It may not be generally realized that the speed of brushless fans commonly found in computers can be controlled down to just two or three revolutions per second if necessary. This could allow their use in other applications such as displays, lighting effects, or even driving the “Nipkow Disk” described in Ingenuity Unlimited December 1999.

The motors within these fans generally consist of an outer revolving armature containing permanent magnets surrounding a static inner coil assembly. The coils are switched sequentially to produce the rotation by an internal electronic circuit, using Hall  Effect devices to sense the position of the magnets. Although the motor speed can be reduced to some extent by lowering the supply voltage, a point is quickly reached where the voltage becomes too low for the electronics to operate, so it simply stops.

Much lower speeds can be achieved with a pulse-width modulated (PWM) supply where the power is applied as brief pulses of the full supply voltage, with a constant frequency but a variable width. The circuit of Fig. 3 has been used successfully to achieve this.

How It Works

The circuit works as follows. Opamp IC1a acts as an integrator and IC1b as a comparator with hysteresis set by resistors R4 and R5. Together these two circuit elements form an oscillator with a triangle wave output. Note that the triangle wave from the output of IC1a is applied to the inverting input (pin 2) of IC2, whilst the non-inverting input of IC2 is connected to a control voltage set by VR1, the speed control which provides a range spanning zero to full power. The value of VR1 is shunted by resistor R7 to counteract the wide tolerance typical of these controls. IC2 acts as a comparator and drives output transistor TR1 (a general purpose npn medium power transistor), which in turn powers the brushless motor. Diode D1 counters any back-EMF thatmay be present.

The opamp used for IC2, e.g. the 3130 should have an output capable of reaching negative rail so that the transistor is turned completely off when it is low. No external compensating capacitor is necessary for the opamp in this switching application.Some other opamps which could be used in this position include the 3140, half an LM358 or perhaps one of the new  generation of rail-to-rail CMOS types. (Also see our Opamp Selector Chart in Circuit Surgery, March 2000 – ARW.)

Motors used with this circuit should be 12V types. When tested, a 75mm fan from a scrapped 386 computer and a 47mm Pentium CPU fan both worked without any problems. The motors were surprisingly tolerant of high pulse rates, frequencies up to 100Hz being accepted with no performance loss. The larger fan produced some audio noise at higher frequencies, the smaller unit much less, and this gencould be minimized by adjusting the frequency through the values of R3 and C1. The values shown produce a frequency of about 33Hz for good performance and minimal noise. Experiment if necessary by attaching extra mass to the motor’s armature to increase inertia, otherwise the rotation may be slightly jerky. A drop of dry lube oil on the bearing may also help.

Article reproduced by permission of Wimborne Publishing. www.epemag.com