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Designing magnetic solenoid driver circuits

Posted: 09 Sep 2013 ?? ?Print Version ?Bookmark and Share

Keywords:magnetic solenoids? pulse width modulation? PWM? diode? oscillator?

Several hurdles must be overcome to drive magnetic solenoids in valves, relays and other actuators with utmost efficiency and minimum power.

During energising, the current required to bridge the air gap in the magnetic circuit is usually much higher than what is later needed to hold the device when the magnetic circuit is closed.

Without a suitable driver circuit, magnetic solenoids are often simply powered by the energising current, with an unnecessarily high level of power dissipation as the result.

Furthermore, where standard, unregulated driver circuits are used the solenoid must be suitable for the relevant supply voltage, which in turn defines the solenoid current through the solenoid's internal resistor. Different supply voltages thus require solenoids specifically adapted to them.

Possible approaches towards a more flexible, power-saving driver unit range from simple single-transistor circuits with currents reduced by RC circuits to integrated circuits with pulse width modulation (PWM).

Standard integrated solutions only provide a 'controlled' PWM (figure 1). This makes it relatively easy to reduce the hold current versus the energising current and the overall current draw by lowering the duty cycle. However, the resulting current still depends on the supply voltage. This means that both the solenoid and PWM control (frequency, duty cycle) have to be adjusted to suit the applied supply voltage. In some integrated systems, however, attempts are being made to 'track' the PWM control depending on the supply voltage.

Figure 1: Driving a solenoid with simple PWM.

Using a current-controlled PWM circuit with current reduction takes the two main aspects of solenoid driving into consistent account. As with controlled PWM the inductivity behaviour is exploited to store the current. This is measured and used as a controlled variable. In principle circuits like this are similar to those in switching converters. The switch (T1) is cyclically activated by an internal oscillator with a fixed frequency and powers the solenoid (L1) up to the set cut-off current. When this cut-off current is reached, T1 is again switched off. The energized solenoid freewheels through the diode (D1), i.e. the solenoid current is slowly reduced until the next activation. On average the solenoid is operated at a current which is just below the set cut-off current (figure 2).

Figure 2: Current-controlled PWM solenoid drive.

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