Considering this solid state relay:


The output voltage can be from 24 to 380V. How can I control this voltage? Is it proportional to the input voltage?

What I need is a voltage regulator that can be controlled from a Raspberry Pi in order to control the speed of a fan. Currently, I use something as below that works as expected.

enter image description here

I am completely new in electrical engineering, so please be kind with me...

  • 4
    \$\begingroup\$ A relay is an on/off switch. (Have a look in Wikipedia under 'relay'.) Theoretically you could control a fan speed by switching in on/or off fast (or slow) but that is not the right way to control a fan. I strongly suggest you do not use it for fan control. \$\endgroup\$
    – Oldfart
    Sep 13, 2019 at 12:59
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    \$\begingroup\$ The "FOTEK" unit in the photo is a fake. See protosupplies.com/…. \$\endgroup\$
    – Transistor
    Sep 13, 2019 at 16:38
  • 1
    \$\begingroup\$ @Transistor How can that be? It has the CE mark of quality! </sarcasm> \$\endgroup\$ Sep 14, 2019 at 3:54

3 Answers 3


I think you've misunderstood how a solid-state relay works.

  • The coil, or input, will work on 3 to 32 VDC.
  • The contacts, or output, are rated for 24 to 380 VAC.
  • When the "coil" receives its required voltage (SW1 closes below) it closes the internal "contact". In a normal relay, this is an electromechanical process. In an SSR it is an electronic process. Either way, this keeps both sides isolated from each other.

General Relay Diagrams:


simulate this circuit – Schematic created using CircuitLab

The voltage that the coil operates on and the voltage at the output depend on what voltage you feed into either side. There is no conversion going on in a relay. It is just a switch. I wouldn't suggest using what you've displayed as the main component of a voltage regulator.


A Solid-State Relay (SSR) is just that: a solid-state (semiconductor) version of a relay. A relay is an electro-mechanical switch that either connects or disconnects contacts together under control of an electro-magnet within it.

So your SSR is an on-off switch.

Unlike an electro-mechanical relay, it has no moving parts or electro-magnet. Its functions are carried out by solid-state components. It has a lot of benefits over an electro-mechanical relays. One useful to its applications is that the contacts cannot spark as they open or close because there are no contacts or moving parts.

And yours is only for use on AC voltages between 24 Vac and 380 Vac. If you pass DC through it, it can switch its contacts on but can't switch them off. The DC supply would have to be removed elsewhere.

  • 3
    \$\begingroup\$ Thanks for the lessons. I understand now my naivety. \$\endgroup\$
    – albar
    Sep 13, 2019 at 13:31
  • \$\begingroup\$ OOI, Can you shed some light on how to understand the rated minimum voltage? Surely when the relay is "closed", the voltage ought to be close to 0. Does the minimum mean that the "open" relay might start to conduct if there is not sufficient voltage across it? \$\endgroup\$ Sep 14, 2019 at 18:23

You may misunderstand how fans work

You need to check the spec sheet on the fan in question, but a great many fans do not regulate speed rheostatically. In fact, in residential wiring a very common mistake is to try to control fan speed with a lamp dimmer, which is itself not rheostatic and is a triac leading-edge or trailing-edge device.

You should be using a fan speed control which is compatible with the type of fan. For instance some fans have several wires going into a multi-position switch (e.g. The "pull the chain for the next speed" types). Others want a fan speed control. Yet others want variable frequency drive.

"How does a consumer/residential fan speed control work" might be a good question.

  • 1
    \$\begingroup\$ Thank you for your answer. However my fan speed is currently perfectly controlled by a manual voltage variator (as shown in my question). I just want the same thing, but driven by a Raspberry Pi, not by hand. I think I will use a digital potentiometer. \$\endgroup\$
    – albar
    Sep 15, 2019 at 9:12
  • \$\begingroup\$ @albar "It works" doesn't mean it's right. Lots of things work which are unsafe, especially if the electrical is AC mains. \$\endgroup\$ Sep 15, 2019 at 15:28

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