How to select right type of solid state relays for varied loads?

Rupali Jadhav Menon explains various types of solid state relays (SSRs) and provides tips for proper selection of SSR for varied loads
How to select right type of solid state relays for varied loads?

Rupali Jadhav Menon explains various types of solid state relays (SSRs) and provides tips for proper selection of SSR for varied loads

Given below is a short overview guiding design engineers and technicians regarding selection of solid state relays (SSRs) output switching functions.

Switching AC loads: 1-Phase & 3- Phase

Engineers using electromechanical relays are generally accustomed to energising a coil and waiting for the arcing & clanking to occur shortly thereafter. The nature of a solid state relay (SSR) is more subtle for an AC output there are three variants of SSRs available. The ON-OFF switching (ie 1A: ‘zero-cross’ J-type, 1B: ‘random’ K-type) and linear switching or control ‘proportional control’ (2A: Digital proportional control and 2B: Digital burst firing control).

One of the biggest advantages of solid state relays over an electromechanical relay is its ability to switch ‘OFF’ AC loads at the point of zero load current, thereby completely eliminating the arcing, electrical noise and contact bounce associated with conventional mechanical relays and inductive loads. This is because AC switching solid state relays use SCR’s and TRIAC’s as their output switching device which continues conducting, once the input signal is removed, until the AC current flowing through the device falls below its threshold or holding current value. Then the output of an SSR can never switch OFF in the middle of a sine wave peak. Zero current turn-off is a major advantage for using a solid state relay as it reduces electrical noise and the back-emf associated with the switching of inductive loads as seen as arcing by the contacts of an electro-mechanical relay. Consider the output waveform diagram below of a typical AC solid state relay.

Zero-crossing On-Off switching SSRs

The zero crossing or a ‘synchronous’ solid state relay is the most common type of SSR. The switching of the relay from a non-conducting to a conducting state occurs when the AC mains voltage reaches the zero-crossing point of the sine-wave. This minimises the surge current through the load during the first conduction cycle and helps reduce the level of conducted emissions placed on the AC mains. Figure 1A shows the input and output signals on a zero-crossing solid-state relay.

Note that the output of the SSR does not stop conducting until the load current reaches the next zero-crossing point of the AC sine-wave. However, this is not related to the ‘zero-crossing’ function of the SSR. It is due to the fact that the SCRs in the output circuit cannot turn off until the load current falls below their specified holding current (typically less than 100 mA). This is a characteristic of all AC output solid-state relays, regardless of the switching type.

Zero-crossing relays are ideally suited for most commercial and industrial loads, such as resistive heating elements, lamps and ballasts, and any other load with low initial impedance or capacitive characteristics. They are also preferred in applications with some level of capacitance as they can minimise surge currents during the first conduction cycle; Pf > 0.75.

Random Turn-On SSRs

Also known as ‘asynchronous’ or ‘instantaneous’ solid state relays, these relays turn on immediately after the application of the control signal as seen in Figure 1B. In most cases the output is fully conducting load current in less than 100µS the input and output signals on a random turn-on solid-state relay.

Random turn-on solid state relays are commonly used in applications where precise control of power to the load is required (phase-control applications). They are also commonly used with inductive loads, where the phase shift between voltage and current can cause problems with zero-crossing relays.

Random turn-on relays are usually recommended for Inductive Loads (motors, contactor coils, transformers)- pf <0.75 - as the phase shift between voltage and current may cause problems with some zero-crossing relays. However, most Satronix zero-crossing SSRs today work well with motor loads, but traditionally it would be better to use random turn-on relays with motor loads. Phase-angle and burst-fire relays are not suitable for inductive loads.

Proportional control solid-state relays

The most common types of proportional solid-state relays on the market today are phase-angle controllers and burst-fire relays. These solid-state relays provide proportional power to the load (from 0% to 100% in most cases) based upon the value of an analogue signal applied to the input. This can be a 0-5V, 0-10V, 4-20mA, resistive value, or other varying signal that translates into a desired load-power level. These relays are often found in heating applications requiring extremely precise temperature levels, and lighting applications requiring the gradual increase and decrease in the brightness of a room or area.

Figure 2A gives a simplified diagram of the output waveform on a phase-angle controller/SSR. In this example, we have a 5V analogue signal applied to the input of a 0-10V SSR (50%). The corresponding output waveform shows the SSR turning on at the peak of each AC half-cycle, effectively applying 50% power to the load. If we gradually increased the analogue input from 5V to 10V, then we would see the shaded areas in the waveform diagram slowly disappear until we reached 100% power to the load.

Burst-fire controllers are similar to analog SSRs in that they provide proportional power to the load. However, instead of conducting partial load current during each half-cycle, burst-fire controllers provide a continuous train of full AC cycles to the load. The number of ‘on’ and ‘off’ cycles determines the percentage of power applied to the load over a fixed time period, which is controlled by the value of the analogue signal applied to the input. The advantage of this is that it reduces the level of conducted emissions placed upon the AC mains as the relay is turning on and off at the zero-crossing point of the sine-wave. The disadvantage is that burst-fire SSRs are typically not suitable for lighting applications, as the variance between on and off times can create an unwanted flicker effect.

Figure 2B shows the output waveform on a burst-fire controller/SSR. In this example, we have a 5V analogue signal applied to the input of a 0-10V SSR (50%). The corresponding output waveform shows the SSR providing five full AC cycles to the load, then turning off for five full AC cycles (50% power, effectively). If we gradually increased the analogue input from 5V to 10V, then we would see the shaded area in the waveform diagram slowly disappear until we reached 100% power to the load. Phase-angle and burst-fire relays are also perfectly suited for resistive loads, especially where precise temperature control is required. However, if there is capacitance in the load then we only recommend burst-fire relays.

Three-phase reversing SSRs AC and DC

The three-phase reversing SSRs have a technological advantage over traditional EMRs and mechanical reversing contactors. They are a three-phase solid state switch which control two phases of the load while the third phase is given directly for forwarding and reversing of three-phase induction motors. Very popular and widely used as solar tracking operation of solar panels to follow path of the Sun through the day. Apart from many other applications like forklifts, they are widely used in cement plants or such applications which are dust prone areas. There are two variants one is to switch AC induction motors other DC reverser solid state contactor to reverse the polarity of a variety of DC loads like motors, solenoids, plating baths, electro magnets, electrolytic cells, clutches and brakes.

Dual solid state relays: Simply put two single-phase SSRs (NO) in a package of one with 4 different variants for input output. Also, NO and NC SSR in one package. But one should not forget that if one SSR dissipates 20 watts of power then a dual SSR of same rating will dissipate 40 watts, so suitable heatsink arrangement for heat dissipation must be in place. Dual SSRs can switch 2 phase also 3 phase load in Delta or Wye/Star connection without neutral. Another advantage apart from less panel space is a dual SSR dissipates 33% less power than 3-phase SSR since switches two phase and the third phase is given directly whereby reducing the size of the heatsink too. Typical applications include medical and laboratory equipment, cooking equipment and beverage dispensing equipment.

PCB mount SSRs and interface relay boards: Highly recommended in the place of conventional mechanical relays. Used for their compact size and especially for their longetivity once being soldered on to a complex circuit board. DIN Rail mount sleek models and Interface SS relay boards with easy pluggable SSR variants with fuse protection too available. Short circuit protected solid state relay: This one is a invention of Satronix and these are rated single phase SSRs which are short circuit proof which trip when a load shorts, they restart automatically once the load short circuit is removed. Universally very high-end load monitoring and feature rich variants too available now in this kind.

Hybrid or non heat solid state relay

The output circuit of the hybrid SSR contains both a solid state and a mechanical device. The beauty of it being the advantages of both these relay types come into play giving way to a new generation 3-phase hybrid SSR model which does not require heatsinks. The SCR device ouput energises immediately on application of the input signal thus eliminating arching typical with EMRs and contactors switching at high line voltages or heavy surge current drawn during starting of a typical load. The EMR is parallel to the SSR and operates once the load energises hence power dissipation drops to a few watts at most depending on the load current. Resultant being a win-win situation of higher life expectancy and no heat dissipation management required.

Switching DC loads 1-phase: DC output solid state relays are frequently utilised to switch loads such as electric valves, heating elements, fans, solenoids, EMR electromechanical coils etc. and are designed only to switch DC voltage sources. Apart from the advantages of the normal AC SSRs are coupled with an increase in battery powered applications such as automotive and renewable energy systems, other advantages specifically with mosFET outputs make them desirable due to low-impedance Rds-on which can be as low as 0.005Ohms in high current models.

Rupali Jadhav Menon is the CEO at Satronix (India) Pvt Ltd, a leading company in power electronics and renewable energy. Working with Satronix since 1988, she is financial and marketing strategist. Established in 1985, Satronix is one of the leading manufacturers of factory automation components and power electronics products in India. It was Satronix that introduced solid state relays in India about 30 years ago and has since become a brand synonymous with SSRs. For details, contact Rupali Jadhav Menon on Email: rupali@satronixindia.com

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