A solid-state relay is basically an electronic switch or relay, but without any moving parts. They perform the same basic function – switch power to / from an electrical load – but do so in a completely different manner. Switches and electromechanical relays contain mechanical contacts that change state (open or close) when a button is pushed, an actuator toggled or, in the case of electromechanical relays and contactors, a voltage is applied to their coil. Crydom solid state relays utilize semiconductors to perform the switching function, which eliminates arcing, mechanical wear and acoustical noise.

The output semiconductors of solid state relays are not perfect switches. The SCRs of AC solid-state relays have a forward voltage drop of about 1Vrms during normal conduction, which means that they generate heat at a rate of about 1 Watt per ampere of load current. This isn’t an issue for load currents below 7 or 8 amps since the base plate of the solid-state relay will effectively transfer some of that heat into the surrounding ambient air. However, once the output current exceeds 8 amps, the base plate can no longer efficiently transfer the heat generated. This can result in excessive internal temperatures that can degrade the life-expectancy of the solid-state relay and lead to premature failure.

An external heat sink is basically an extension of the solid-state relay’s base plate. Heat generated by the SCRs is transferred through the base plate and into the heat sink, which is subsequently cooled by the surrounding ambient air. How well the heat sink transfers this heat is dependent upon its thermal impedance rating, typically expressed in °C per Watt (°C/W). If a solid state relay is mounted to a 1°C/W heat sink, then the temperature of the relay’s base plate will increase at a rate of about 1°C for every ampere of load current that it’s switching. As the efficiency of the heat sink increases, the °C/W rating decreases. A heat sink that’s 30% more efficient than the 1°C/W heat sink referenced above would have a 0.7°C/W rating, for example, which means that the base plate temperature of the solid-state relay would be 30% less at the same load current.

The specified load-current rating of a solid state relay is dependent upon several factors, including the size of the SCR die in the output circuit, the lead-frame design, wire-termination capacity, and other various design parameters. However, the actual load-current rating of a solid state relay is dependent upon the efficiency of the heat sink to which it’s mounted. Solid state relays generate heat at a rate of about 1 Watt per ampere of load current. This heat must be transferred away from the internal SCRs, through the relay’s base plate and into the surrounding ambient air to prevent it from overheating. Mounting a solid state relay to a heat sink increases its ability to transfer this heat, and how efficiently the heat sink transfers the heat determines how much current the solid state relay can continuously carry.

A common mistake made by many engineers around the world is to use solid-state relays with higher current ratings whenever there’s an issue with the temperature of a solid-state relay with a lower current rating. For example, a Crydom 50 amp solid state relay generates heat at a rate of 1 Watt per amp and is rated to 50 amps when mounted on a 1°C/W heat sink. A Crydom 125 amp solid state relay also generates power at a rate of 1 Watt per amp, which means that its maximum allowable current rating when mounted to the same 1°C/W heat sink is also approximately 50 amps. The internal temperature of the 125 amp solid state relay will be slightly cooler due to the efficiency of it’s design, but not enough to provide a significant increase in the actual current rating. In order to carry more than 50 amps, a larger, more efficient heat sink (or forced airflow) is required.

Advantages;

· Life expectancy – Solid-state relays can operate reliably for decades in most applications. Theoretically they can operate reliably for centuries but, since the first solid-state relay was invented in 1972, that theory has yet to be proven. However, there are Crydom solid state relays that have been in the field since the 70s that are still operating normally and reliably.

· Silent operation – Solid-state relays do not generate acoustical noise when switching power to/from the load since they contain no moving parts. Therefore, they are the preferred solution in many commercial or residential applications where acoustical noise can quickly become an annoyance.

· Shock & vibration resistance – Since solid-state relays have no moving parts, they are not prone to false triggering under load in harsh environments subject to shock and vibration.

· Fast operation – Instantaneous turn-on solid state relays can switch power to a load in less than 100 microseconds of receiving an input signal. This makes them ideal for phase-angle control applications where tight control over power to the load is required.

· PLC compatibility – As opposed to large contactors or other mechanical relays, solid-state relays can switch heavy loads with only a few milliamps of input current.

· Environmentally friendly – Crydom solid-state relays contain no mercury. Also, the abbreviated life expectancy of electromechanical relays means that hundreds, if not thousands of EMRs will fail and have to be disposed before a single end-of-life solid-state relay failure.

Disadvantages;

· Initial Cost – The initial purchase price of a solid-state relay may be seen as prohibitive in some applications. However, due to the limited life of EMRs, the total cost of ownership of a solid-state relay (the impact on cost over the life of the product in which it is used) is often significantly less than an equivalent rated EMR.

· Size – All solid-state relays dissipate power in the form of heat during normal operation and therefore require an external heat sink in order to operate reliably.

· Galvanic isolation – Some applications require the load to be physically disconnected from the AC mains when in the off state. In these cases, the addition of a series electromechanical relay to provide off-state galvanic isolation might be the only viable solution. Energizing the EMR first in the sequence places the burden of switching full-load current on the solid-state relay and significantly reduces, or eliminates altogether the arcing of the EMR contacts, as does removing the input to the solid-state relay just prior to de-energizing the EMR when switching to the off state.

HBControls primarily uses Crydom AC solid-state relays with inverse-parallel SCRs in their output circuit. Many low-cost solid state relay manufacturers utilize triacs in the output of their relays in order to reduce cost and simplify the manufacturing process. However, triac-output solid state relays have a higher forward-voltage drop than SCR output solid state relays, which means they generate significantly more heat at a given load-current. Triac-output solid state relays also transfer heat through the base-plate less efficiently and, therefore, require larger or more efficient heat sinks than an SCR-based solid state relay.

Solid-state DC relays use low-impedance MOSFETs in their output circuit. However, a limited range of economical bipolar transistor output solid state relays are also available.

“Zero crossing” refers to the point in the AC sine wave at which the solid-state relay begins to conduct load current after the input signal applied. If an input signal is applied when the AC sine wave is close to its peak (outside of its “zero-crossing window”), the solid-state relay will not turn-on until it reaches the zero-crossing point. The zero-crossing function gives solid state relays a significant advantage over electromechanical relays and contactors in applications with loads that have a low initial resistance. Since the output of the SSR will not turn on until the sine wave is close to zero, inrush currents are kept to a minimum.

Instantaneous, or “random turn-on” solid-state relays will begin to conduct load current within 100µS of the input signal being applied. While this could be problematic in low-resistance applications, it’s a required feature in phase-control applications where the solid state relay is being precisely controlled to only provide a percentage of AC power to the load. Typical applications include light dimming systems, or heating applications that demand exact temperatures. They’re also used with highly inductive loads due to the phase-shift between the output voltage and current.

With HBControls, you don’t have to worry about calculating heat sink requirements for solid state relays. Nor do you have to source multiple components, apply the exact amount of thermal compound or worry about whether the proper mounting torque is being applied when assembling the solid-state relay to the heat sink. All of this is done for you. HBControls power controllers are solid-state relays pre-assembled onto efficient, custom-designed heat sinks. Their actual load-current rating is as-specified within a 40°C ambient temperature. Basically, they’re plug-and-play.

However, for those that like math, the formula is, Tbp = Tamb + (P x (Rhs + 0.2°C/W)), where Tbp is the desired maximum temperature of the solid-state relay’s base plate, Tamb is the ambient temperature inside the panel, P is the power dissipated and Rhs is the thermal impedance of the heat sink.

Most of our standard solid-state relay power controllers are available on the website. There are thousands of possible configurations available, either as options for existing power controllers or as an application-specific, customized solution. Please contact our technical support team @ 800.879.7918 / support@hbcontrols.com if you can't find the exact solution for your application, would like to further discuss power dissipation and solid-state relays, or simply learn more about other products, such as HBControls DC output power controllers or AC and DC compact power controllers. We look forward to hearing from you.