Design Feature: September 1, 1995

Mechanical actuators that develop a force from magnetic interaction operate efficiently with high power until seated (the pull-in state). You should then reduce the power to a lower holding level. These rules are especially true for long-stroke solenoids. Currently, the only available solenoid-driver ICs are Motorola's MC3484 Series and National Semiconductor's LM1984, which are both designed to drive automotive fuel injectors. Upon activation, either IC ramps the current to a peak value and switches to a holding current that is one-fourth as large. This ratio is fixed and does not satisfy all driver applications. Fig 1 shows a universal solenoid driver based on the 556 timer chip.

The second portion of the timer chip, IC_1B, is configured as a conventional astable oscillator. You establish the duty cycle by setting IC_1B's output high to control the charge time of C₃, given by t_ON=0.693R₂C₃. The following expression yields the discharge time:

The first portion of the 556 timer, IC_1A, discharges C₂, and setting the control-level signal on reset pins 4 and 10 disables the solenoid. When the solenoid energizes, C₂ and C₃ are essentially in parallel and begin charging together. The long charging time forces the first pulse of the astable oscillator to be long. The length of this first pulse allows the solenoid to ramp up to the maximum current established by the supply voltage, V_CC, and the resistance of the solenoid coil.

When the voltage on C₃ reaches two-thirds of the 5V supply level, diode D₁ removes C₂ from the astable oscillator. The removal of C₂ establishes the normal duty cycle the circuit requires to maintain the holding current at the desired average value. The values in Fig 1 yield an initial charge time of 75 msec, followed by a 50% duty cycle at approximately 13 kHz.