Work has commenced on the LowBrau controller with the installation of a pair of solid state relays (SSR) into a polycarbonate waterproof electrical box.
As the name suggests a solid state relay works very much like a standard electromechanical relay – using a low voltage, low current signal to switch a high voltage, high current on and off. Where a mechanical relay uses an electromagnet to physically make and break the contact of a switch a SSR acts more like a transistor to achieve the same outcome without any moving parts.
Inside an SSR there is a light operated switch consisting of an LED operated by the signal and a thyristor switching the load. This arrangement has some excellent advantages.
First, the LED side is low current and TTL which means that the SSR can be connected directly to the arduino without any need for transistors, resistors or any other components.
Second, the SSR can be switched incredibly quickly (milliseconds to microseconds) which means that pulse width modulation (PWM) allows for not only an on or off state, but also to vary the effective output level of the load by rapidly oscillating and varying the gaps of ‘off’ between the spikes of ‘on’ – in my case this will allow me to tail off the heat of the element to avoid overshoots.
Third, the SSR is fully opto-isolated (meaning that there physically is no connection between the low voltage circuitry of the controller and the household supply) and does not suffer from any of the back-EMF noise issues of electromechanical relays.
Lastly, my SSRs were quite cheap (~$3 each) so I bought two: one to modulate the heating element and the other to switch the pump.
Unlike a standard mechanical relay, though, SSRs do produce a fair amount of heat. This will be particularly so for the SSR controlling the heating element because it will be performing rapid switching on a 2.4kW load! To deal with this the SSRs have been mounted to a large aluminium heatsink which covers the back side of the controller box. Thermal grease between the SSRs and the heatsink ensure good transfer.
It is also worth noting the importance of properly earthing the heatsink (along with anything else conductive on the outside surface of the control box) as a loose connection could easily turn the aluminium heatsink into a dangerous conductor.