SOLID STATE RELAYS FEATURE Will Darby, managing
director, Carlo Gavazzi UK, outlines how to select the most appropriate
Solid State Relay (SSR) for your application
T
he cables connected to the starter motor are among the largest diameter cables in your
vehicle, and they are big because a starter motor needs a lot of power to turn a combustion engine. If the cables were connected directly to the ignition switch on the dashboard, you would need a giant switch to handle the load. Instead, the ignition switch or starter button uses a much smaller amount of current which controls a relay. When you turn the key it energises the relay which closes the starter motor circuit. A relay is an electrically controlled switch which allows you to control
Relay useful information
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.
a large amount of current without having to run that current through the controlling device. An electromechanical relay (EMR) uses a physical moving part to connect contacts within the output element of the relay; while Solid State Relays (SSRs) do not have physical moving parts and are switched electronically when an external voltage is applied to the unit’s control terminals. SSRs are inherently smaller than electromechanical relays, they are
silent in operation, switch faster and generally have a longer lifespan. They are also less sensitive to vibration, humidity and impact. 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.
ENSURING A COMFORTABLE TRAVELLING ENVIRONMENT Thanks to their speed and reliability, Carlo Gavazzi’s SSR’s were selected to control the heating, ventilation and air conditioning systems in the passenger carriages of a state-of-the-art express train. Robust SSRs enabled the heater elements to be continuously switched on
and off to ensure a comfortable environment. At the same time, the fans and vents are continuously controlled to ensure excellent air quality. The relays cater for a nominal control voltage of 96V DC and provide over voltage protection. They can also cope with current and voltage transients and temperature fluctuations within each carriage’s control panel. In addition to meeting the carriage 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. A compact SSR of the required rating from the company’s RJ1A Solitron Mini range was therefore supplied. The RGC1A series SSR is a DIN-rail mount relay featuring zero-
switching operation, ensuring quick response times as well as a high current rating within a unit measuring just 110mm x 17.8mm x 103mm. These 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.
SELECTING THE RIGHT SSR A wide variety of relays are manufactured by Carlo Gavazzi. To decide which one is the most appropriate for your application: 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 8.3ms. As a consequence, Zero Crossing SSRs create less electromagnetic noise when
/ DESIGNSOLUTIONS
• 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.
• 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 the 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 offers an online heatsink selector tool, go to
https://bit.ly/2NZ3y34.
Carlo Gavazzi UK
www.carlogavazzi.co.uk
DESIGN SOLUTIONS | FEBRUARY 2019 23
Carlo Gavazzi’s SSR’s were selected to control the heating, ventilation and air conditioning systems in the passenger carriages of a state-of-the-art express train
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