Electro nonmechanical relays characteristics and how-to DIY

With the replacement of vacuum tubes by transistors, switching between logic states with a minute amount of power consumption and in a relatively smaller space became possible. This article will focus on logic switching.

Not much has changed with electrical switching technologies, although there have been some innovations around various techniques to switch electrical loads. Still, the size and power of electrical switching devices are much bigger than their logic switching counterparts.

The most popular electrical switching component know is a relay. On its own, it consumes much less power than the heavy-duty load it can drive. A relay’s driver and load are isolated from each other to ensure against mishaps to the control system.

We can classify relays into two categories.

Electro-mechanical
Electro-nonmechanical or simply SSR (Solid state relays)

Generally, relays are electromechanical, whereby a solid wire makes contact and allows current to pass through it, usually short-circuiting the wire at the output. The relays working on this principle are termed electromechanical relays and are the most familiar.

Electromechanical relays perform best in cases where delay doesn’t matter, and switching frequency is low. But what about the cases where the delay is critical and high switching activity is required? This is where electro nonmechanical relays are well suited.

Electro nonmechanical relays switch the output without any mechanical part involvement. They use multiple techniques to drive the output without any physical contact.

Solid state relays (SSR)

Unlike normal relays, solid state relays don’t have any mechanical parts, producing no noise during switching. The removal of the mechanical element increases the switching speed allowing the relays to work on higher frequencies. Solid state relays adhere to all other relay properties (isolation, heavy load driving, etc.) from their mechanical family members.

Most common solid-state relays work on the optoisolator principle, where input and output are separated based on light.

What is an optoisolator?

Opto-isolation or opto-coupling is a technique in which a light dependent resistor (LDR) activates an output when its surface is exposed to a light source.

In the case of solid state relays at the input, we have an LED. At the output, we have a light-dependent resistor. The light-dependent resistor at the output is part of a circuit which is composed of several other electronic components such as a transistor, diode, MOSFET, SCR, diac, or triac.

For example,

An internal structure of an SSR AQY217GS Mouser part number 769-AQY217GS
Figure 1From datasheet

The above SSR has an LED at the input and a MOSFET at the output. The LDR is not shown in the above circuit diagram, but it lies in between the base of the two MOSFETs. When the light from the LED falls on the LDR, the LDR resistance decreases, and a short circuit allows the current to flow between the output channels. The above SSR can handle a load current of few milliamperes.

Now take an example of another SSR A2425-10 available on Digikey.

Figure 2From datasheet

The above SSR internal structure is separated using an optoisolator. At the output side, we now have several other components, such as the optotriac and SCR. The SSR above can handle current in amperes. As the SSR output power handling increases, the internal structure becomes increasingly more complex.

The key advantages of SSRs over traditional relays are:

Non-mechanical switching.
Almost no switching noise.
Switching frequency is high compared to traditional relays.
Optoelectronic isolation.
Low input power consumption (just an led at the input. whereas traditionally a coil).
Drive heavy loads, used in heavy industrial processes where heavy-duty motors are working on high frequencies
Works on both AC and DC voltage.

The key disadvantages of SSRs over traditional relays are:

The SSR heats up too quickly, so an extra heat sink is required to absorb the heat. This increases the bill of materials.
While it is not necessarily a disadvantage, it does matter in certain applications. Because of the size and weight of SSRs, it isn’t easy to mount them on a PCB (printed circuit board), and extra space is required to house them.

How to build your own SSR?

Yes, it’s possible to build up an SSR from scratch. A simple SSR circuit is shown below.

Figure 3 Made on circuitlab.com

The central part of your custom SSR will be a photocoupler. I suggest you choose a photocoupler with an output driven by a TRIAC as in the above figure.

SSR inputs can be controlled with a microcontroller or a digital circuit, as shown in the above circuit. More heavy-duty applications include inputs from a PLC (programable logic controller) or VFD (variable frequency drivers). A Schmitt trigger circuit can also drive the SSR, although it’s best to use a Schmitt architecture when power is critical.

You can directly attach your loads or extend the circuit according to your requirements at the SSR pins’ output. For testing purposes, a bulb at the output is preferable, as shown in Figure 3, where a bulb is driven by a TRIAC working on an AC power supply.