1. DC Solenoids

When a DC power supply is connected, the current flows in one fixed direction. Since a coil inherently resists sudden increases in current, the current rises gradually. As a result, the burden on the power supply and components is relatively small.

However, when the power supply is turned off, the energy stored in the coil appears as back electromotive force (back EMF), which attempts to drive current in the opposite direction.

This can damage components, so DC solenoids are typically protected by connecting a diode in parallel to safely dissipate the current.

2. AC Solenoids

With an AC power supply, the current changes periodically. If the power is switched on at a moment when the voltage is at its peak, the magnetic circuit of the coil is not yet stabilized, which can cause a sudden surge of inrush current. When the power is turned off, the back EMF appears as surges or noise, which may affect the circuit and surrounding components.

Therefore, AC solenoids are commonly protected with surge suppression circuits using resistors and capacitors.

3. Pumps (Motors)

Pumps driven by motors also generate inrush current. A motor requires a large amount of current when starting from a stop, resulting in a surge several times greater than the current during steady-state operation. This is the pump’s inrush current. Such high current can lead to breaker tripping or increased stress on wiring.

To mitigate this, soft-start circuits or inverter control methods are employed to gradually ramp up the current.