru it to raise the FET gate to 12 V when the opto is on, but not waste too much extra current. I'd aim for the zener drawing around ½ mA when the FET is on.
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