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I have problems triggering a SSR (Solid State Relay) from an industrial robot controller.

Gear

Context

  • Beckhoff modules are OK: output is approx. 26 V rated for 500 mA, it can light up LEDs
  • SSR is OK: it triggers using an external DC power supply, and consumes 24 V 20 mA
  • SSR also work when powered directly by the robot controller PSU (24 V 10 A) and consumes 24 V 20 mA

Problem

When powered by the Beckhoff the SSR isn't triggered. On the control side I measure approx. 5.4 V and 2.5 mA.

Troubleshooting

I switched outputs on my Beckhoff, switched SSR (even to another brand), switched robot controller: always the same result, SSR doesn't trigger through the Beckhoff.

Has anybody got a clue as to what's happening here?

Edit: I corrected my original post because I messed up my current measurements.

My Beckhoff outputs 24 V 500 mA, but somehow when I plug my SSR the voltage drops to 5.4 V and current 2.5 mA.
Also the SSR works fine when plugged upstream directly to the robot PSU (24 V 10 A).
It seems the situation is caused by the Beckhoff.
Wiring has been triple checked and is consistent with 3 other robot controllers we have.

Could the voltage drop be a safety feature of the Beckhoff?

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  • \$\begingroup\$ DC "SSR" may need to be on a particular side of the load, unlike a true relay. If it is only dim, you should verify what voltages you get and where. What voltage do the LEDs need? \$\endgroup\$
    – Abel
    Commented Mar 5 at 3:24
  • \$\begingroup\$ Sorry I was unclear about the LED : it's an on-board LED on the SSR that lights up when the relay is triggered. \$\endgroup\$
    – AntoineG
    Commented Mar 5 at 20:33
  • \$\begingroup\$ @AntoineG - Hi, You wrote an "answer" but it was actually more info, not a solution, so it has been added to your question as an edit (i.e. an update) instead. || Since you asked the question, then unless you are writing the full & final answer to your own question (i.e. unless you don't need further help & you have solved the problem yourself using a different solution than any other answer) please don't use the box labeled "Your Answer" below. Instead, to add clarification, please edit the question (unless that would invalidate an answer). || Please see the tour & help center. TY \$\endgroup\$
    – SamGibson ♦
    Commented Mar 5 at 20:44
  • \$\begingroup\$ Noted ! Thanks for explaining ! \$\endgroup\$
    – AntoineG
    Commented Mar 5 at 21:17
  • \$\begingroup\$ The answer might be in there if part of it means that 0V must be shared and it works. I wouldn't worry about the onboard indicator LED if the relay is doing its job- see if you can control an external load properly. \$\endgroup\$
    – Abel
    Commented Mar 6 at 1:57

1 Answer 1

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Wiring is shorted or incorrect. Make double sure you know where the 24V and ground for that EL2809 is. It's routed via power contacts on the bus. With the system off, measure continuity (in resistance mode!) between where you think the 24V and 0V source is, and the power contacts on the downstream (right) side of the 2809. The SSR control input must be connected between one 2809 output and the 0V power contact bus.

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