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.

enter image description here

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?

  • 2
    \$\begingroup\$ 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)? \$\endgroup\$ – The Photon Sep 6 '19 at 22:14
  • \$\begingroup\$ Yes. We are able to apply reverse voltage without blowing them up. \$\endgroup\$ – Eric Sep 7 '19 at 0:32
  • \$\begingroup\$ What is the expected reverse current per LED string? \$\endgroup\$ – The Photon Sep 7 '19 at 1:44

Your test method should work if you have a current-limited supply for the 12V. The supply provides 12V up to the maximum possible current for a working device, and limits above that. Many 12V switching supplies would probably work, but you might want to use a lab supply with programmable voltage and current and measurement . Some might be able to reverse output voltage in a programmable manner, or you might need to add relays. The one I linked can be interfaced with the PLC via Ethernet and uses standard SCPI commands, but there are many other options.

If you want good measurement of the leakage/shunt you'd probably want to add another LAN interfaced bench meter in series with the output so the current could be measured to uA or below.

This would be a pretty standard type of test setup (a few rack-mount devices wired to a test fixture with some operator interface).

Of course a bespoke custom PCB design could replace the off-the-shelf test instruments at considerably less parts cost, and that would be the first impulse of many engineers, but by the time you're done paying engineering time and maintenance you might be better off buying the calibrated traceable instruments.

The reverse LED leakage of all the LED strings in parallel would limit how accurately you could measure the shunt resistance "short". It's also possible to measure in the forward direction with a relatively low voltage but I think reverse would be more accurate at a given test voltage. Typically, at room temperature, that leakage is very low for most LEDs, but you'd have to check the data sheets and maybe make some measurements. It would be best to do the reverse test first while the LEDs are cold.

So the PLC or computer would need a program that would configure the one or two test instruments for each phase of the test plan, read the results for each and then present a summary to the operator. Maybe with some additional operator interface buttons, lights, beepers or whatever.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.