Real World Applications of Solid State Relays

Solid state relay circuit

What are solid state relays?

Solid state relays (SSRs), or OCMOS FETs, are a type of switching device that has no moving parts (as opposed to the regular relay). These have the same function as electromechanical relays; allowing high current and/or voltage through a load in response to a low input voltage and current.

Solid State Relays vs Electromechanical Relays

Solid state and electromechanical relays both have extremely high input impedances, which act to isolate the input from the output. But despite the similarity, SSRs consist of no moving parts, giving them some special functions.

Elecrtomechanical relay
Fig 1 Elecrtomechanical Relay
Solid state relay circuit
Fig 2 Solid State Relay

SSR pros

  • Since there are no inductive coils, they do not generate electromagnetic interference with the load.
  • Silent in operation, (although I personally like a clicking sound of relays).
  • Have no moving parts so they are not subject to mechanical wear and tear.
  • Do not have switch bouncing.
  • Have higher frequency switching capabilities.
  • The number of safety components is much greater in SSRs than in electromechanical relays.

SSR cons

  • Unlike the electromechanical relay in Fig 1, where there is a clear break between the movable contact and the stationary contact in the off state, the solid state relay will still have a ‘closed’ circuit output even there is no input, which leads to some leakage currents.
  • If the SSR breaks down (for example at the triac, which is the symbol that looks like 2 inverted diodes) in the optocoupler results in a short circuit (permanently switched on). Whereas a breakdown in the electromechanical relay usually results in an open circuit (permanently switched off), which makes it dangerous in a lot of situations; especially when building one yourself, where over-voltage and over-current is required.

How the solid state relay circuit works

The SSR relies on different forms of diodes and transistors for its operation as a DC-DC, DC-AC, or AC-AC switch. The AC-AC switch just converts the input into a DC waveform and then proceeds to regulate it, making it into the aforementioned switches.

Starting from the input side, which is the DC control, on the left-hand side of the SSR circuit, there is a diode D1 which is connected in reverse bias. When the diode is turned on, it will indicate that the input connections are the wrong way around.

The next part which is the overcurrent protection prevents too much current flowing through the diode inside the optocoupler. The input will have a DC voltage range, by which if the range is compromised, the transistor Tr1 (which has a limiting resistor R2 biasing it just under cutoff) will pull it into the saturation region. This will make the transistor act as another resistive source; lowering the amount of current flowing into the optocoupler.

The optocoupler is an optical isolation device and is the basis for the high input resistance. It works by shining a light (in this case, an LED) on a photo-detecting device which can turn on and off.

The triac is a type of switch that has 4 modes of operation, as listed below. All that is needed to know is that in quadrant 1, when there is a forward positive voltage past some threshold, a positive current runs. When there is a reverse voltage, a reverse current runs. This makes SSRs applicable to AC circuits; as shown in the figures below.

Triac ciruit 1The four modes of operation of a triac for solid state relays

four modes of operation of the triac in the quadrants

Triac 4 operating modesThe TVS diode is a form of a voltage surge protection that removes transient spikes by clamping the output voltage to a voltage range. You can think of it as 2 zener diodes in which one is forward and another is reverse biased.

The RC snubber just removes high-frequency transients from switching. Another thing to note is even when the SSR is off there is leakage current flowing through this circuit so you must ALWAYS turn the circuit off.

As shown above, the triac will work for AC outputs. For DC outputs, the triac can be replaced with high power MOSFETs that would just switch the load on and off. Also notice that the load is connected on the neutral side. If connected on the live side, an open circuit would occur, and if you touch the load you may create a path for the current to flow and you will at the very least be in the hospital.

SSR Considerations 

Like electromechanical relays, SSRs have the ability to be switched on by micro-controllers with low voltage and current. The main advantage of this is the ability for fast switching. Whereas regular electromechanical relays suffer from contact bounce as mentioned earlier, they typically has a maximum switching rate of around 10ms (100Hz); SSRs can switch at a higher frequency.

There are several variables that require attention when buying an SSR for micro-controller use. The first thing is finding the SSR with the correct DC input voltages. Depending on the other requirements, a level-up translator can be used as an additional interface.

One of the biggest benefits of the SSR is its ability to turn on at zero voltage, which reduces back emf from large loads like heaters. So if the SSR is running large inductive loads, you will probably need to look at SSRs with this feature first. But this will also mean that if a pulse train is being used, the frequency must be low enough to facilitate the load frequency. E.g. if the load frequency is 50Hz, your switching frequency should at maximum be around half (i.e if it just crosses the zero mark then you have to wait half a cycle) though ideally, it should be less.

Then you must look at variables such as the maximum output power that can be handled by the SSR, and whether or not your load is AC or DC. The SSR inhabits internal resistance which means that it can heat up a lot during usage.

Triacs also have something called phase control, which means that you can control at what point in the waveform you switch the load on and off. This is useful because it allows for maximum power control. For example, if you have a sine wave, the maximum voltage will occur at 90 degrees, so you might want to limit the phase between 75 degrees and 105 degrees, then adding 180 for the negative part from 225 to 285. This phase control can only really be done in SSRs with no zero on mode. To control the phase, apply an impulse on the gate side of the triac when the voltage through the load is at the point that you would like it to be. But you have to be careful as this method of switching the triac on and off rapidly from ground can create high transient voltages.

Another thing to look at is how the SSR will be mounted, there are 3 different types of mounts the PCB, panel and DIN rail mount.

Summary of things to consider

  • Control Voltage (input voltage) – find the correct range of operation (max turn-on and min turn-off voltage).
  • Max input current can also be given by the input impedance requirement
  • Max turn-on time and turn off time – maximum response time between an on or off from input to the output.
  • Load current – Does the SSR need a heat sink? Check the minimum as well for correct operation.
  • Zeroed SSR, or non-zero (for phase control regulation)
  • Type of mount – PCB, panel or DIN rail.

Applications of SSRs

Latching solid state relay circuit

One application of the SSR is a latch, this is useful for things such as kettles, where an input pulse would indicate a start, and latch onto that state until it is interrupted. Following that example, if the temperature of the water has reached 95+ degrees Celsius, the SSR would indicate that the stop sequence should run.

phase-controlled dimmingAs stated earlier, you can change the pulse sequence in order to change the amount of the output wave used. A good thing to add here would be a phase detector so that when even when using a non-zero SSR, the dimming is done such that the pulse turns off when there is a zero crossover. This prevents transients from occurring.

Despite this being a very broad overview of a simple SSR and some random applications, if you ever require a high load switching and a circuit that isn’t very bulky, SSRs are the way to go.