Solid State Relays 9 Answers for Beginners

I can’t forget the factory I worked at in my after graduation period. It was a plastic pipes factory. It depends on solid state relays to control production lines heaters. Many of solid state relays in the control panels switching on and off very fast.

The most important task in the factory was to check theses relays hourly. We used current clamp meter to check if the SSR is working or not. It has been more than 10 years ago when I left this factory to a better working chance. I decided to make some searching analysis about SSR, and I found some basic, but important, questions you should know.

In this article I will answer these important questions. Let’s get started.

Solid-State Relay working principle?

 “A Solid-state relay is an electronic switch that turns ON or OFF on the base of a specific input. When an external voltage of a certain value is applied on the control terminals of the relay, the relay turns the power supply through itself to OFF or to the zero value.”

Solid-state relays are used as the “Control switches”. These switches have a behavior just like the Electromechanical Relays, but unlike the mechanical relays, the control switches never “break or cut-off” the circuit. It does not move its actual contact points or melt the bi-metallic strip.

The working steps of the principle that is followed by the Solid-State Relay can be listed in some points:

  • The relay has a light-transmitting electronic component that transfers signals between two circuits that are not electrically connected. This is generally pronounced as a photo-coupler. This photo-coupler detects the voltage at the input control terminal and transmits an electric/optical signal to the second circuit that is actually the control circuit for the output. The voltage that activates the photo-coupler is called The Pickup Voltage.
  • The switching circuit on the output side becomes active after receiving the signal transmitted by the photo-coupler.
  • Now, the Switching circuit on the output side allows us to control the output.
  • To turn the system back to a normal/non-relay-controlled scenario, just lower the applied voltage at the Control switch to the Dropout Voltage at the input side of the Solid-State relay, and the control switch connected to the output side of the relay will be turned off, resulting in the loss of our control on the output using the relay.

Difference between relay and solid-state relay (SSR)?

RelaysSolid-State Relay
The relays are “Control switch” that does the switching process by changing the position of the contact points.Solid-State relay is the “Control Switch” that does the switching process without using any kind of “contact” points or movable parts.
The relay uses “the electro-mechanical” coil or other parts for the switching process.The signals of photocoupler become the reason for the switching.
The energize or de-energize process makes or breaks the circuit.The input voltage at the control terminals makes (close) or breaks the circuit.
The operating voltages range from 12-120V.The operating voltage ranges vary for different mounting types.
These relays are inexpensive.Solid-state relay is relatively expensive.
It is a mechanical device.The solid-state relay does not have any parts that move.
Very much feasible for the AC circuits.More reliable for the electronic circuits. But still works for AC circuits
The parts of these relays wear out and start adding more bouncing effect than new relay after a certain time.These relays do not age.
These are age-limited and have to be replaced after some time.These relays last very much longer than the elctro-mechanical relays and render long last reliability.
These relays have coils and flaps for switching purposes.These relays have thyristors, TRIAC(s), and transistors.
Noisy switching is done, adds noise when used in the electronic circuit.Does not add noise and has low EMI (Electro-Magnetic Interference).
Switches take time to respond to the input signals.Switching is done by light signals and the speed of switching is way much more than other relays.
Types:(on the basis of working principle) General-purpose relays Machine control relays Reed relaysTypes: (on the basis of working principle) Zero-Switching relays Analog-Switching relays Peak-Switching relays Instant ON relays.

Which type of relay should I use?

Most and the best reason to choose solid-state relay over the electro-mechanical relay is that is its fast switching. The solid-state relay switches way faster than the magnetic coil-driven relays. The solid-state relays are preferred over the Elector-Mechanical relays for the below reasons:

  • There are many reasons due to which solid-state relays are more efficient than general relays. There is no mechanical wear and tear in the solid-state relays that allow us to use the relay over and over again with the ideal conditions.
  • There is no noise added by using the solid-state relays while the bouncing effect of the electro-mechanical relays adds a high amplitude, high frequency, and high-power noise signal that can add quantization error if used in the communication systems.
  • The solid-state relay is insensitive to the position in which it is placed. The effect of height or angle does not bother the solid-state relays while mechanical relays must be fixed and placed in a stable equilibrium state.
  • The operation in the solid-state relay is quiet and electro-mechanical relays generate sound(noise) while switching between states.
  • The current needed by the coil of the Electro-mechanical relay is way higher than the solid-state relay’s trigger input current, making the solid-state relay more efficient in power also.
  • As, the solid-state relays do not use magnetic coils that may get affected by the environmental conditions, in harsh conditions electro-mechanical relays are not the option we should vote for.

Why Does Solid-State Relay has no Mechanical Vibration?

The mechanical vibrations are generated by the movement of the mechanical parts of the system. In the Solid-State relay there exists “no mechanical part” that moves to toggle the switching process as a result of applying a voltage at the input terminals of the relay.

Do Solid-State relays fail open or closed?

The solid-state relay devices fail “closed”. When a solid-state relay fails due to an abnormal value of the current applied at the input terminals the solid-state relay acts as a short circuit.

This is a disadvantage of using the solid-state relay that when the input is abnormal and solid-state relay fails, we expect to cut off the power to avoid any kind of damage to the system for which the relay was being used but here the case is the opposite.

For this reason, In my pipe factory days, we were taking the hourly check of the SSR panels seriously, to prevent any damage due to the unexpected power connection.

Failing closed was one of the main serious issues that cost the factory too much before making a regular hourly check.


Does Solid-State Relay get hot?

The solid-state relay passes current through itself and the current when passes through an electrical device generates heat that eventually leads to the generation of heat in the relay.

The heat sink is not a precautionary measure but a necessity for the solid-state relay to work properly and not meltdown. 1-1.5 watts per ampere heat is generated in the SSR.

The SSR that passes current up to the value of 5 amperes does not need any designated heat sink. Convectional current or simple pressure of air can concatenate the heat generated by the 5-ampere current.

The heat generated by the current beyond this limit of current needs particularly designed heat sinks that can actively remove the heat from the system through the process of heat transfer while keeping work done equal to 0.

How do you cool a solid-state relay?

In some cases, the solid-state relays do not need any particular heat sink as the ambient conditions are cold and heat transfer through the system is well enough that the system does not start melting. Engineers have observed that up to 90% of the problem occurring in the Solid-State relays are caused by insufficient heat transfer through the heat sink.

Most of the time, we need heat sinks to concatenate the heat developed in the relay. Choice of the right heat sink improves the reliability and the life of the solid-state relay.

Some important factors that we keep in mind while choosing the heat sink have 2-factor types:

  • Using Internal factors.
  • Using External factors.
  • Internal Factors

The internal factors are the

  • Size and
  • Shape

of the solid-state relay.

More the surface area more will be the heat transfer rate and the heat rejected from the SSR will be effectively removed into the atmosphere. The shape of the relay that has more surface area has greater heat transfer through its surface, thus the heat rejected from the SSR is quickly removed to the atmosphere.

  • External Factors
  • Ventilation conditions
  • Installation Density
  • Volume of SSR
  • Ambient temperature

The ventilation conditions are very helpful in the heat removal from the SSR. Better ventilation conditions provide more airflow rates to the SSR that allow more heat transfer.

More SSR(s) installed nearby generate more amount of energy than a single SSR thus they need to be placed apart at a particular distance so that they do not add heat to each other’s surroundings.

The volume of SSR relates as did the surface area in the internal factors. More surface area leads to more volume and the heat rejection rate is more so greater volume cools down rapidly.

Ambient temperature plays important role in heat sinking. In some areas, the temperature is too high that the SSR that is flowing current greater than 4 amps needs to be dealt with particularly designed heat sinks.

Some cases need particularly designed heat sinks. Some of these sinks are:

  • MG-I: dimensions:(50x60x50) mm, for load current no more than 15A.
  • MG-W: dimensions: (70x125x50) mm, for load current no more than 26A.
  • MG-T: dimensions: (80x70x80) mm, for load current no more than 26A.
  • MG-L: dimensions: (150x88x35) mm, for load current no more than 26A
  • MG-H: dimensions: (150x80x80) mm, for current no more than 75A with actual load current <35A.
  • MG-F: dimensions:120x105x93, for load current no more than 120A.

These values of dimensions do not include the dimensions of the fan.

Why does a solid state relay fail?

SSR fails when its inner components get damaged due to the high current rush towards the internal components of the circuit it is attached to.

Malfunction caused by external Surge:

The high current towards the circuit passes through the SSR, if this current is greater than the rated value of SSR, the components of SSR get damaged. Some other reasons are discussed as:

Installation area:

If the ambient temperature is so high that the heat generated by the relay and the heat from the surroundings add up to a value of temperature that melts the internal circuitry of SSR.

Reverse Voltage of the Load:

The solenoids or we may say the inductive loads send reverse current that may damage the SSR. In that case, we have to use an SSR that can concatenate the sum of current flowing through the input terminals and the reverse current to avoid the failure of SSR.

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