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Selecting a solid state relay

02 December 2019

There are many circumstances in which a high current or voltage load needs to be controlled by the operation of a low-power circuit. In these cases, a relay can provide the necessary isolation between the high-power and low-power parts of the system. Martin Wyatt outlines how to select the most appropriate solid state relay for different applications

If you were to try and turn on the headlights in your car directly using the headlight switch it is more than likely that you would exceed the amperage rating of the switch, melt wires, and risk an electrical fire. That is why your car’s headlight circuit incorporates a relay. The relay enables a low amperage circuit connected to the dashboard switch to be used to turn on or off the higher amperage headlights circuit. Relays work by allowing a small electrical signal to control a high power signal.

There are two main types of relay: electromechanical relays and solid state relays. An electromechanical relay (EMR) uses a physical moving part to connect contacts within the output element of the relay. The contact’s movement is generated using electromagnetic forces provided by the low-power input signal to enable completion of a circuit containing the high-power signal.

By contrast, Solid State Relays (SSRs) do not have physical moving parts, they are switched electronically when an external voltage is applied to the unit’s control terminals. An SSR uses a low power electrical signal to generate an optical semiconductor signal, typically with an octocoupler, that transmits and energises the output circuit. When activated, the input optical signal acts as the “switch” that allows a mains voltage power to pass through the SSR’s output component to turn a device on or off. This article is about the selection of SSRs.

An SSR’s lack of moving parts enables it to switch much faster than electromechanical relays

An SSR’s lack of moving parts enables it to switch much faster than electromechanical relays. And, because SSRs have no moving parts or contacts that can wear out they generally last longer and require less maintenance than EMRs.

It is this speed and reliability that made Carlo Gavazzi’s SSRs the relay of choice to control the heating, ventilation and air conditioning systems in the passenger carriages of a state-of-the-art express train.

A carriage’s heating, ventilation and air conditioning system has to work hard to ensure a comfortable travelling environment. This means that the heater elements are continuously being switched on/off. At the same time, the fans and vents are continuously controlled to ensure excellent air quality is maintained to keep passengers comfortable throughout their journey.

Robust solid state relays were required to enable the heater elements to be continuously switched on and off to maintain the comfortable passenger environment. The relays had to cater for a nominal control voltage of 96V DC and provide over voltage protection. In addition, the SSRs had to be able to cope with current and voltage transients and temperature fluctuations within each carriage’s control panel. The engineering team at the train manufacturer carried out reliability tests in multiple scenarios which proved the robustness and suitability of the Carlo Gavazzi product.

In addition to meeting the manufacturer’s performance and reliability criteria, Carlo Gavazzi’s relays also had to meet strict space constraints to ensure they would fit within the carriage’s very compact control panel. Carlo Gavazzi was able to supply a compact SSR of the required rating from its RJ1A Solitron Mini range.

The RGC1A series SSR is a DIN-rail mount relay featuring zero-switching operation. This ensures it has quick response times as well as a high current rating within the compact unit, which measures 110mm x 17.8mm x 103mm. The RGC1A SSRs are able to withstand 1,200V non-repetitive peak voltage. Standard variants are available to operate at control voltages of 4-32V DC or 20-275V AC / 24-190V DC.

How to select a solid state relay

Carlo Gavazzi makes a wide variety of relays, to decide which one is the most appropriate for your particular application follow this how-to selection guide for solid-state relays

Determine the load type

The type of load to be switched will affect the choice of SSR:

- Resistive loads such as heating elements and light bulbs, where electrical energy is converted to heat or light, are best switched with a Zero-Crossing SSR in which the output is activated at the first zero crossing of the alternating line voltage sine wave, often in less than 8.3 milliseconds. As a consequence, Zero Crossing SSRs create less electromagnetic noise when they turn on. Proportional control of the resistive load can also be achieved with an analogue-switching SSR that can provide an output that is proportional to the input signal with a range 4 - 20mA or 0-10V DC.

- Inductive loads such as solenoids, pump and fan motors (anything which is built around a large coil of wire) work best with an instant-on SSR. This type of relay is activated immediately after applying the control voltage, often in less than 0.35 milliseconds.

Inductive loads work best with an instant-on SSR

- Transformers and other heavy inductive loads should be switched with peak-switching SSRs. In this type of relay the output is activated at the first peak of the line voltage (and close to the zero crossing point of the current) of the SSR.

Identify load voltage and current to be switched

Determine whether you need to switch AC or DC voltage and current for your load - this will depend on the specification of the load.

For motors, you will also need to know the maximum kilowatt rating at the application voltage.

For AC loads, SSRs usually switch one phase at 120V or 240V or at 240V, 480V or 600V for three-phase applications

Determine the control voltage or input signal

The critical control voltages for an SSR are the voltages at which the load is energised and de-energised. Unlike electromechanical relays, which are typically controlled by a fixed voltage, SSRs have a range of inputs: VAC, VDC or dual VAC/VDC. To proportionally control the load, you will need to determine the optimum output-switching type for the load, ideally with the help of an SSR manufacturer such as Carlo Gavazzi. Proportional output types include phase angle, distributed full cycle, burst full cycle, soft start and burst full cycle with soft start.

Establish the maximum ambient operating temperature

An elevated ambient temperature can reduce the current rating of the SSR, so the relay may need to be de-rated accordingly. Similarly, chassis-mounted SSRs may require a heat sink to maintain optimum performance. To select an appropriate heat sink both the ambient temperature, load current and the relay’s mounting orientation are needed. For DIN rail-mounted SSRs, the heat sink is already pre-selected, rated and attached to the SSR

Select an appropriate mounting configuration

SSRs are available in DIN rail, printed circuit board (PCB) and chassis mounting configurations.

- Carlo Gavazzi’s DIN rail mount SSRs already incorporate a heat sink making them ready to be wired up immediately. Proportional control versions are also available in this format.

- PCB mounted SSRs are generally limited in load size as a result of space constraints and the challenge of dissipating heat.

- Chassis mount SSRs are often installed in an electrical cabinet or attached to a machine or appliance. If the load is under 5A a heat sink is often not required. If SSR is attached to a metal mount, which will help dissipate heat, loads up to 8A can be accommodated without a heat sink but it is always worth checking with the manufacturer. To help SSR specifiers, Carlo Gavazzi offer an online heatsink selector tool, go to https://bit.ly/2NZ3y34

Martin Wyatt is BDM at Carlo Gavazzi UK

Key Points

  • An electromechanical relay (EMR) uses a physical moving part to connect contacts within the output element of the relay
  • SSRs have no moving parts or contacts that can wear out so they generally last longer and require less maintenance than EMRs
  • Unlike EMRs, which are typically controlled by a fixed voltage, SSRs have a range of inputs: VAC, VDC or dual VAC/VDC