SSR Relay | Practical Guide

Switching Frequency of SSR ON/OFF SSR Relay is characterized by delays associated with switching on and off. Inductive loads (transformers, electric motors) also have the same delays when switching on and off. When selecting an SSR, this should be taken into account.

Interference of the pulsing control signal SSRs require very little power for proper operation. Therefore, the control circuit should be free from interference that could cause incorrect relay operation) A combination of resistor and capacitor can help remove unwanted noise in the INPUT input circuit.

Example of a noise/interference reduction circuit. The value of the resistor (R) cannot be too high as the input voltage (E) will not reach the appropriate level. The higher the value of the capacitor (C) the longer the delay associated with the SSR switching off. The delay is a result of the capacitor discharging.

When selecting the values of R and C, the above table can be used, taking into account the switching frequency.

Signal interference from electromagnetic induction.

It is best to avoid placing the power line too close to the SSR as this can cause interference. To reduce the interference on the control signal, the appropriate cables can be used in combination with an RC filter.

- Twisted Pair cable for electromagnetic interference. - Shielded cable for static interference. An RC filter should remove the remaining noise at the input.

Leakage current - countermeasures. Control circuit (INPUT)

An SSR relay controlled by a transistor output may not switch off completely due to leakage currents of the transistor in the control output, resulting in a small leakage current flow. The solution to this problem is to use a so-called "bleeder" resistor as shown in the diagram. The resistance should be chosen so that the VR voltage is 0.5 V or less.

The permissible value of the SSR current in the OFF state is usually not given in the catalog card, so the formula can be used to calculate it:

Fig.Output circuit (INPUT)

For devices with high current sensitivity, the circuit from the following scheme can be used

Using a protective diode for the SSR against the opposing electromotive force induced in inductive loads is the most effective, but it may extend the switching time of the circuit. To shorten the switching time, connect a Zener diode and a rectifier diode in series.