voltage comparator driving an optocoupler rather than EM relay
voltage comparator driving an optocoupler rather than EM relay
(OP)
I have put together circuits that use an operational amplifier to drive a transistor-relay circuit. If I want to drive a optocoupler could I hook that optocoupler (-) pin right to the opamp output? Effectively, the opamp is driving only 15mA led.
Thanks
Thanks





RE: voltage comparator driving an optocoupler rather than EM relay
You must also not violate the LEDs reverse voltage spec! You can prevent this with a diode in parallel with the LED.
RE: voltage comparator driving an optocoupler rather than EM relay
RE: voltage comparator driving an optocoupler rather than EM relay
Now an LED is not a proper diode and so they pay no attention in their design to PIV ratings. Hence they are usually pretty low. They may be only 5 volts or so. If your opamp jams out 15Volts well then ...PIFFT. However if you opto's LED has a 25V PIV by some freak of nature then you wouldn't need a reverse diode to protect the LED.
RE: voltage comparator driving an optocoupler rather than EM relay
RE: voltage comparator driving an optocoupler rather than EM relay
RE: voltage comparator driving an optocoupler rather than EM relay
Then you don't need that extra diode I mentioned.
Often you would want to buffer the signal in a case like this because the comparator input may have feedback that will tweak the analog signal a little.
RE: voltage comparator driving an optocoupler rather than EM relay
RE: voltage comparator driving an optocoupler rather than EM relay
RE: voltage comparator driving an optocoupler rather than EM relay
Because a comparator usually needs hysteresis to work reliably. Another words, once the comparator has flipped the inputs must reverse back PAST the spot where the flip occurred. This hysteresis would mess up the linear aspect of a follower. The LM393 does not appear to provide any hysteresis so they may expect you to provide it externally (with resistors) so you might be able to use one as a unity gain follower. Try it.
Conversely you will probably have problems with your comparator part of the circuit with the required hysteresis.
RE: voltage comparator driving an optocoupler rather than EM relay
RE: voltage comparator driving an optocoupler rather than EM relay
RE: voltage comparator driving an optocoupler rather than EM relay
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