Five Key Advantages of Solid State Relays

Modern electrical control systems have been greatly enhanced by the invention of solid-state devices such as diodes, transistors, and thyristors. For large load devices like heaters and motors, solid-state relays can be a huge advantage over traditional mechanical relays.

Although not ideal for every circumstance, they have many appealing benefits and some key situations in which they hold the upper hand.

The solid-state relay is a fairly complex device, but it has a simple purpose - to activate a single output load when energized. They are efficient and effective at the job, allowing in some cases, extremely large loads to operate with very precise control voltages.

The basic construction of the solid-state relay consists of a control (input) side with two terminals. A voltage applied to these input terminals activates an internal LED, paired with a current-limiting resistor for protection, which usually allows around 10-20 mA of input for the LED activation.

The input voltage can adopt ranges of low voltage DC, low voltage AC, and line voltage AC, but each voltage is a different relay model. The light from the LED strikes a photosensitive transistor or thyristor (usually an SCR, TRIAC). This allows it to control either DC loads, or AC loads respectively but never both.

 

Five Advantages of Solid State Relays

The solid-state relays are not ideal for every situation, and they are not designed to be a drop-in replacement for mechanical relays in all cases. But there are a few key advantages that might tip the scales in favor of the choice of solid-state for control of a load device.

 

Lower Power Consumption

The first of these factors which encourage the use of solid-state controls is the lower power consumption required to energize them. The activation of a solid-state relay is the internal LED with an integrated current limiting resistor, so the entire control circuit only uses a few milliamps. 

 

A bench test with a solid-state relay showing a current input of 15mA at 24 volts, which leads to 360 mW of power consumption and input resistance of 1.6 kΩ.

 

When I recently connected a voltage supply and ammeter to a Crydom DC60S3 solid state relay, I measured an input power of 75mW at 5 volts, and 360mW at 24 volts. By comparison, a small PCB mounted Schrack mechanical 5-volt relay consumes about 400 mW of power which is over 5x more, a tested Magnecraft 24 volt ice cube relay used 1.44 W, about 4x more.

 

Ensuring Long Lifespan

The next major benefit to using solid-state relays is the long lifespan. Since they contain no moving components, there is no mechanical fatigue failure like there would be in mechanical devices. This leads to a longer life and less replacement cost, especially when they are switched at a higher frequency. 

A Crydom DC output relay is stated to survive over 21 million hours (2400 years) of use when the load uses 50% of the rated power. No bench apparatus can really be devised for verification of this rating. In comparison, a mechanical relay is based on the number of switching cycles in its lifespan.

A datasheet for a particular Omron 24 volt relay states 500,000 minimum switches at a rate of 2 seconds per switch.

 

Solid-state relay with a single in-line package PCB mounting style. 

 

Fast Switching Speed

A third major advantage, especially for the DC output relays, is the extremely fast switching speed. Mechanical relays are limited in speed, or switch frequency, because of the time it takes to switch contacts open and closed. With no moving parts, the solid-state is faster.

In those DC loads, it can be used to change duty cycles and send Pulse Width Modulation (PWM) control signals to large load devices, along with the electrical isolation that cannot be achieved with a transistor alone.

That Crydom solid-state relay states an allowable pulse width modulation frequency of over 1 kHz (1000 switches per second), although this frequency limit will decrease with larger load currents.

In contrast, the Omron 24 volt relay has an absolute maximum possible switching frequency of 5 Hz (5 switches per second). At this slow speed, it is impossible to maintain precise control over the average voltage sent to the load device.

 

Heightened Value of Dielectric Strength

The fourth benefit of the solid-state relay is the heightened value of dielectric strength or resistance to arcing between the input control and output contacts.

One of the reasons for using a relay as opposed to a high-current transistor is that there is absolute isolation between two input terminals and two output terminals. This isolation means that if a large voltage surge affects the load terminals, the control device will be completely unaffected.

 

A set of PCB mount solid state relays on a power supply board.

 

A solid-state relay may have a resistance of potential voltages as much as 4,000 or 5,000 volts. However, a mechanical relay may only have a dielectric strength to resist voltages up to 2,000 or 3,000 volts. Although this may seem like an unreasonably high voltage, remember that inductive devices like motors can generate reverse voltages many times higher than their operating voltages when de-energized.

 

Resistive Input

Finally, a fifth and final appeal to the use of solid-state devices is the fact that the input is purely resistive, as opposed to the inductive loading of the mechanical relay. Coil-based devices which generate magnetic fields suffer from a reverse-generation of voltage, called ‘flyback’ when the control voltage is suddenly removed. This large voltage can be fatal to sensitive controls and cause arcing to occur.

 

The back view of a solid-state relay showing the large metal pad meant to be mounted to a heat sink - this will dissipate the heat generated be large load currents.

 

Fortunately, the solution to inductive flyback is fairly simple - connecting a diode in reverse polarity in parallel with the coil. So the problem is by no means unsolvable, and may not always present enough of an argument to use solid-state relays.

Solid-state relays are the preferred option when very precise high-speed switching is desired for just one single load device. They are efficient, and have a long operational life, and are ideal devices for those operating circumstances.

 

---

Do you use solid-state relays?

 

All images used courtesy of the author.