Solid – State Relay (SSR) – Buy or DIY?

With a few mouse clicks you can easily purchase Solid State Relays (SSRs) online.

Or you can make one at home with a handful of different electronic parts. The question is– should I buy it or make it myself?

Which one works best for you? If you are an active electronic hobbyist, you can build a solid-state relay with ubiquitous and inexpensive parts. If so, you will what is inside your build and what is going on there. Moreover, you can improve its performance with small tweaks.

Another important thing to note is that many of the online products are often cheap fakes or clones of famous brands. For example, a 6A rated device may be available as a genuine 60A product. Are you ready to trust in that?

I expect a big NO!

Let’s start building a compact Solid-State Relay…

Before I get into the actual construction project, let me bore you with a few theories.

What is a solid-state relay?

In electrical terms, a relay is a relatively simple switching device that is used to automatically close or open a set of contacts between two circuits. This process is triggered by an electrical input or control signal of some kind, in response to which the relay switch usually moves from an ‘off’ to an ‘on’ position.

In a standard electromechanical (0r electromagnetic) relay (EMR), this switching process is mechanical, but a solid-state relay (SSR) has no mechanical or moving parts. A solid-state relay does it without any physical movement!

As said previously, the key characteristic of a solid-state relay is that it needs no moving parts to perform the task of opening or closing contacts on a circuit. Unlike an electromechanical relay, there’s no positional change of any component within the solid-state relay when it switches between off/on (open/closed) states. Below you can see the basic block diagram of a common solid-state relay.

Here, the incoming electrical control signal (input) is converted into an optical one inside the opto-triac. This optical signal is then fired across a small gap of open space within the opto-isolator to where it’s received by a photosensitive triac (opto-triac), which in turn converts and sends on the signal to the next electronic component – ​​the triac – which drives the final device (load).

Note that the photo-triac consists of an infra-red LED and a triac in one package. The LED is switched on and off by a low-power DC control circuit and this switches the triac which can be used to control AC devices up to mains voltages. The opto-triac provides electrical isolation between the DC control circuit and the AC output circuit.

Triac is a semi-conductor switch that can be turned on by a pulse on the gate or trigger pin. Once turned on they stay on until the current drops below the hold-on value. By delaying the turn-on point until sometime after the voltage crosses zero volts (the zero-cross point) the voltage can be adjusted although it is no longer sinusoidal.

In the below figure (part of a new variable phase angle control project), the upper trace shows the delayed trigger close to the end of the cycle. The resultant effective voltage is low. The lower trace shows the trigger close to the start of the cycle. This will result in close to full voltage. The relationship between phase angle delay and resultant RMS voltage is graphed on the right.

The schematic symbol shown before represents a “random” opto-triac. Following is the schematic symbol of a “zero-cross” opto-triac. Note that the zero-cross detection circuit inside the zero-cross opto-triac will wait until the voltage is very close to zero before switching on the triac.

The zero-cross (ZC) switching technique minimises switching noise and electro-magnetic interference (EMI) to neighborhood equipment.

Let us move to the construction project. The next paragraphs describe the entire construction of a simplified solid-state relay using a handful of cheap electronic components, perhaps lying around your workbench.

How to Build A Solid-State Relay?

First off, here’s a proven design of an adaptable solid-state relay (3~32V DC control input → 230V AC switch output):

At this point, note that zero-crossing SSRs are widely used when a simple on/off function is required for the load, while random turn-on and phase angle types are primarily used for dimming applications. Depending upon the application, zero-crossing, random turn-on, and phase angle SSRs are all suitable for everyday applications. However, burst fire SSRs are not advised for certain lighting applications as the on/off period may cause unwanted flicker in the lamps. Also note that phase-angle and burst-fire SSRs are not good for inductive loads.

On a side note, burst-fire SSRs are like phase-angle SSRs in that they provide proportional power to the load. However, instead of full AC continuous cycles current during each half, burst-fire technique provides train of full AC cycles to the load. The number of on/off cycles determines the percentage of power applied to the load over a fixed time, which is controlled by the value of the analogue signal applied to the input. Below you can see a simplified burst-fire waveform at 50% power (part of another ongoing project).

Getting back into our schematic, the design principally revolves around an opto-triac (OK1) and a discrete power triac (T1). Front end of the circuitry (the input section) is a tricky constant voltage/current source tailored to drive the light emitting diode sited inside the opto-triac.

Moreover, when the SSR is used to drive inductive loads, excessive voltage changes will occur within a short period when the triac is turned on and off. As a result, the SSR circuitry makes mistakes in firing time. An optional snubber circuit (R6-C2) is therefore included in this design to suppress excessive voltage changes.

Closing Thoughts

This is a relatively complete guide on how to build a solid-state relay prototype and get it functioning with AC230V. I hope this will be a solid starting point for anyone wishing to build a solid-state relay from the ground up.

You are free to modify this crude design to build an industrial-grade solid state relay.

Lab Snaps

Nothing but a couple of casual snaps of my breadboard SSR prototype!

Addendum

In many triac datasheets, you can see the “4Q triac” sign. What does this really mean?

A 3-quadrant (3Q) triac cannot be turned on in the fourth quadrant (T2 negative, G positive) which is beneficial while dealing with inductive loads, as ringing caused by sudden turn-off could potentially turn on a 4-quadrant triac, necessitating snubbers and other protection means to prevent spurious conduction.

See, 3Q triacs offer a cost-effective solution with excellent trade-off of surge current versus immunity and commutation. The 3-quadrant triacs cannot be turned on in the fourth quadrant which is favorable in applications with inductive loads, therefore an RC network is not all important (if the datasheet limits are respected).

Credits & References

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