DIY high current Solid State Relay schematics

Solid State Relay -- SSR

In this article you learn how to make a DIY SSR using basic electronic components like Triac and Phototriacs which can be used for high current electrical switching. Solid State Relay or SSR is a very common component in the electronic industry because it is being used to interface between control systems like microcontrollers and microprocessors and high current high voltage devices like motors and heaters. In comparison with former electro-mechanical relays (EMR), a solidstate relay (SSR) does not make any sound or noise while switching. The current is also much higher than the electro mechanical ones due to components you can pick yourself.

SSR Applications

With the help of an SSR you get a low input voltage on one side and you can switch high voltage on the other side, for example you can open a 220v door lock with a 5v input signal from your remote controller. Usually you can switch between 24 and 380 volts AC on the other side which is good enough for most applications. There is a red LED on the case which glows when you turn the power on.

Make one yourself for your projects

You can make an SSR yourself with a low price using the proper industrial type components like Triacs and Phototriacs. The ready to use industrial SSRs in the market are built with resin inside to provide proper isolation between the low voltage control segment and the high voltage parts. The terminals are also completely sealed for electrical isolation and protection against motion.

The circuit diagram of SSR

ssr circuit diagram

This circuit is based on a reversed engineered solid state relay which you can do it yourself. There is a 6K8 resistor in series with the LED just as an indicator. Its brightness will vary depending on the control voltage. Then we’ve got one primary resistor of 750 ohms and then it branches out and there’s a 1uF smoothing capacitor, a 180 ohm resistor in series with the LED inside the phototriac and also there’s a Zener diode to limit the max voltage on the phototriac. The Zener diode limits the current through the LED because as the voltage goes up beyond it’s nominal 5.1 volts, it starts to conduct and that way it stabilize the voltage of that node. With the help of the Zener diode, you can connect it to higher voltages like 32 volts but if you plan to connect it to a voltage like 42 volts, it probably makes the resistor quite warm.

You can actually do the math for this circuit. 32 volts minus the 5 volt zener would be 28 volts, divided by 750 ohms, there would be 37mA, times by the voltage being dropped across the resistor, you need a res more than 1 watt to handle the dissipating. It’s better to drive it with 24 volts and I guess that’s the standard industrial control voltage.

The Triac is a bta600 which is a very standard triac rated for 12 amps and so. You can use other high current triacs as well. Always check the datasheet for the specification, like the transient peak current which is usually about 10 times higher. Also keep that current in mind when you wanna design the PCB in order to choose the right track width. Also take isolation into consideration when you design the board.

There is a snubber circuit on the triac too keep it safe from the transients. It is a 47 ohm resistor and a 10 nanofarad ceramic capacitor. You can also design a snubber networks with other elements yourself. It also helps to avoid false triggering. The pulldown resistor is also important to get a reliable triggering.

When you trigger the optotriac, current will flow through the 47 ohm resistor on the other side into the gate of the main BTA triac and as soon as this triac turns on, it immediately clamps that and it latches on. Check the specification sheet of the optotriac you use. It should be a zero crossing point detection opto-isolator. It means the optotriac will trigger at the start of the AC sine wave and that’s kinda important to prevent inrush current and EMI harmonics.