How to Select a Right Solid State Relay?
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.