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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?

The defining advantage of this configuration is that the current is regulated independent of the supply voltage. Only the hold and energising currents have to be set to match the solenoid. Here, the duty cycle of the PWM is automatically regulated depending on solenoid parameters, such as inductivity and internal resistance, and the supply voltage.

In this manner, with this driver circuit the same 6 V solenoid can be used within a wide voltage range (10 to 36 V), for example, as the solenoid current no longer depends on the supply voltage.

Figure 3 shows the practical design of a controlled PWM driver circuit as described above in the form of integrated circuit iC-GE. The energising current is set by RACT and reduced to the hold current configured with RHOLD after typically 50 ms. By reducing this to two-thirds of the energising current, for instance, the power dissipation is cut by approximately 50%.

The Zener diode in the PWM output stage is activated when the driver is switched off and enables faster demagnetisation of the solenoid due to the higher freewheeling voltage.

Figure 3: Integrated system for a current-controlled, intelligent magnetic solenoid driver circuit.

The PWM output stage uses an internal 80kHz oscillator as a time base, and thus is beyond audible range.

External resistors RACT and RHOLD permit the respective energising and hold currents to be set within a range of 100 mA to 1 A. Alternatively, iC-GE100 is available for a current range of between 10 and 100 mA. This enables solenoids within 10 mH to 10 H to be driven in many different applications.

Besides current reduction and regulation this device also includes diagnostic functions which can signal solenoid defects, fractured wires, and over-temperature in the iC to the LED output (blinking frequency typically 2.4Hz).

Another advantage over the regulated driver circuits on the market to date is the separate configuration of the energising and hold currents, plus the ability to extend the actuation timethe time needed for the solenoid current to be regulated down to the preset hold currentusing a simple capacitor.

Other interesting features include the following:
???Flexible error display (DIAG), either with an LED driver or as a logic output
???Option of synchronising the driver with the load current of the relay to be driven, for instance (SYNC), so that this can be switched in zero load current to preserve contacts

To sum up, it can be said that utilising special ICs in applications with electromechnical relays, valves and magnets can reduce power consumption independent of the supply voltage. At the same time, the system becomes more reliable, and errors can be quickly detected.

About the author
Uwe Malzahn, Dipl.-Ing., studied electrical engineering at the University of Darmstadt, Germany. He is an application engineering manager responsible for actuator and laser driver iCs at iC-Haus in Germany.

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