The solid state relay (SSR) is an electronic switching element in which a larger current or voltage can be controlled by a control signal. SSR function is similar to electromechanical relays, except that there is no moving part in this element and it has smaller dimensions than it. The absence of a moving part in the SSR increases the life of the component and is used in circuits with a high number of keystrokes.
Electromechanical relays have two parts, the coil (steering circuit) and the contact (power circuit). In solid state relays (SSR), there are also these two parts, which are usually isolated from each other by an optocoupler (optical isolator). In solid state relays, semiconductors are used to switch loads instead of using electromechanical force.
The input of solid state relays is often DC, but their output, like electromechanical relays, can be AC or DC. These relays are available in a variety of AC and DC models. AC models are available for both three-phase and single-phase electricity. Pay attention to the information written on the relay to observe its allowable voltage and current limits. If we want to use SSR in DC loads, in the internal building, MOSFETs are used in parallel with each other to increase the tolerable current of SSR.
Three parameters must be considered to select an SSR, which are listed below:
- Range and type of control voltage for disconnecting and connecting SSR, which are two modes: DC control input and AC control input.
- Voltage that is connected to two output ends (to both ends of the switch), which is also DC or AC modes.
- Maximum current passing through the electronic switch at the output.
Solid state relays or SSRs are divided into the following four categories in terms of switching type:
Zero Crossing Switching
- Can be used for resistive, capacitive and inductive loads.
- During excitation, it turns on or off when the voltage level exceeds zero.
- The advantage of connecting in the passage of zero is that when the load is connected to the relay, it prevents the creation of noise and also prevents damage to the relay and the load. Instant-On
- Can be used for selfie loads. (This relay can be used to control the phase.)
- As the name implies, this relay turns on as soon as the control is activated and turns off as soon as the excitation is stopped.
Peak Switching
- Can be used for selfie loads with iron core.
- The output of this type of SSR starts conducting when the control circuit is excited, when the voltage range is at its highest.
Analog Switching
- Can be used for resistive, capacitive and inductive loads. (This relay can be used for phase control)
- One of the widespread uses of this type of relay is in soft starters and various types of motor speed controllers. Other uses include temperature control of furnaces. In fact, the most widely used type of SSR is the analog type.
Other types of SSRs have been developed, some of which include:
- DC Switching SSR
- Low Noise SSR
- System Monitoring SSR
- Full Cycle SSR
SSRs include the following three categories in terms of type of stimulation:
- Photo Coupled SSR
- Transformer-Coupled SSR
- Hybrid SSR
The most important benefits of SSR are the following:
- Relatively long service life (about 50 times of mechanical relays)
- Dimensions much smaller and lighter, in power similar to mechanical relays
- High security in flammable places (due to lack of spark generation during switching)
- Requires very little current set up
- The SSR electronic relay function is soundless.
- In the control part, it does not generate inductive current.
- SSR relay is more resistant to shock and vibration
- This electronic relay is suitable for explosive places.
- SSR electronic relays have much faster response time than electromechanical relays and contactors, and their switching times is in the microsecond or millisecond range.
The parameters that were considered when selecting SSR are as follows:
- Range and type of control voltage for disconnecting and connecting SSR, which are two modes: DC control input and AC control input.
- The voltage that connects to the two ends of the output (to both ends of the switch), which can also be DC or AC.
- Maximum current passing through the electronic switch at the output.
- Single-phase or three-phase SSR.
Common types of solid state relays:
- ON/OFF
- Voltage Regulator
- Motor Reverse
- Rail PCB
SSR applications:
- SSR can be used in almost any circuit that requires high switching speeds.
- For example, engine speed control, soft starter, heater and the like.
- Use SSR as contacts of an electronic contactor.
- Heat control with PID output pulse output
- Control of the movement of a DC motor by means of four SSRs closed in the form of a bridge (H-Bridge)
- Heaters
- Lighting circuits and light regulation
Important points in SSR relay wiring:
- Consider the following tips for wiring SOLID STATE RELAY:
- When wiring, make sure that you use up to 50% of the rated SSR power to drive selfie loads and 70% of the rated power to drive non-selfie loads.
- Keep in mind that the SSR relay also passes some current does not cause the other equipment to operate momentarily.
- If the steering or power circuit is D.C, polarization can cause SSR to malfunction or be damaged.
- The power circuit of the relays must be completely electrified foe repairs or replacement of equipment. Never stop the steering circuit and use breaker or switch with a sufficient number of steps before SSR.
- SSR relays with an output of 5 amps and above require heatsink. Do not install the relay on a plastic, wooden, etc. surface without a suitable size heatsink.
- Use suitable equipment such as wire head, tin-plated wire, wire show, and …. In SSR wiring.
- Use suitable and noise free ground system in electrical panels. This increases safety, reduces accidental operations, reduces electric shock and so on.
- Solid state relays must be protected against overcurrent and short circuit by installing appropriate systems. Be sure to use a suitable fuse or breaker.