Precautions For Selection Of Solid State Relay

When using solid-state relays, users should pay attention to the selection of performance parameters and load capacity of solid-state relays.

The following describes the precautions in the use of solid-state relays from four aspects:

1. Load type, output current and surge current

The steady-state current flowing through the output terminal of the relay shall not exceed the rated output current at the corresponding temperature specified in the detailed specification of the product, and the surge current that may occur shall not exceed the overload capacity of the relay. When the user selects the solid relay, he must consider that the relay can withstand this surge current on the premise of ensuring steady-state operation.

After referring to the load surge current and the overload capacity of the relay, the recommended value of the derating coefficient of the rated output current of the solid-state relay by the steady-state current of various loads at normal temperature.

For example, if the load of solid-state relay is electromagnet and the working current of electromagnet is 1.4A, the rated current is calculated to be 1.4a/0.5 = 2.8A according to the derating coefficient, leaving a little margin, and 3a solid-state relay is more suitable .

2. Output voltage, transient voltage and DV / DT

The DC solid-state relay is only applicable to control the DC power supply and load, and the AC solid-state relay is only applicable to control the AC power supply and load. The AC DC universal (bidirectional) solid-state relay can be applied to control the AC, DC and bidirectional square waves.

The voltage of the load power supply shall not exceed the rated output voltage of the relay, nor be lower than the specified minimum output voltage. The maximum voltage peak that may be added to the output terminal of the relay must be lower than the transient voltage of the relay.

When switching AC inductive load, single-phase motor and three-phase motor load, or when these load appliances are powered on, the output terminal of the relay may have a voltage twice the peak value of the power supply voltage.

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 the larger DV / DT value is added to the output terminal of the relay. Therefore, the relay with higher DV / DT should be selected.

2. Input properties

The input voltage of resistive input solid-state relays can generally be divided into two grades, which are suitable for 10-30V of low-voltage input signals. There are also constant current input solid-state relays with a large input voltage range, which is 3-32v. The input current of solid-state relays is generally about 10mA, which can be compatible with TTL circuits.

3. Other properties

Including the output voltage drop, output leakage current, zero AC, insulation resistance, dielectric withstand voltage and other electrical characteristics of the solid state relay. The solid-state relays have both zero crossing and non-zero crossing modes. When selecting solid-state relays, there should be margin to prevent load short circuit, and fast fuses should be selected. When controlling inductive loads, the starting characteristics of loads must be considered. When the current is greater than 40a, fans or water cooling should be added, and the wind speed should be greater than 6m / s.

Notes: 

1. When the load is 10A or above, a radiator must be installed. When the load is 40A or above, a fan must be added for forced cooling or water cooling.

2. When using inductive load, please connect a varistor at the output end in parallel, and the value is 1.6-1.9 times of the load voltage.