3 Basics of Driving Solenoid Loads

Because solenoids are inductive loads, they store energy in the magnetic field when current flows through the coil. Whether disabling the solenoid or using PWM to maintain a specific current level, any circuitry used to drive a solenoid must never abruptly stop the flow of current. Doing so will cause a large voltage spike due to the energy leaving the solenoid. This is apparent from the expression that defines the voltage characteristic of inductors: V = L*di/dt.

As mentioned in the previous sections, freewheeling diodes allow for the current to recirculate when the driving FET disables. This keeps the voltage across the solenoid equal to the forward voltage drop of the diode. When disabling a solenoid, the current will recirculate until the energy stored in the inductor dissipates as heat in the series resistances of the diode and solenoid. Because the current decreases slowly, freewheeling diodes should be used in systems when PWM or current regulation schemes are used to implement peak and hold control for power savings.

Some circuits require solenoids to disable quickly to minimize latency in the system for valves or actuators. Clamping circuits may be integrated into the driver or added externally to help dissipate the energy. For instance, adding a Zener diode in series with the freewheeling diode will help to dissipate energy quickly from the solenoid. In this case, the voltage drop across the solenoid when the driving FET is disabled will be equal to the Zener clamping voltage plus the diode drop. Because this voltage is much higher than the freewheeling diode alone, it will dissipate the stored inductive energy much fast.