Solid State Relay for microcontroller

For a specific project with the Arduino – that I will publish later– I needed to switch a few 230 Volt appliances. This always makes me having to decide between a relay, a triac with optocoupler, or a ready made Solid state relay.
SSR’s can be had for fairly little money with the 39MF22 only at 1.80 euro. For that money I am not buying mechanical relays and don’t mess with triacs.

The circuit is quite simple:

R1 is there to limit the current through the LED and the 39MF22. The LED shows when the switch is active. The 39MF22 comes in a DIL-8 but misses pin 7. The switch is between pin 6 and 8. Pins 1, 3 and 4 are the ground connectors and need to be connected. It is advisable to connect them on a PCB to a little mass of copper so that can act as cooling. There is no snubber circuit as I prefer to have that closer to the load. One could however use a 100nF capacitor in series with a 100 ohm resistor.

As I needed 4 switches, I just built the circuit 4 times on a PCB. Initially I had considered mounting the circuit and the Atmega328 on one PCB, but I opted for a separate PCB out of safety concerns as it is switching 230 Volts.

The PCB is quite simple. (The lay-out for heat transfer can be found here.):
Solid State Relay with 39MF22 PCB


After finishing, I inserted the entire PCB in a plastic peppermint dispenser.

Solid State Relay with 39MF22

The 39MF22 can handle 900mA and has an isolation voltage of 4kV. The datasheet can be found here.



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