I have been testing with the SSRF-240D25R to see his application on shutting down a load current that in my case is mains powered from the grid.

In order to see if the relay stops the currents flow as soon as I stop feeding the control side, I did a test where I put a LED on the load side and turned on the control side. I expect to see the LED turn on when I feed the control and turn off as soon as I turned off the control side. In the end, only one part of the experiment went as expected the LED turned on but stayed on long after a shut down the control side. I can´t see what might be the problem even though it should be something simple to resolve.

Here is an electric scheme of the montage:

enter image description here

In summary, the SSRelays+Detect is the 12 V that I use to feed my control side, to activate the Mosfet Q1 I apply 3.3 V (SSRelaysDrive), voltage converted from the 12 V to bias a voltage regulator and in this experiment I used my STM32 Nucleo, "MCUB", as the load.

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    You cannot test an SCR or Triac based SSR with a DC load. When you turn the LED on, enough current will flow to exceed the holding current of the SCR (and only one of them is on). – Jack Creasey Sep 14 at 16:13

Your datasheet tells us that this is a "25A SIP Solid State Relay With Paired SCR Output, Integral Heatsink". This sounds as though it is a pair of back to back thyristors and will behave rather like a triac. Thyristors and triacs work well on AC circuits but are not much use in DC circuits unless the DC is rectified mains with no smoothing.

enter image description here

Figure 1. Triacs (or thyristors in this case) once switched on remain on until the current through them falls below the hold-on value. With a steady DC supply this never happens. Image source.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 2. AC test circuit.

Try repeating your test will a small transformer - say 12 V or so - and a couple of back-to-back LEDs with current limiting resistor.

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