To control the load current on the bobbins of a motor (and, consequently, the field they generate), various techniques can be used; it is possible for example to commutate power continuously to the bobbin between two voltages, or, if power to the bobbin is a square wave, its frequency can be changed.

There is a technique by which every positive impulse (of the wave) is cut before its end: this technique is called PWM or pulse width modulation. In all these cases the final result is that of changing the average current value which circulates in the inductor. We will look into the latter technique in more depth: PWM. To carry out this type of technique we need some block s that act upon the impulse, the choppers. Each chopper is formed by a comparator, usually an OP-AMP, a flip-flop and a resistor which provides the voltage (to the comparator) relative to the current flowing in the load.

The chopper's operation can be summed like this: if we connect the set foot of the flip-flop to a square wave oscillator, the flip-flop will be set in correspondence to each positive transient of the same, so that the filp-flop's output is taken to the logic one. This brings to the enabling of a section of the driver's bridge because every flip flop's output is connected, through an appropriate combinatory network, to the transistors' bases. Thus at this point the power current to the bobbin is increased and, consequently, the same occurs of the tension present at the ends of the Rs resistance.

The tension's value however cannot grow indefinitely because when this reaches the d.d.p. Vref reference value, the comparator activates resetting the flip-flop and disabling the bridge until it another oscillator impulse arrives. We can conclude that the flip-flop output is the signal modulated with PWM and that Vref voltage determines the bobbin's peak current.

November.2001