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Solid State Relay Working: How Does A Solid State Relay Work?

An automotive solid state relay diagram
An automotive solid state relay diagram

The solid-state relay working principle is based on one, the switching action of semiconductors, and two, the isolating action of an optocoupler or high-frequency transformer in transformer-based SSRs. This is what we will be covering in this write-up, but not before briefly going over the basics of these relays and their internal construction or circuit.

What are Solid State Relays (SSRs)?

Solid-state relays are defined as electronic switching devices that switch loads on/off when small external voltages are applied across their control terminals. Here are some facts about them.

  • They typically consist of a trigger switch, which is triggered by the application of voltage (AC or DC), and the main switch, which controls the load current.
  • Unlike conventional electromechanical relays, SSRs have no moving parts and are, therefore, non-arcing and more reliable. They also switch faster and can connect to logic circuits, among other benefits.
  • A solid-state relay can use an optocoupler for isolation and it can use a transformer. Some use a combination of both and are known as hybrid SSRs.

So, how do solid state relays work? Continue below to find out.

Solid state relay internal circuit
Solid state relay internal circuit

Solid State Relay Working Principle

In order to understand the solid state relay working principle, it’s important that you’re familiar with its circuit and components. The solid state relay circuit comprises of an input or control circuit, a drive circuit, and an output circuit or switch.

Note that, for the purposes of this article, we will be discussing the most common type of SSR, which is the optocoupler (or photocoupler) based relay.

1. Solid State Relay Input Circuit

The input circuit is where the control voltage is applied. It’s also called the control circuit. The voltage can be AC, DC or a combination of both (AC/DC. In some types of SSR, the input is designed with +ve and –ve logic inputs, meaning it can accept TTL or CMOS connections. Here is how a solid state relay works at the input side.

  • When there is no voltage applied, the LED is turned off and there is no current flowing through it. This means that the transistor in the output circuit is also turned off, and there is no current flowing through the load.
  • When the control voltage is applied, the LED is turned on, allowing current to flow through it. This, in turn, activates the phototransistor in the output side of the relay circuit.
  • A solid state relay control voltage can be either AC or DC, but it must be within the specified operating range of the SSR. This is often in the range of 3V to 32V. If the voltage is too high or low, the SSR will not work properly.

2. Solid State Relay Drive Circuit

The drive circuit is what activates the optocoupler, which in turn, switches on the transistor in the output circuit. The transistor then controls the load current. Here is the solid state relay working at this stage explained.

  • The SSR drive normally consists of the following mini circuits: the isolation or coupling circuit, the function circuit and, finally, the trigger circuit.
  • The isolation coupling circuit is used to electrically isolate the control and load side of the SSR. This is important because it prevents any voltage or current from flowing between the two sides, which could damage the SSR.
  • The solid state relay galvanic isolation, as the isolation circuit is also called, mostly uses an LED or infrared diode to couple or trigger the transistor on the load side. Note that some relays use transformer isolation or both transformer and LED.
  • The function circuit can comprise rectifiers, protection, detection, and several other circuits. The trigger circuit, as its name suggests, is used to trigger the output circuit. The output with then, using semiconductor devices, control the on/off function of the load.

3. Solid State Relay Output Circuit

The solid state relay output circuit is the part that is concerned with switching the load power supplies on/off. It mainly consists of an output transistor or chip, a transient voltage suppressor, and may also incorporate a feedback circuit. Let’s see how it works:

  • Upon receiving the trigger signal from the drive circuit, the output high-power transistor is turned on, allowing current to flow through the load. This circuit is, therefore, usually directly connected to the power supply of the specified load.
  • The components used are mainly MOSFETs, IGBTs, and thyristors (SCR or Triacs). Additionally, the solid state relay output can be a DC type, AC type, or both AC and DC type.
  • The solid state relay output (when rated AC) can be a single-phase type or a three-phase type. These can also be zero-crossing or random-turn-on types.
Using a solid state relay
Using a solid state relay

How to Use a Solid State Relay

Knowing how a solid state relay works can help you troubleshoot and fix any issues you may have with it. However, understanding how to properly use one is just as important. Here, therefore, are a few tips on how to use a solid state relay:

1. Always pay attention to the input/output voltage and current ratings of the SSR. Applying too much voltage or current would damage the SSR.

2. When using AC voltage, make sure the SSR is properly grounded. This will help prevent any electrical shocks and other incidents.

3. Make sure the solid state relay heat sink, if needed, is properly mounted and has good thermal contact with the case of the SSR. This will help to dissipate any heat that is generated by the SSR.

4. When installing an SSR, make sure the ambient temperature is within the specified range. Solid state relay are normally designed to operate within a certain heat levels. Operating the SSR outside of this range could damage it.

5. Finally, always consult the datasheet for the specific SSR you are using. This will provide you with information on how to properly use and install the SSR.


Solid state relays are becoming increasingly popular due to their many advantages over traditional electromechanical relays. They are smaller, more rugged, have a longer life span, can switch faster, and other benefits.

Having learned how a solid state relay works and how to properly use one, you can now take advantage of all the benefits they have to offer. That also means that you can apply them in a wide range of applications, both commercial and industrial.

Picture of William

I am William, Electrical Engineering Author. Dedicated to writing technical articles on Timer Relay, Monitoring Relay, Surge Protection Device and other electrical devices. With 7 years of writing experience, I am committed to providing accurate and in-depth expertise to my readers.


    “Gracias x la informacion, muy util

    Muy util,gracias

    excelente informacion,paa soliitar masacerca de estos dispositivos.

    Exelente explicación

    Gracias por tu tiempo invertido en darnos una gran explicación. Un saludo

    Muy importante y beneficiosa información, gracias por ayudar

    Hola, se podría sustituir un relé de un coche de 12v DC 40A, por un relé de estado sólido con una entrada de 3~32v DC y de salida de 5~60v DC, modelo SSR-40DD?
    Muchas gracias y un saludo.

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