Solid State Relays as the name suggests is solid state in nature, that is it's construction is based on totally electronic components. There are no mo...
Solid State Relays as the name suggests is solid state in nature,
that is it's construction is based on totally electronic components. There are no moving parts as in their electromechanical counterpart. This relay is also controlled by a small amount of power and can control large amount of power. But the main difference is that the input & output is made up of semiconductors like Optocouplers, Diode & Thyristors. This type of construction is having many advantages over the traditional electromechanical relays (EMR) which we will see later on.
Let us see what it is made up of. Solid State Relays can be broadly classified into main three types. This classification is mainly based on input circuit that is employed in the design. This input circuit decides how isolation is obtained between input & output.
1. Transformer coupled SSRs
2. Reed Relay Coupled SSRs
3. Photo coupled SSRs
Transformer coupled SSRs - This type of SSRs consists of small transformer at the input side. This input power is supplied to a small transformer's primary side & the output of secondary side is used to trigger a TRIAC or SCR, which in turn control the output power. This way isolation is obtained through transformer.
Reed Relay Coupled SSRs - This type of SSRs consist of small reed switch at the input side. The input power is applied to a coil surrounding reed switch with or without using an amplifier. The contacts of reed switch are used to trigger the output circuit having TRIAC or SCR or a power transistor. So here the isolation is achieved through use of reed switch.
Photo-coupled SSRs - This type of SSRs use the light source to provide isolation between input & output. Input power is applied to visual or infrared light source like LED (Light Emitting Diode). This produces light which passes through a optically clear barrier to a receiving component which may be photosensitive diode or transistor. This produces a current which is used to trigger the output TRIAC or SCR to control connected load. Here the light beam couples the input & output and same time provides isolation also.
SSRs provide following advantages over electromechanical relays.
1. At relatively smaller size, the power handling capability is more.
2. Because of no moving parts, the life is much longer.
3. Because they are made totally of electronic components, can be operated at a very speed so fine tuning of power output becomes very easy.
4. Input power is very small so any microcontroller can control SSR directly.
5. Isolation is also beyond 2500 Volts which is very good.
6. They do not generate any noise.
Though there are advantages, SSR is to be used with proper precautions as the output consists of Thyristors like TRIAC or SCRs.
As they are easy to use and now much reliable, they are used extensively in power control circuits. The can provide very fine tuned control capability so that the output power is exact, what you have planned.
