A solid-state relay (SSR) is an electronic switching device that controls power to and from an electrical load without using any moving parts. Like a mechanical switch or electromechanical relay, its primary function is to turn a load on or off in response to a control signal. Traditional switches and electromechanical relays rely on mechanical contacts that physically open and close when actuated, either manually or by energizing a coil. In contrast, solid-state relays use semiconductor devices to perform the switching function. Because there are no mechanical contacts, SSRs eliminate contact arcing, mechanical wear, and acoustical noise, resulting in quieter operation and significantly longer service life.

HBControls Power Controllers use semiconductor-based solid-state relays to switch power to or from AC and DC electrical loads. These relays most commonly employ SCRs for AC load switching and MOSFETs for DC load switching. In certain applications, other semiconductor devices—such as diodes or IGBT modules—may be used when specific electrical or control requirements dictate.

AC output solid-state relays are not ideal switches and exhibit a forward voltage drop (Vf) of approximately 1 Vrms when conducting load current. As a result, they dissipate power in the form of heat at a rate of roughly 1 watt per ampere of load current. For example, an SSR switching a 50-amp load will dissipate approximately 50 watts of heat during normal operation. If this heat is not efficiently removed, the temperature of the relay will rise rapidly. For illustration purposes, if a solid-state relay dissipating 50 W were suspended in free air with an effective thermal resistance of approximately 20 °C/W, the baseplate temperature would theoretically rise by 1,000 °C above ambient. In practice, the device would fail long before reaching this temperature, but the calculation highlights the necessity of proper heat sinking. HBControls Power Controllers utilize highly efficient heat sinks to safely dissipate this heat and prevent the solid-state relay from overheating during normal operation. Datasheets for each controller specify the maximum allowable load current at a given ambient temperature, eliminating the need for additional thermal calculations or heat sink selection.

Solid-state relays dissipate heat in direct proportion to load current, and their long-term reliability depends on how effectively that heat is removed. As a result, the maximum allowable load current of an SSR is directly tied to the thermal performance of the heat sink used in the application. For example, a 90 A solid-state relay can carry 90 A of load current only if it is mounted to a heat sink capable of dissipating approximately 90 W of heat in the given ambient environment. If the heat sink cannot safely dissipate that amount of power, the load current must be reduced to prevent the relay from overheating. In most applications, it is recommended to keep the SSR baseplate temperature below 80 °C, although many devices can operate safely at baseplate temperatures up to 100 °C. If a relay is installed in a 40 °C ambient environment and the baseplate temperature is limited to 80 °C, the allowable temperature rise is 40 °C. To dissipate 90 W of power while staying within this limit, a heat sink with a thermal resistance of 0.44 °C/W or lower is required (40 °C ÷ 90 W). If the same relay were instead mounted to a 2 °C/W heat sink, the allowable power dissipation would be limited to 20 W (40 °C ÷ 2 °C/W). Assuming a dissipation rate of approximately 1 W per ampere, the maximum allowable load current would be reduced to 20 A. Without any external heat sink, most AC output solid-state relays can carry only 5 to 7 amps of load current in a 40 °C ambient environment. While the relay baseplate provides some heat dissipation, its effectiveness is limited by size and surface area. HBControls Power Controllers utilize highly efficient, application-matched heat sinks to safely dissipate this heat during normal operation. Datasheets for each controller specify the maximum allowable load current at a given ambient temperature, eliminating the need for thermal calculations or heat sink selection.

Advantages Long life expectancySolid-state relays contain no moving parts and are therefore not subject to mechanical wear or contact erosion. As a result, they can operate reliably for decades in most applications. While life expectancy is typically specified as MTBF rather than number of operations, many solid-state relays installed in the 1970s remain in service today. Silent operationBecause solid-state relays switch electronically rather than mechanically, they produce no acoustical noise. This makes them ideal for commercial, residential, and noise-sensitive environments. Resistance to shock and vibrationThe absence of moving parts makes solid-state relays highly resistant to shock and vibration, eliminating false triggering or contact bounce in harsh industrial environments. Fast switching speedInstantaneous (random turn-on) solid-state relays can switch power to a load in less than 100 microseconds after receiving an input signal. This fast response makes them well suited for applications such as phase-angle control where precise power regulation is required. PLC compatibilitySolid-state relays can switch high load currents using only a few milliamps of control current, making them easy to interface directly with PLCs and low-power control circuits. Environmentally friendlySolid-state relays contain no mercury and typically have a much longer service life than electromechanical relays. Over the lifetime of a system, far fewer devices require replacement and disposal compared to mechanical switching technologies. Disadvantages Higher initial costThe upfront cost of a solid-state relay is often higher than that of a comparable electromechanical relay. However, when factoring in maintenance, replacement, and downtime, the total cost of ownership is frequently lower over the life of the application. Heat dissipation and sizeSolid-state relays dissipate power as heat during normal operation and therefore require an appropriately sized heat sink. This can increase the overall size of the solution compared to a standalone mechanical relay. Limited off-state galvanic isolationSome applications require complete physical disconnection from the AC mains when the output is off. In such cases, a solid-state relay alone may not provide sufficient galvanic isolation. A common solution is to use a solid-state relay in series with an electromechanical relay—allowing the SSR to handle load current switching while the mechanical relay provides physical isolation. This approach significantly reduces contact arcing and extends the life of the mechanical relay.

Zero-crossing solid-state relays are the most common type used in AC applications. As the name implies, these relays wait until the AC mains waveform passes through its zero-voltage crossing before switching from a non-conducting to a conducting state. This behavior minimizes electrical noise and conducted emissions by avoiding turn-on at higher voltage levels. Instantaneous (also called random or asynchronous turn-on) solid-state relays switch from a non-conducting to a conducting state immediately upon receiving an input signal, regardless of the instantaneous phase of the AC waveform. Typical turn-on times are less than 100 microseconds, making these relays suitable for applications requiring precise timing or phase-angle control. Both zero-crossing and instantaneous solid-state relays will continue to conduct load current after the input signal is removed until the AC waveform naturally crosses zero. As a result, turn-off time depends on the point in the AC cycle at which the control signal is removed, but will rarely exceed 8.33 milliseconds on a 60 Hz mains or 10 milliseconds on a 50 Hz mains.