OpenSSR (Open Solid State Relay)

I’d like to start an Open Source Solid State Relay project because I find myself needing to switch high power loads fairly often.
The existing solution is often to get a chinese clone Fotek SSR which is quite bulky and has questionable quality.

The aim is to get the cost under £5 for the 10A and 20A versions and under £8 for a 35A version.

I have made a careful selection of parts which will give a AC/DC switching capability, AC (42V) or DC (60V) at 10A, 20A or 35A. All through-hole parts.

The circuit is full of protection: current limiting (to 10A/20A/35A), over-temperature cut-out, over-voltage protection.

My current schematic is:

To be added to schematic:

  • Status indicating LEDs
  • Correct R3 to be calculated to maximise switching time

Tentative specification:

  • 60V DC, 42V AC
  • Current limiting to 10A/20A/35A
  • Over-temperature protection
  • Over-voltage protection
  • 7-35V V+ on input side
  • Reverse polarity protection on input side
  • 2V or better input threshold voltage on Vctrl
  • High switching speed; supports PWM upto 10khz

I approve. You may continue. /royalwave


Schematic revision to incorporate LED for on-state, and added some components to reduce V+ inrush current during switching.

Don’t forget to check the protection diode current ratings for Q2 & 3, these can sometimes be lousy compared to the FETs max current.

If you add a bias resistor from Q1 base to gnd you can better set bias threshold in conjunction with R2, Q1 will turn on when the base-gnd voltage is about 0.6v.

You could also consider adding zero cross switching to reduce noise and stress


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Great suggestion with the extra bias resistor, I’ll get that in there.

Hopefully the protection diodes will never be forward biased?

The bipolar (AC) switch formed by Q2 & 3 relies on the body diode of the MOSFETs to operate so you will need to make sure they can cope with the current or add external higher capacity diodes (Schottky preferably).

Interesting, I thought the body diode only applies when the MOSFET is off… when the MOSFET is on, it’ll conduct in both directions. Hence why there are 2 MOSFETs - to ensure there is no diode drop in an AC scenario.

I have got a draft layout, any thoughts?

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PCBs are being made!

This is really a proof-of-concept version because I rather suspect I’ll end up increasing the size of the PCB to space out Q2 and Q3 more to enable the fitting of decent TO220 heatsinks.


Just wondering what software you’re using for schematics and layout.

Proteus by Labcenter Electronics

I’m still trying to wrangle KiCad. Looks like I’ll need to read some more in-depth documentation before I embark on getting PCBs fabricated.

I’ll be around on Thursday and Sunday’s sessions if anyone wants a hand with KiCad.


@Cylindric is probably going to re-draw my OpenSSR design in KiCad as a learning exercise, perhaps you could join forces?

The proof-of-concept PCBs have arrived! The slot cutout at the top came out really well, considering that the OSHPark recommended minimum slot width is 100th and this slot is 50th.


Tidy looking board. Must remember these people if ever I need a PCB.

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I have heard good things about Ragworm as well. Never used them personally but they have been at various Maker Faires and Pimoroni are using them for the new plug and play platform.

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Unfortunately Ragworm PCBs are orange, I rather insist on purple PCBs :wink:

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From what I have seen Laens purple PCBs kick Ragworm’s into the gutter, no real comparison quality wise.