What are Solid State Relays SSRs?

A Solid State Relays (SSRs) acts as a non-contact switch which switches on or off when a small external voltage is applied across its control terminals. The electronic device is composed of a solid-state electrical element. It can switch on and off with no contact and no spark. The lack of movable parts in SSR’s allows them to switch much faster than electromechanical relays. Due to this same reason they last longer and require less maintenance.

Advantages of SSRs over Electromagnetic Relays:

Solid State Relays as mentioned before the last longer and require less maintenance due to lack of any moving parts which would have caused wear and tear of the device. SSRs also show higher reliability, have a longer life span, show faster switching speeds, are smaller in size. 

Uses of Solid State Relays:

Solid State Relays have a number of uses. For example, they are widely used in CNC machines (Computer Numerical Control Machines), in remote-controlled systems, and in automated industrial devices. These include all types of industries i.e. Chemical, Medical, Food and Beverage, Plastics, Packaging, Lighting, etc. They are also used in motion control devices. 

How do Solid State Relays work?

Here we will discuss the basic working principle behind Solid State Relays. We can first and foremost divide SSRs into two types on the basis of their application: AC SSRs and DC SSRs. 

We will understand the working principle behind SSRs by taking an AC SSR as an example. The main body of the SSR consists of a coupling circuit, a triggering circuit, a zero-crossing control circuit, and a switching circuit.  The SSR itself has only two input terminals and two output terminals.

With only a small voltage at the input terminal, the SSR can control the on and off state of the output terminal. The coupling circuit provides a channel between input and output terminals but cut off the electrical connection between input and output terminals to stop the output from affecting the input. Optical couplers are used in the coupling circuits which have good action sensitivity and high response speed and can withstand high input and output voltages. A light-emitting diode is used as a load at the input terminal. It can be directly connected to the computer output interface and can, therefore, be controlled by logic levels “1” and “0”.  

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Next, the trigger circuit is used to generate the desired trigger signal to drive the switching circuit. A zero-crossing control circuit is used to control any radio frequency interference that may have been generated. It also prevents the interference of higher harmonics and the pollution of the power grid.

Next, we have a snubber circuit which is used to prevent the impact and interference to switching component Triac from the surges and spikes from the power supply. Now if we talk about DC SSRs, they do not make use of a zero-crossing control circuit and snubber circuit inside and a large power transistor is usually used for the switching component. Their principles of working are however the same. 

How to select the best SSR for your application:

1: Load Type

First, determine your load type. A resistive load type will electrical energy into heat and light (such as in heating elements and light bulbs). This type of load is best switched with a zero-crossing SSR in which the output is activated at the first zero crossings of the voltage sine wave often in less than 8.33 msec. Inductive loads can be used which are those that resist changes in current such as solenoids, coils, pumps etc. Instant-on SSRs are activated immediately after applying a control voltage, often less than 0.35 msec. The heavier inductive load types such as transformers should be switched with peak switching SSRs. In these SSRs, the first peek of the line voltage of the SSR activates the output. Now if we talk about less common capacitive loads they resist changes in voltage and are partially found in rapid charge and discharge situations such as those in flashbulbs.  

2: Number of Poles 

Next determine the number of poles or lines of voltage that are connected to the load. If you are using a DC load you will require a one-pole VDC SSR. For a single phase AC load, on the other hand, you’ll need a one-pole VAC SSR. And if you are using three-phase AC loads, consider if you want to switch two or three poles of your AC voltage via a two-pole or three-pole solid state relay. 

3: Load Voltage and Current 

Next, you will have to determine the maximum AC or DC voltage and current for your load. You can find these on the specs of your motor, heater, or other devices. SSRs usually switch one phase at 120 or 240 VAC—or 208, 240, 480, or 600 VAC for three-phase applications for AC loads. 

4: Control Voltage or Input Signal 

Now you need to determine the control voltage which is required to energize your load and drop out voltage, below this voltage the load will de-energize. These voltages will control your SSR. SSRs do not have one fixed controller voltage; instead, they have input ranges which include VAC, VDC or dual VAC, VDC. If you need to proportionally control you load you will require some additional specs to choose the correct solid state relay. This is done through a 0-10 VDC or 4-20 mA control signal. You should also determine the optimum output switching type for you load and application.

5: Ambient Temperature 

Next, determine your ambient temperature. Your SSR’s maximum current rating depends on the ambient temperature when it is mounted. Higher temperatures can reduce the SSR’s current rating. A heat sink is usually required with many SSRs to ensure optimum performance. In order to specify the right kind of heat sink you should know the ambient temperature as well as mounting orientation. 

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6: Pick a Mounting Type 

SSRs are available in a number of mounting configurations. PCB mount SSRs use limited load size due to less space and heat dissipation constraints. Chassis mount SSRs will require a heat sink to achieve the SSR current rating. The SSR can be typically mounted without a heat sink if the load is under 5A or 8A if the mounting surface is of metal type. 

You can also choose a DIN rail mount model if you don’t want to deal with sizing and mounting. DIN rail mount models snap directly onto the DIN rail and are ready to be wired up and used. Some of the more advanced type SSRs are available in the category and design.