A solid state relay (SSR) is just what it sounds like; an IC that acts like a mechanical relay. They allow you to control high-voltage AC loads from lower voltage DC control circuitry. Solid state relays, have several advantages over mechanical relays. One such advantage is that they can be switched by a much lower voltage and at a much lower current than most mechanical relays. Also, because there’s no moving contacts, solid state relays can be switched much faster and for much longer periods without wearing out.
They accomplish this by using infrared light as the ‘contact,’ a solid-state relay is really just an IR LED and a phototriac sealed up into a little box. Thanks to the fact that the two sides of the relay are photo-coupled, you can rely on the same type of electrical isolation as in mechanical relays.
SSRs do not have actual coils and contacts. Instead, they use semiconductor switching devices such as bipolar transistor, MOSFETs, silicon-controlled rectifiers (SCRs), or triacs. The solid-state relay has no moving parts, it is resistant to shock and vibration, and it is sealed against dirt and moisture.
SSRs can be used to control ac or dc loads. If the relay is designed to control an ac load, a triac is used to connect the load to the line. Solid-state relays intended for use as dc controllers have a power transistor, rather than a triac, connected to the load circuit. When the input voltage turns on the LED, a photodetector connected to the base of the transistor turns the transistor ON and connects the load to the line.
The control voltage for SSRs can be direct current or alternating current and usually ranges from 3 to 32 V for the dc versions and 80 to 280 V for ac versions. Maximum load circuit amps of up to 50 A are possible at input line voltage ratings of 120,240, and 480 Vac. In most applications, SSRs are used to interface between a low-voltage control circuit and a higher ac line voltage.
Many SSRs used to control ac loads have a feature known as zero switching. Zero switching ensures that the relay is turned ON or OFF at the beginning of the ac voltage wave at the zero crossover point. Zero voltage switching is often needed to reduce in-rush current and radio frequency interference (RFI).
The SSR has several advantages over the EMR, such as :
– The SSR is more reliable and has no longer life because it has no moving parts.
– It is compatible with transistor and IC circuitry and does not generate as much elecctromagnetic interference.
– The SSR is more resistant to shock and vibration, has much faster response time, and does not exhibit contact bounce.
The SSRs do have some disadvantages, such as :
– The SSR contains semiconductors that are susceptible to damage from voltage and current spikes.
– Unlike the EMR contacts, the SSR switching semiconductor has a significant ON-state resistance and OFF-state leakage current.
– Sparkfun web. “Solid State Relay – 8A”. 18 Mei 2014. https://www.sparkfun.com/products/10636.
– Petruzella, Frank D. Industrial Electronics. 1996. McGraw-Hill International Editions. pg. 205-207.