Solid state relay - operation issues - Crydom - 84134750

I bought a solid state relay - Crydom - 84134750.
The control side of this SSR is 24VDC by a wago PLC, on the other side the Consumer is a DUAL DC pump - GEO-DUPLO-PLUS 12V - 7.1A
since this consumer is inductive, I connected a diode - 1N5408 1000V, 3A - to protect the SSR. I put the system under voltage and
the pump also works without control. Why is this possible, how to protect against it?


Thank you for your question. We are working on this and should have a response shortly.

Thank you

Ryan Urness

A diagram or photograph showing the connections within the system would be helpful.

Incorrect connections are a common cause of this type of behavior. Many DC-output relays will conduct continuously if the output is connected backwards. If you have a clamp-on type current meter capable of DC measurements available, these can be a useful tool for determining the presence and direction of current flow in this sort of system.

Hello balintjoe,

How is your pump project progressing?

I have one parting thought that may be of interest to you and our readers. Based on your description, I assume you used a connection such as this snippet from the datasheet:


The 1N5408 diode may be under specified for this application. My thinking goes like this.

  1. Suppose the motor is running at its full rated current of 7 A.

  2. The SSR turns the motor off.

  3. For a brief period of time, the full 7A is commutated through the diode.

To be sure the 1N5408 is rated for approximately 200 A for a few milliseconds. You may be better off with a diode such as this:

1n3889.pdf (

This is a larger stud mount device. Notice that the anode and cathode may be interchanged - 1N3889 vs 1N3893R. This allows us some flexibility with regards to diode mounting. For example, if you look at the Crydom schematic above (right) you can envision the diode being bolted (anodes side) into the chassis.

Something to think about.

Do you agree with the diode specifications? Have I over specified the device rating for this application?

What is the best choice when we consider cost vs reliability?

Best Wishes,


P.S. Could you post pictures of your project? It’s good way to learn from each other.