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Why is this transistor required

rishi2628

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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    There should be some notion of design margin.  Even if, say, the optocoupler Q is rated at 30mA and the relay coil is rated at 30 mA, the buffer Q should still be employed.  There is such a thing as specsmanship.  There is such a thing as design (process) variation.  There is such a thing as not being a stunod.  For every 10 degree K more heat (temperature rise, heat is not measured in Kelvins) you put into a device you halve its lifetime.

     

    We don't have to race to the bottom of being cheap if our applications are valid.

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    There should be some notion of design margin.  Even if, say, the optocoupler Q is rated at 30mA and the relay coil is rated at 30 mA, the buffer Q should still be employed.  There is such a thing as specsmanship.  There is such a thing as design (process) variation.  There is such a thing as not being a stunod.  For every 10 degree K more heat (temperature rise, heat is not measured in Kelvins) you put into a device you halve its lifetime.

     

    We don't have to race to the bottom of being cheap if our applications are valid.

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