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
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
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.
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.
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.
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.