Solid State Relay(SSR) Circuit Diagram Explained

SSR or Solid State Relay is a compact semiconductor static device that switches or on/off the electrical or electronic signals when a control voltage applied to its input terminals. An Electromagnetic Relay or EMR also provides the same function but it consists of Coils, moving parts whereas an SSR has no coil and moving parts. SSR works with the help of Electrical and optical properties of Solid-state semiconductor devices. SSR has very high insulation between its input and output terminal which provides more electrical safety to electrical circuits and operations.

Generally, most of the SSRs are designed to operate with DC input supply. But the outputs are both AC and DC possible. A Solid State Relay has so many advantages over an Electromagnetic Relay such as very fast switching, no latching, low power loss, high electrical insulation, etc. 

The main part or component of an Electromagnetic Relay is the electromagnetic coil which attracts the metal contacts to connect and disconnect the circuit. On the other hand, the main components of a Solid State Relay are a Photo emitter and a photodetector. When an input voltage applied to the SSR, its photo emitter emits lights that fall on the photodetector. When the photodetector detects the light, it sends the signal to the output control circuit for the switching of electrical or electronic signals.

Internal Circuit of SSR(Solid State Relay)

Here you can see the internal circuit diagram of SSR that rated with input supply 3-32V DC and output capacity 240V, up to 40A AC.

Internal Circuit Diagram of solid state relay(SSR), SSR Circuit

Here, you can see the SSR has two main components - 1. Photo Emitter 2. Photo Detector
The Photo Emitter is basically an IR LED which emits the IR rays when a DC voltage(3 to 32V) applied across it. The photodetector is a Photo Transistor. It allows current to flow through it when the light emits from the photo emitter falls on it. Photodiode also used in some solid-state relays. Here, the Phototransistor is able to conduct alternating current.

The Photo Emitter in the SSR has a polarity, so we cannot connect it to the power supply in any polarity. It should be connected with proper polarity otherwise it will not work. On the other hand, the photo detector is a Phototransistor that is able to conduct alternating curent so we can connect it with any polarity.

So you can see here in the Solid State Relay, no moving contact mechanism and there are no connections between the input and output circuit. The input and output circuits are completely isolated. As both the photo emitter and photodetector are semiconductor devices, so they work very fast and produce very low power loss.

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Solid State Relay(SSR) Circuit Diagram with Load and Input Power Supply

Solid State Relay(SSR) Circuit Diagram, circuit diagram of SSR

Here, you can see in the above circuit diagram, the input of the Solid State Relay is connected to a 12V Battery or DC Power Supply. So when you use the solid-state relay in AC electrical or electronic circuits you must connect a rectifier circuit with the input of the SSR. If you connect or provide an AC power supply to the SSR, the light emitter or LED will flicker according to the frequency of the supply. So you can not get the continuous output. When you connect an AC load to the SSR, it should be connected like a normal single-pole switch.

For example, if you want to connect a single-phase motor. The neutral of the motor is to be connected directly to the neutral of the power supply. The phase terminal of the power supply is to be connected to one terminal of the SSR and the phase terminal of the load is to be connected to another terminal of the SSR. You can understand the connection with the help of the above circuit diagram.

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Solid State Relay(SSR) Circuit Diagram Explained Solid State Relay(SSR) Circuit Diagram Explained Reviewed by Author on June 04, 2021 Rating: 5
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