Operation of PC817

Question

I am studying a DC-DC converter circuit design which has PC817 photocoupler in it. I am confused with the way PC817 behaves. My friend says that it is an analog photocoupler. I have tested it myself and it appears to work as digital photocoupler. Here are the test cases and results I got:

1. For input voltage less than 2V (10k ohm in series with supply), the output is 4.3V.

2. For input greater than 2V (10k ohm in series with supply), the output drops to zero volts.

3. For voltages closely around 2V (10k ohm in series with supply) the output varies between zero and 4.3V.

(4.3V is due to voltage sources at output side of IC)

I don't know how it is a analog optocoupler. Can anyone explain exactly how this IC operates?

datasheet

Answer 1

Any opto-coupler can be used to couple analogue signals but the distortion will be unlikely to impress everyone. The give away for this device is the following graph: -

• For a forward current of 1mA collector current is about 75% of 1mA i.e. 750uA
• For a forward current of 10mA collector current is about 130% of 10mA i.e. 13mA

This tells me the device is fairly non-linear and should be used for transferring digital signals.

Answer 2

Everything is analog if you look closely (until you look closely enough that quantum mechanics rears its head).

Perhaps your friend is comparing the photo transistor-output PC817 to the type of opto coupler that has an logic output type IC or a photodiode driving a transistor on the output. For example this one. They're typically specified in Mbaud and are more suited for high speed logic signals.

Answer 3

For input voltage less than 2V (10k ohm in series with supply), the output is 4.3V.

For input greater than 2V (10k ohm in series with supply), the output drops to zero volts.

For voltages closely around 2V (10k ohm in series with supply) the output varies between zero and 4.3V.

The "input" side of an optocoupler is an LED. Here's a typical optocoupler symbol:

For an analog optocoupler, if you vary the input current, the output current will vary approximately linearly (the graph in Andy's answer shows how much error there will be in this linearity).

It sounds like you tried to control the optocoupler with a varying voltage.

Since the input is an LED, that will tend to cause the input current to change very little at low voltages, and then change very quickly when you reach the forward turn-on voltage of the LED.

Try adding a series resistor (maybe 5k-10k) to the input side to get a more linear response to a voltage input (but you will still get very little output current for input voltages below about 2 V).