Precautions for the use of solid state relays
Precautions for the use of solid state relays
1. Series and parallel connection of SSR input terminals
The input pins of multiple SSRs can be connected in series or in parallel, but if each SSR must meet the high level, the zero-crossing type trigger current is greater than 5mA, the operating Dumb Type is greater than 10mA, and the low-level voltage is less than 1V. That is, the parallel drive current must be greater than the sum of multiple SSR input currents; when connected in series, the drive voltage must be greater than the sum of multiple open circuit voltages (calculated at 4V).
2. Matters needing attention in RF absorption circuit and circuit leakage current and SSR test
The function of the RC absorption circuit is to absorb the surge voltage and improve the static dV/dt index, but the RC circuit inside the SSR brings the leakage current in the shutdown mode. Generally, the SSR leakage current of 2A-6A is suitable for the load above 10W, such as the motor. Almost no effect, SSR leakage current above 10A In addition, if a large inductive load is actually applied, an RC sink circuit can also be connected in parallel at the two outputs of the SSR to protect the SSR.
For users with small load power such as intermediate relays, contactor coils, electromagnetic attraction micro-power motors, etc., solid-state relays with leakage current less than 1mA can be customized (add e to the corresponding SSR model).
The solid state relay used for power expansion (add e after the model) must have no RC circuit inside. This is due to the malfunction caused by the charging and discharging of the RC circuit.
When the resistance at both ends of the solid state relay is measured to be close to zero within the resistance range, it means that the thyristor inside the solid state relay is damaged. In addition, in order to judge the quality of the solid state relay, a circuit with a load must be used.
3. The selection of solid state relay voltage level and overvoltage protection
When the voltage peak applied to the AC terminals of the solid state relay exceeds the higher voltage peak that the SSR can withstand, the components in the solid state relay will be damaged by voltage, and the SSR will be destroyed. The SSR can be protected by proper selection of voltage levels and shunt varistors.
4. Selection of solid state relay current level and overcurrent protection
Overcurrent (or load short circuit in the worst case) is the main cause of permanent damage to the internal output thyristor of the SSR. (Fast fuses and air switches are one of the overcurrent protection methods, and fuses can also be selected as small-capacity SSRs; many loads will generate large surge currents at the moment of switching on, but due to insufficient heat dissipation, the surge Inrush current, like overcurrent, is one of the main reasons to damage the internal output thyristor of SSR. Therefore, it is extremely important to ensure a certain current margin when selecting solid state relays.
5. SSR heat generation and heat dissipation The larger heat generation when the single-phase SSR is turned on is calculated according to the actual working current of 1.5W/A. The heat dissipation design needs to consider the ambient temperature, ventilation conditions (natural cooling, fan cooling) and SSR setting density. A thin layer of thermal grease must be applied between the solid state relay and the heat sink mounting surface.
6. Grid frequency
SSR is applied to commercial power grids of 50Hz and 60Hz, and is not suitable for situations with large low-frequency and high-order harmonic components. For example, when there are multiple loads at the output end of the inverter and they need to be switched separately, if SSR is used as the switch, it may not be able to be reliably cut off by high-order harmonics, and the high-order harmonics may also cause the RC absorption circuit inside the SSR to overheat. rupture.
The above are the precautions for the use of solid state relays. If you want to know more, you can contact us at any time!
