Tech Tips / Application Guidelines

Tech Tips / Application Guidelines

How to Select a Right Solid State Relay?

Fri, Dec 15 by
When selecting solid state relay (SSR), it should be based on the actual application conditions and SSR performance parameters, and especially take into consideration the overcurrent and overvoltage conditions in the use and the load capacity of SSR, which is helpful to achieve the long life time and high reliability of solid state relays. Then, follow the questions and find the answers to choosing a right SSR.

1. How to select a solid state relay based on load types?
There is no problem for SSR to switch on/off the normal loads, but some special load conditions should also be considered so as to avoid the unnecessary damages to the device caused by excessive impact current and overvoltage. In the use, the steady-state current flowing through SSR output should not exceed the rated output current under relevant temperature as stipulated in the product specifications. The possible inrush current cannot exceed the overload capacity of the relay. Generally, there should be some margin.

The rated current of SSR is selected according to different load types.The instantaneous current of resistive load, inductive load and capacitive load is large when starting. Even for the load with pure resistance, the resistance value is small in cold state because of the positive temperature coefficient, so it has a large starting current. For example, the starting current of asynchronous motor is 5 to 7 times as large as rated value, and the starting current of DC motor is larger. Moreover, the inductive load has higher back EMF. This is an indeterminate value, varying with L and DI/DT, usually 1 to 2 times higher than the power supply voltage, which is superimposed with the power supply voltage. Thus there exists a voltage 3 times higher than the power supply voltage. Capacitive load has potential risk. When starting, the capacitor (load) is equivalent to a short circuit because the voltage at both ends of the capacitor cannot be mutated.

Therefore, when selecting solid state relays, users should carefully know about the surge characteristic of load, and then make a decision. SSR can bear the surge current in the case of ensuring its stable working. Generally, the ordinary SSRs can be selected based on the 2/3 of its rated current value. The enhanced SSRs may be selected according to the parameters provided by the manufacturer. In the harsh conditions such as industrial control sites, it is recommended to leave enough voltage and current margin.

2. How to select a right SSR according to the circuit's power voltage, transient voltage and dv/dt?
DC solid state relay is only suitable for controlling DC power and load, AC solid state relay is only for controlling AC power and load, and AC/DC universal(bidirectional)solid state relay is applied to AC, DC and bidirectional square wave control.

The voltage of load power supply can't exceed rated output voltage of solid state relay, and can't be lower than stipulated minimum output voltage. The maximum value of voltage peak which is possibly added to solid state relay should be lower than the value of its transient voltage. When switching the AC inductive load, single-phase and 3-phase motors, or energizing these loads, the output side of SSR may appear twice the voltage peak of the power supply.

For inductive and capacitive loads, when AC solid state relay turns off at the zero current, the power voltage is not zero, and adds to solid state relay output end with a large dv/dt value. Therefore, high dv/dt solid state relay should be selected.

3. What are the requirements for the input ends of solid state relay?
ATO provides two types of solid state relays, DC and AC input control. DC control input all use constant current source circuit, with input voltage range of 3-32V DC, convenient to connect with TTL circuit and microcomputer interface. The positive and negative polarity of control terminals should be paid attention to under installation. AC control input of solid state relay is also available with control voltage ranging from 70 to 280V AC.

4. How to protect the overcurrent, overvoltage, overheating of solid state relay?
Over current and short circuit may cause permanent damage to the internal SCR of solid state relay in the use. In this case, installing fast fuse and air switch in the control loop can be taken into account for the protection. So, solid state relays should be selected with output protection, built-in RC snubber circuits and MOV, which can absorb surge voltage and improve dv/dt tolerance. It is also feasible to connect RC snubber circuits and MOV in parallel at relay output end to achieve output protection.

The load capacity of solid state relays is greatly affected by the ambient temperature and its own temperature rise. In the installation and usage, good heat emission conditions should be guaranteed. In general, for the SSR with rated operating current more than 10A, radiators should be equipped with. For more than 100A, a radiator and a fan should be equipped with for forced cooling. In the installation, it should pay attention to good contact between the bottom of the relay and the radiator, and consider the amount of thermal grease coated appropriate to achieve the best cooling effect.

Protection of Solid State Relay Circuits – Fuse Selection

Absolute protection of a solid state relay from a shorted load or line condition requires more thought than simply providing a common circuit breaker or fuse in the circuit. Compared to electromechanical switching devices, the solid state thyristor switching elements used in the output section of a Solid State relay have very short thermal time constants. Consequently, extreme current levels and surges caused by load or line faults, even if only applied over extremely short time periods, may cause the thyristor devices to permanently fail. Standard fuses and circuit breakers simply cannot react quickly enough to prevent the fault current from exceeding the maximum levels that the thyristors can withstand.

Fortunately for the system designer, solid state relay manufacturers provide within their datasheets a specification value that designates the maximum current vs. time that the thyristors can handle.

This value is commonly listed as "maximum I²t for fusing", (amperes squared seconds). Equally fortunate is that fuse manufacturers have certain types of fuses that also carry an "I²t" value. These fuses are generally called "Semiconductor" or "Ultra Fast Acting", and are specifically designed to completely open within their published "total clearing I²t" value.In the most simplistic sense, (assuming that the appropriate solid state relay has been selected for the particular load parameters), the fuse selection can be made by considering:
1. The I²t rating of the selected solid state relay.
2. The fuse voltage rating to accommodate the system voltage.
3. The fuse current rating, (considering normal running load, start-up surges, operating temperatures, etc.)
4. The I²t rating of the fuse. Basically the "total clearing I²t" rating of the fuse selected must be below the I²t rating of the selected

solid state relay, and above the expected "normal" current surges of the load. See Fig. 1. It may happen on some occasions that the "normal" current and voltage ratings required of the fuse push its I²t rating close to or beyond the I²t rating of the solid state relay. If this is the case, a higher I²t rated solid state relay can be selected. As stated previously, this is a very simplistic and general method of determining adequate fusing for solid state relays. There are several other items that should be considered if one needs to "dial-in" a perfectly ideal fusing solution. These factors include among others, the available fault current from the overall system, the amount of load surge cycling that will affect the cumulative heating of the fuse itself, and the peak "let-through" current of the fuse prior to clearing. Fuse manufacturers such as Cooper Bussmann, Littelfuse, etc., publish extensive notes detailing the calculations and methods of using those factors.