# Detecting a short in an enclosed circuit with a PLC

I have an LED circuit that may or may not have a short. It looks somewhat like the image below. The short is labeled 500 ohm in the drawing. The short can actually be anywhere from < 1 ohm to millions of ohms. There can be anywhere from one set of three LED's to 30 - 40 sets of three LED's in parallel. Two sets of three LED's are shown in the image below.

I need to have front line employees test thousands of these for the short. I want to make a PLC that will do the following:

1. Provide power for the circuit
2. Detect if the wires from the PLC are not connected to the circuit
3. Check if the wire from the PLC are correctly connected to the circuit.
4. Tell the operator if there is a short, the tester is not connected or if the tester is connected and the part does not have a short.

I was originally and wrongfully thinking that I could have the PLC do a series of two alternating tests:

Test A. Power the circuit normally (positive to positive, negative to negative) and check voltages or amps being drawn.

Test B. Power the circuit reversed (positive to negative, negative to positive) and test for voltage.

I am now officially over my head. Does anybody have any suggestions of a test algorithm that will determine polarity of the circuit, if there is a short and if the wires are making connection?

• A megohm short (producing ~12 uA excess current) is going to be very difficult to detect when your normal operating current varies from 50 to 2000 mA. What is the expected reverse current for these diodes? Can you apply 12 V reverse voltage without blowing them up (Many LEDs are not tolerant of reverse voltages)? – The Photon Sep 6 '19 at 22:14
• Yes. We are able to apply reverse voltage without blowing them up. – Eric Sep 7 '19 at 0:32
• What is the expected reverse current per LED string? – The Photon Sep 7 '19 at 1:44