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10350 Views 3 Replies Latest reply: Feb 28, 2014 1:05 PM by davecuthbert RSS
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Oct 23, 2013 3:00 PM

protecting optocoupler input

I manufacture a device that uses a 0-5V TTL data input to a 4N35 optocoupler.  I have a 47 ohm resistor and T 1-3/4 LED in series with the optoupler to give me a visual input status and current limit.  All works fine in normal use, but it blows the resistor, LED, and/or resistor when customers accidently connect the 24V power supply input to the data input connector.  I need to come up with reliable overvoltage/overcurrent protection on the data input.  A fuse is not an option since the device will be potted in epoxy.  I was thinking that replacing the 47 ohm resistor with a PTC would be a good option.  Is there a better way to do this?

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