I replaced some old electromechanical relays by those solid state relays from Wieland to improve the switch speed of my system but I can't manage to switch off the solid-state relay.

Here is the relay schematic as presented in the datasheet:

Solid state relay schematic

I am trying to control the relay with a digital output comming from a PLC (24V) connected in A1 while with the ground is in A2. My objective is to control the release of pulses coming from an encoder (+13) and going to a camera (14). When the PLC output is turned on, the LED in L1 brights up and it shuts down when the PLC output is turned off. Now, even when the light is switched off (or when I remove the cables) pulses pass from +13 to 14.

I have a quite basic knowledge in electronics and was expecting I could use the same circuit as with my electromechanical relays. I am aware of the problems related to triacs when people buy AC solid state relays instead of DC ones but I don't think this is the problem here as the constructor clearly state it is made for DC.

  1. Do you have any idea why the relay never switches off ?

In addition, I suppose it is probably not relevant but...

  1. What does represent the half circles on the top of +13 and below -A2?

  2. In the datasheet, the Nominal input current is between 4 and 6 mA. Does that mean that a bigger current would pass the relay no matter the PLC output? Since that value was not specified at the seller's site, I'd be quite surprised that such a discriminating value would not be better advertised.

EDIT: Here are some voltage measurement I made as requested by user Transitor. Without any load (relay not connected to the pulse divider):

  • When the PLC output is off, from pin A1 to A2: 200.0 mV

  • When the PLC output is on, from pin 14 to 0V: 0.5 mV (so ~0V)

  • When the PLC output is off, from pin 14 to 0V: 0.0 mV With a load:
  • When the PLC output is off, from pin 14 to 0V: 3.7 V
  • When the PLC output is on, from pin 14 to 0V: 10.27 V --> that should be interesting as I suppose a circuit could be used to lower the voltage so that my device is not detecting it when the plc output is off. Still I don't know why this small voltage exists.
  • 1
    \$\begingroup\$ Measure the voltage from pin 14 to 0 V with the input on and the input off and add the results to your question. Add a make and model and a link to the user manual for the camera. \$\endgroup\$
    – Transistor
    Jan 8, 2019 at 22:49
  • \$\begingroup\$ Actually, it is not connected to the camera directly but to a pulse divider (motrona it251 - input 4 to 6). If I remember correctly, I measured a potential equal to ~10 mV (between 13 and 14) while it was 0 with the input off. I'll make more measurement when I'll be back at the office in the morning. \$\endgroup\$ Jan 8, 2019 at 23:21
  • \$\begingroup\$ Input 4 to 6 on the HTL input? Do you mean you have three inputs and that the 0 V line is connected to 10 as GND? \$\endgroup\$
    – Transistor
    Jan 8, 2019 at 23:29
  • \$\begingroup\$ Yes, it uses a A, B, and Z signal input (pin 4,5,6) then generates A, \A, B, \B, Z and \Z (pin 1,2,3, 7,8,9) powered by 24V DC (pin 11). I assume the signals are passing through the SSR because the pulse divider is also equiped with a LED indicator which blinks extremely fast when I turn on the conveyor belt (connected to my encoder) and stops as soon as the conveyor is stopped. \$\endgroup\$ Jan 8, 2019 at 23:46
  • \$\begingroup\$ Yes, it sounds as though the PLC side is OK. Let's see what the measurements show up. Make sure that the opto power ground is connected to pin 10 on the converter. Bed time in Ireland. \$\endgroup\$
    – Transistor
    Jan 8, 2019 at 23:54

2 Answers 2


The solid state relay you're using is designed for switching moderate current loads on and off, from a power supply, and not for switching signal lines.

Unlike a mechanical relay, your solid state relay behaves as if there were a diode in series with the relay contacts.

You can likely easily adapt it to do what you want but you'll need to understand in which direction the current is flowing in your existing circuit.

What's most likely is that the encoder signals to the camera by pulling the input to the camera low by connecting it to ground.

If that's the case you need to reverse the connections on pins 13 and 14.

It's also possible that your camera has a very high impedance input, and that you need to pull the camera input low or high as appropriate with a resistor to compensate for the leakage current through the solid state relay.


The circuit as shown is a bit problematic:

If, and I presume that's the case, the voltage from the PLC is 24 V AC, (otherwise you wouldn't need the rectifier), then your rectified voltage would be zero twice per source period (i.e. at twice the frequency). That would neither be good for the relay nor the switched load.

However, you didn't report anything switching on and off 100 times a second, so I presume there's something to "smoothen" the output of the rectifier – a capacitor – not shown in the circuit. It might be built in to the rectifier or the relay!

So, that capacitor charges when the PLC turns on the control voltage. After you turn of the control voltage, it has no path to discharge, aside from through the relay.

That relay, being a solid state one, will have a high input resistance, and thus will take forever to discharge the capacitor. As long as the capacitor's charged, there's voltage across it, and thus, the relay won't turn off.

The solution is easy: add a load in parallel to the relay. The load size depends on the size of the capacitor, but for an easy start, simply try with a low-wattage incandescent bulb (e.g. from a truck).

  • \$\begingroup\$ My interpretation of the OP's diagram is that the bridge rectifier and LED are built into his SSR, and that the SSR is capable of operating on AC or DC. The question also states that the LED turns on & off as expected as the PLC output changes state. As such, I doubt that the problem is on the input side. \$\endgroup\$
    – brhans
    Jan 8, 2019 at 23:11
  • \$\begingroup\$ Indeed the seller advertises "Input voltages of 24Vdc up to 230V ac" while the constructor just advertises " input: 24 V DC, output: 4,4 - 5 3 V DC/0,5 A" which is confusing... However, the PLC output is definitely 24 V DC (I measured 22). The diagram indeed represents the SSR in-built circuit. \$\endgroup\$ Jan 8, 2019 at 23:34

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