Bootstrap Circuit Speeds Solenoid Actuation

The circuit in this Design Idea bootstraps a large capacitor in series with the solenoid to provide a large actuation voltage (Figure 1). This higher voltage provides substantially more current to operate the solenoid (Figure 2), speeding the operation of the solenoid. You can also choose operating voltages or solenoid specifications that result in lower continuous current through the solenoid, reducing dc power consumption and resulting in a cooler-running solenoid with better reliability.


Figure 1

When there is a 0V input to the circuit, both transistors are off. Resistor R₁ slowly charges the left side of capacitor C₁ to the 24V power-supply voltage. D₂ clamps the right side of capacitor C₁ to 0.6V. When the input signal goes high, both the Q₁ and the Q₂ transistors turn on. This action quickly drives the left side of C₁ to ground. Because voltage cannot change instantaneously across a capacitor, the right side of C₁ goes down to -23.4V. D₂ steers the solenoid current into the capacitor until it discharges, at which time the solenoid current conducts through D₂ to ground. D₁ prevents a voltage-overshoot spike when the circuit turns off, and current suddenly stops flowing in D₁. It clamps the bottom leg of the solenoid to 24.6V until the current decays in the solenoid.

The time constant of the circuit depends on the inductance of the solenoid and the value you choose for the capacitor, which you can calculate with the following equations:



In these equations, e is the mathematical constant, ω is the radian angular frequency, and t is time in seconds. In addition, L is inductance and R is resistance.