Why is the transistor Q3 required in this circuit of a optocoupler and a relay. Can't I directly attach the power pin of the relay to the opto coupler pin 3 and ground the other pin of the relay. I tried it and it works. Just want to know why is it not implemented and why is the transistor required.
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The simple answer is the opto coupler is only rated for 50mA while the relay is either 72 or 90mA.
Under some circumstance you may be able to drive it (although it may not pull in correctly), but these have issues being drivn by a Raspberry Pi.
The 3v3, series resitor and LED n the input side combine to be just on the edge of triggering even with the transistor.
The answer is to add a second 1K across the resistor (R5 in the schematic).
Mark
