Quick Test for Optocoupler needed

I need to test a Sharp PC817 Optocoupler.

Can I apply a 1.5 volt AA cell to pins 1 and 2 (pin 1 the positive side)... then simply check the change in resistance with a VOM on the output side (pins 3 and


Will that work as a quick test?



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Are you trying to blow the input? You are shorting a battery with a photodiode. Put at least 300 ohm in series. Or read the documentation on the optocoupler.

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Sjouke Burry


The LED in the opto, like pretty much all LEDs, must be current-limited. In this case, the absolute max is 50 mA. It looks like 10-ish mA would be a good target so, assuming your cell is 1.5 V (measure it) and the LED drops a "typical" 1.2 V (although it may be as much as 1.4), then using a 22 ohm resistor between the cell and the anode terminal should give you around 10 to 15 mA through the LED.

On the other side, the opto is set up in an "open collector" configuration. This is very common with optocouplers. When it's not "turned on," the transistor is off and "floats" unless it is pulled-up to some level. When it's fully on, it acts like a transistor switch and connects the collector side to the emitter side (neglecting for the moment max current limits and Vce at saturation).

Set up your test like this (use Courier or another fixed-width font):

___ .-----------------------------|___|----. | 330 | | | test point | _/ ___ | o-------o/ o-------|___|--. o---------o | switch 22 | | | | | .-o --- | |/ | - 1.5V V -> -| | | - |> | | | | | | | PC817 | | | | | ' GND GND GND GND (created by AACircuit v1.28.6 beta 04/19/05

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and connect a voltmeter at the test points. When the switch is open then you should see pretty much the full battery voltage across the opto. With the switch shut, the opto should turn on and you'll see somewhere around 0.5 V or less on the meter.

Rich Webb     Norfolk, VA
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Rich Webb

If you're trying to determine if the coupler is operational, connecting

1.5 directly will make it non operational in no time. You need to current limit the source.. If you have a current limited supply that you can set to around 20 ma's, then you can use that method how ever, if you don't, use a resistor in series.
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No, of course not. It isn't a relay.

You can connect the LED cathode to the transistor emitter, and treat the resulting three-terminal device as a transistor; a curve tracer should show the operating characteristic (and what you want to check will be the 'current transfer ratio' which corresponds to the transistor 'beta' measurement). The important part of this, is the LED current is specified at 5 mA for this measurement (so a multimeter's 'beta check' feature won't likely work, you really need a semiconductor curve tracer).

I presume, by 'quick check', you would prefer something simpler? You could wire up an oscillator to drive the LED input, and look for the output to switch an idiot light on and off... it'd be easy to use a self-blinking LED for the input, and a simple LED for the output, with output limit resistor and a 9V battery...

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