Design Ideas: July 18, 1996

Solenoid valves commonly control the flow of low-pressure gases and fluids in biological applications. A typical valve may operate at 12V and draw 1A. In systems using such valves, these power requirements can cause cost and space problems, even though a well-regulated supply is unnecessary. One solution for minimizing power consumption takes advantage of the fact that less than 20 msec after you apply the activation voltage to the solenoid, you can subsequently control the solenoid using much less power. The circuit in Figure 1 carries out this function simply and allows for control from a logic input.

Op amp IC_1A amplifies the 5V logic input to 12V. The output of IC_1A drives the positive input of op amp IC_1B, which is configured as a difference amplifier. When you first apply a logic signal to the circuit, the output of IC_1B's difference amplifier swings to 12V. The difference amplifier drives IC, a simple rectifier, which, in turn, drives power amplifier IC₂. IC₂ then applies 12V to the solenoid and activates it. The resistor in parallel with the solenoid linearly charges the 10-µF capacitor until the voltage across the capacitor reaches the rated value of the 5V reference diode. IC_1D monitors this ramping voltage and applies the voltage to the negative input of IC_1B, which reduces the solenoid voltage to 7V. IC_1C prevents the circuit from applying a negative voltage to the power amplifier when the logic input is 0V.

You can alter the resistor, capacitor, and reference-diode configuration in parallel with the solenoid to provide the ramp height and slope that suits your application and solenoid type. In Figure 1's circuit, a ±15V source powers IC₁, and a nonregulated 16V source powers IC2. (DI #1894)