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PWM Control - PWM Control Pulse-width modulation control...

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PWM Control Pulse-width modulation control works by switching the power supplied to the motor on and off  very rapidly. The DC voltage is converted to a square-wave signal, alternating between fully  on (nearly 12V) and zero, giving the motor a series of power "kicks". If the switching frequency is high enough, the motor runs at a steady speed due to its fly- wheel momentum. By adjusting the duty cycle of the signal (modulating the width of the pulse, hence the 'PWM')  ie, the time fraction it is "on", the average power can be varied, and hence the motor speed. Advantages are, 1. The output transistor is either on or off, not partly on as with normal regulation, so  less power is wasted as heat and smaller heat-sinks can be used.  2. With a suitable circuit there is little voltage loss across the output transistor, so the  top end of the control range gets nearer to the supply voltage than linear regulator  circuits.  3. The full-power pulsing action will run fans at a much lower speed than an  equivalent steady voltage.  Disadvantages:
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1. Without adding extra circuitry, any fan speed signal is lost, as the fan electronics'  power supply is no longer continuous.  2. The 12V "kicks" may be audible if the fan is not well-mounted, especially at low  revs. A clicking or growling vibration at PWM frequency can be amplified by case  panels. A way of overcoming this by "blunting" the square-wave pulse is described  in  Application Note #58  from Telcom.  (a 58k pdf file, right-click to download) . I've  tried this, it works, but some of advantage #3 is lost.  3. Some authorities claim the pulsed power puts more stress on the fan bearings  and windings, shortening its life.  How It Works An oscillator is used to generate a triangle or sawtooth waveform (green line). At low  frequencies the motor speed tends to be jerky, at high frequencies the motor's inductance  becomes significant and power is lost. Frequencies of 30-200Hz are commonly used. A potentiometer is used to set a steady reference voltage (blue line).  A comparator compares the sawtooth voltage with the reference voltage. When the sawtooth  voltage rises above the reference voltage, a power transistor is switched on. As it falls below  the reference, it is switched off. This gives a square wave output to the fan motor. If the potentiometer is adjusted to give a high reference voltage  (raising the blue line) , the  sawtooth never reaches it, so output is zero. With a low reference, the comparator is always  on, giving full power.
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