Power Electronics

What is Inside a Solid State Relay


Solid state relays are switching relays that do not require the use of any mechanical parts. This usually gives them the advantage of having a lifespan longer than that of a common electromechanical relay, and although solid state relays are magnitudes faster than electromechanical relays, they do have some design stipulations.

Solid state relays have taken the world by storm, revolutionizing power distribution in every industry from agriculture-automation to aerospace. But you may be wondering… “How exactly do they work?” This article will cover everything from solid state relay basics to opto-isolators and opto-couplers, photodiodes, and PN junctions.

What is inside a solid state relay?

Solid state relays are typically designed as a simple on-off switch with a power terminal and load terminal that switches when an external control signal is passed to the relay through another terminal. When this occurs, the switching happens very quickly, and the load is given power, usually by means of a MOSFET power-transistor. 

Relays can be designed and used in either an AC or DC switching capacity, but the internal configuration has to be modified to work for either scenario. DC relays can operate with a single MOSFET, with the source and drain connected to power and load of the main circuit and the control signal attached to the pass-gate. The control signal can be very low power, which enables the relay (and the massive load circuit) to be controlled by something as small as an Arduino. Solid state relays can have multiple transistors aligned in parallel to allow for a higher current flow potential, which can be rated into the 100s of amperes. AC switches require at least two transistors because one MOSFET cannot inhibit current in both directions when the relay is in an off state. Two transistors, with their sources connected, are used to block current when off and then pass power when the control signal is switched on within the relay.

How does the ‘switch’ work?

You may be wondering – what kind of switch enables the control signal to power hundreds of amperes? The real beauty of solid state relays versus electromechanical relays lies ultimately in the difference between the switching mechanisms. Solid state relays use what the industry calls opto-isolators or opto-couplers. In human speak, that means a “light-separator.” That’s right – the switch inside a solid state relay is just a beam of light! Generally, there is a very low power LED that shines a beam of light on a photodiode, which nearly instantly allows power to be transmitted through it – or “switched” on.


Figure 1: This is a sample diagram of a typical photodiode. The diagram depicts an LED shining onto a photodiode transistor. This action is what allows current to flow through the transistor.

Opto-isolators are crucial in solid state relays as they are what separate the two or more circuits of the relay. Since relays use small voltage signals to control very large voltage signals, it is extremely important to keep these signals separated. The beauty and revolutionary trait of opto-isolators is that there are no moving parts. For example, in electro-mechanical relays, this circuit separation is made possible by an electro-magnetic field, which is also what is used to ultimately complete the large load circuit.

In a solid state relay, the photodiode is what makes the connection within the load circuit complete. So what in the world is a photodiode? It is a very specialized transistor that uses photons to power the gate, rather than a typical electrical signal. How in the world does that work? It uses a highly specialized silicon P-N junction.

How does this special P-N junction work?

A P-N junction is seen in all sorts of different silicon components for a variety of applications and is what essentially allows for “silicon” to function as a semi-conductor. As a stand-alone element, silicon has very low electrical conductivity. However, when silicon is doped with other elements such as phosphorus and boron, p-type and n-type silicon becomes much more electrically conductive. The area of the silicon where the p-type and n-type silicon meet is called a P-N junction. In an opto-isolator circuit, this P-N junction is known as the photodiode, which ultimately has one main purpose – to generate current in the presence of light.

0517_What inside SSR_PN-Junction

Figure 2: This image is an artistic depiction of a PN junction that illustrates the depletion region of the photodiode.

Light consists of photons, or energy carrying particles, that are the “bread and butter” of photodiode physics. Typically, the light that photodiodes best respond to is around 200nm (ultraviolet) or 1100nm (infrared). These photons create electron-hole pairs in the silicon photodiode’s depletion region. The depletion region is formed when the p-type doped silicon comes into contact with the n-type silicon and the electrons and holes flow into lower potential areas. When light hits the silicon, the photons are absorbed, creating electron-hole pairs. When the electron-hole pairs begin drifting apart, they are swept away by the electric field of the depletion zone. This movement of electron-hole pairs is what creates the flow of current in the photodiode as long as the PN junction is operated in a reverse bias direction.

Now that there is a signal being generated at the output end of the opto-isolator, a transistor or series of transistors can be used to amplify that signal to eventually output very large signals, a method mentioned earlier in the article. The ability to use a very low power signal as the input of the opto-isolator and conversely turn that signal into a very large output signal is the ultimate purpose of a solid state relay.

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