Solid State Relays (SSR)

A solid state relay (SSR) is a digital switch for AC current. With a small DC voltage, it can switch high AC currents.

When using SSRs to switch high currents (for heating elements), they do require a heat sink. For switching a small fridge or 60W heater, the current is so low that a heat sink is usually not necessary.

Compared to mechanical relays, SSRs have a few benefits: 

• They switch on an AC zero crossing, resulting in much smaller inrush currents
• They do not have a limited number of switching cycles, so they are suitable for switching often (as done with PWM).
• They are silent