Replacing vacuum tubes with transistors made it possible to switch between logic states with minimal power consumption and in a relatively smaller space. This article will focus on logical switching.
Not much has changed with electrical switching technologies, although there have been some innovations around various electrical load switching techniques. Still, the size and power of devices used for electrical switching are much larger than those for logical switching.
The most popular known electrical switching component is a relay. By itself, it consumes much less energy than the heavy load it can drive. The driver and load of a relay are isolated from each other to prevent accidents in the control system.
We can classify relays into two categories
- Electromechanical
- SSR (solid state relays)
Generally, relays are electromechanical, in which a solid wire makes contact and allows current to pass through it, usually short-circuiting the wire at the output. Relays that work on this principle are the most familiar. They work best in cases where the delay does not matter and the switching frequency is low. But what about cases where a delay is critical and high switching activity is required? This is where SSRs are suitable. SSRs switch the output without any involvement of mechanical parts and use various techniques to switch the output without any physical contact.
Solid State Relays (SSR)
Unlike normal relays, solid state relays have no mechanical parts, so they do not produce noise during switching. Removing the mechanical element increases the switching speed by allowing the relays to work at higher frequencies. Solid state relays adhere to all other relay properties (insulation, heavy load conduction, etc.) of their mechanical family members. The most common solid state relays work on the principle of optoisolation, where the input and output are separated based on light.
What is optoisolation?
Optoisolation or optocoupling 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, we have an LED at the input and a light-dependent resistor at the output. The light-dependent resistor at the output is part of a circuit made up of several other electronic components, such as a transistor, diode, MOSFET, SCR, DIAC or TRIAC.
For example,
Figure 1. Internal structure of an SSR.
The SSR above has an LED at the input and a MOSFET at the output. The LDR is not shown in the circuit diagram above, but it sits between the base of the two MOSFETs. When the LED light falls on the LDR, the resistance of the LDR decreases and a short circuit allows current to flow between the output channels. The RSS in Figure 1 can handle a load current of a few milliamps.
Figure 2. An example of another SSR.
The internal structure of the SSR shown in Figure 2 is separated using an optoisolator. On the output side, we now have several other components, such as the opto triac and the SCR. The above SSR can handle current in amps; As the power output of the SSR increases, the internal structure becomes increasingly complex.
The main advantages of SSRs over traditional relays are:
- Non-mechanical switching.
- Almost no switching noise.
- The switching frequency is high compared to conventional relays.
- Optoelectronic isolation.
- Low input power consumption (only one LED at the input. While traditionally it is a coil).
- Drives heavy loads, used in heavy industrial processes where heavy motors work at high frequencies
- Works on AC and DC voltage.
The main disadvantages of SSRs compared to traditional relays are:
- The SSR heats up very quickly, so an extra heat sink is needed to absorb the heat. This increases the bill of materials.
- While not necessarily a disadvantage, it is important in certain applications. Due to the size and weight of SSRs, it is difficult to mount them on a PCB and extra space is required to house them.
How to build your SSR
Yes, it is possible to build an SSR from scratch. A simple SSR circuit is shown in Figure 3.
Figure 3 Made from circuitlab.com
The core part of your custom SSR will be a photocoupler. We suggest that you choose a photocoupler with an output driven by a TRIAC, as shown in Figure 3 . The SSR inputs can be controlled with a microcontroller or digital circuit. Heavy-duty applications include inputs from a PLC (programmable logic controller) or VFD (variable frequency drivers). A Schmitt trigger circuit can also trigger the SSR, although it is best to use a Schmitt architecture when power is critical.
You can connect your loads directly or extend the circuit according to your needs at the output of the SSR pins. For testing purposes, a lamp at the output is preferred, where the lamp is driven by a TRIAC operating on an AC power supply.