How to Use Solid State Relay | Connection Diagram

Solid State Relay or SSR is a very useful electronic device. In this article, we are going to know how to use a Solid State Relay, Solid State Relay Connection Diagram. There are different types of SSR available in the market. To use a solid-state relay in a particular application we must select an SSR of proper rating suitable for that particular application. So the selection of SSR is a very important factor.

Solid State Relay Overview

Solid State Relay or SSR is a static semiconductor switching device. Its main function is to switch the electrical signal or power. SSR works on the principle of optical coupling which provides great isolation between input and output circuits.

A Solid State Relay can work with a 3V to 32V DC supply and it can handle a very high current than a normal relay or electromagnetic relay. Even it can control alternating supply also. For example, we can control a 230V power supply by applying a 3V DC power supply to the SSR. Solid State Relay provides very faster switching and it can work very efficiently with any programming device such as microprocessors, PLC which produce digital signals or high-frequency signals.

Solid State Relay(SSR) Block Diagram

Here you can see the internal block diagram of Solid State Relay. There are different types of SSR available, and their circuit diagram is also different. Here only a simple block diagram is given to understand the working of SSR.

Here you can see, four blocks are there. Its input circuit is connected to a LED(generally infrared LED) which produces light when its input circuit is connected to a power supply. Its output circuit is connected with Photodiodes and power MOSFETs. Generally, MOSFETs are back-to-back connected in the opposite direction. In normal conditions, MOSFET does not conduct currents. Once the power supply is given to the input of the SSR, its infrared LED produces lights that fall upon the photodiode. Photodiode will produce voltage and current which turns on the Power MOSFET and the MOSFET will start conducting current.

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Why we should use Solid State Relay(SSR)?

There are so many advantages to using a Solid State Relay. 

1. It provides very faster switching facilities. Because there is no mechanical movement. In fact, it works well with pulse signals.

2. It has no Latching problem. It provides a very high efficiency.

3. It can work with a very low voltage and current than a normal electromagnetic relay.

4. No ARC or sparking occurs during the operation.

5. It can handle a large amount of current than an electromagnetic relay.

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Where we can use Solid State Relay(SSR)?

1. We can use Solid State Relay for high-frequency applications and also for digital signals.

2. We can use Solid State Relay with Microprocessor, PLC, and another programmable device that generates output up to 5V DC.

3. As the Solid State Relay offers more resistance in its output circuit, so it is not suitable for high-impedance circuits, so we can use solid-state relays in low-impedance circuits.

4. Solid State Relay provides more advantages for AC power switching, so we can use motor control, electrical heater controls, etc.

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How to select a Solid State Relay?

The selection of a proper Solid State is very important to use in a particular application, so some important factors should be considered when selecting an SSR,

AC VS DC SSR: It is the first important factor while selecting an SSR. There are two types of SSR available according to switching characteristics AC and DC. If you want to switch AC signals then you must select the AC SSR or if you want to switch DC signals then you should take DC SSR.

If you take an AC Solid State Relay for the switching of DC signal it will not work because they use TRIAC and Thyristors in their output circuit. So it would not turn off the load until the DC current falls to zero even when you remove the input signal from the SSR.

Also, check the type of input signal required for the SSR because most of the SSRs are designed to operate with a DC input signal even they switch the AC power. So if you did not find out the SSR which can be operated at AC input signals, then you required extra electronic components to make the rectifier circuit. You can convert your AC input signal into DC and then apply it to the SSR.

When you select an AC SSR, then must check if is it embedded with surge protection, because, in the case of AC, surge voltage occurs.

SSR Voltage: Proper voltage rating selection is also a very important factor. Most of the SSRs are designed to operate at 3 to 32V DC input voltage. So input voltage is not more important, you can give any voltage between 3V to 32V. Switching voltage is more important. Always select a solid-state relay with a 25% greater voltage rating than how much voltage you want to switch. And also check which type of voltage you want to switch AC or DC.

Switching Current: It also can be called Load current. Any electrical load takes a high inrush current than the average load current at the starting time. So when you select a solid-state relay for a particular load, it should keep in mind how much inrush current can take that load at the starting time.

So when you select the relay, read both specifications means the specification of the particular load and the specification of the SSR. After reading both specifications, match their characteristics means current, voltages, etc then select the SSR for a particular application.

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How to Use Solid State Relay?

First, select a proper Solid State Relay which suitable for your application then arrange the components and materials. Make the proper connection with SSR and Load, input signal circuits. Also, arrange the heatsink arrangement for the SSR if you are working with a high current. 

Solid State Relay Connection Diagram and Wiring

Here one example is given for the use of SSR. Here you can see the Solid State Relay Connection Diagram and Wiring for the automatic control of an electric heater.

Here, you can see the heater is operating with a 230V AC supply. In normal conditions, the temperature meter provides a continuous 12V DC input signal to the SSR and it turns on the heater. Once the heat produced by the heater exceeds a certain limit the temperature meter will stop the input signal to the SSR, so the SSR will get turned off and the Heater also got turned off. So, you can understand here the 12V DC power supply is used to control a 230V AC power supply system.

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