Considerations for selecting solid-state relays

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Considerations for selecting solid-state relays [Copy link]

1. Load type, output current and surge current The steady-state current flowing through the output terminal of the relay during use shall not exceed the rated output current at the corresponding temperature specified in the detailed product specifications. The surge current may not exceed the overload capacity of the relay. When selecting a solid-state relay, the user must consider whether the relay can withstand this surge current while ensuring steady-state operation. The following is a recommended value for the derating factor of the rated output current of the solid-state relay for the steady-state current of various loads at room temperature after considering the load surge current and the overload capacity of the relay. Example: Assume that the load of the solid-state relay is an electromagnet, and the working current of the electromagnet is 1.4A. According to the derating factor, the rated current value is calculated to be 1.4A/0.5=2.8A. With a little margin, it is more appropriate to choose a 3A solid-state relay. 2. Output voltage, transient voltage and DV/DT DC solid-state relays are only suitable for controlling DC power supplies and loads, and AC solid-state relays are only suitable for controlling AC power supplies and loads. AC/DC universal (bidirectional) solid-state relays can be used for the control of AC, DC and bidirectional square waves. The voltage of the load power supply cannot exceed the rated output voltage of the relay, nor can it be lower than the specified minimum output voltage. The maximum voltage peak value that may be added to the output end of the relay must be lower than the transient voltage value of the relay. When switching AC inductive loads, single-phase motor and three-phase motor loads, or when these load appliances are powered on, a voltage twice the peak of the power supply voltage may appear at the output end of the relay. For inductive and capacitive loads, when the AC solid-state relay is turned off at zero current, the power supply voltage is not zero, and is added to the output end of the relay with a larger DV/DT value, so a relay with a high DV/DT should be selected. 3. Input characteristics The input voltage of resistive input solid-state relays can generally be divided into two levels, 10-30V for low voltage input signals, and constant current input solid-state relays with a larger input voltage range, ranging from 3-32V. The input current of the solid-state relay is generally around 10MA, which is compatible with TTL circuits. 4. Other characteristics include the output voltage drop, output leakage current, zero-point AC, insulation resistance, dielectric withstand voltage and other electrical characteristics of the solid-state relay. All solid-state relays have two modes: zero and non-zero crossing. Please indicate the non-zero crossing mode. When selecting solid-state relays, you must leave a margin to prevent load short circuit. You should choose an appropriate amount of fast fuse. When controlling inductive loads, you must consider the starting characteristics of the load. When controlling inductive loads, you must consider the starting characteristics of the load. When the current is greater than 40A, you need to add fan cooling or water cooling, and the wind speed is greater than 6 meters/second. Note: 1. For loads of 10A and above, a radiator must be installed, and for loads above 40A, add fan cooling or water cooling. 2. When using an inductive load, you must connect a varistor in parallel at the output end, and the value should be 1.6-1.9 times the load voltage.