Efficient Fan Speed Controller

Posted on Feb 6, 2014

A partial solution to quietening noisy PCs can be to reduce the speed of internal cooling fans. Low-cost fan speed controllers are available, but they often employ inefficient, heat-generating linear regulators and contain no temperature feedback mechanism. This idea makes use of a readily available, cheap in-car mobile phone charger. The majority

Efficient Fan Speed Controller
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of these use common circuitry and require only minor modifications to operate as efficient fan speed controllers complete with temperature feedback. Most in-car chargers are based on the well-known MC34063 DC-DC switchmode IC. When used for charging mobile phones, the open-circuit output voltage is typically set to between 7V and 9V. This is achieved with a simple voltage divider across the output, the centre point of which connects to the feedback input (pin 5) of the MC34063. To make the output voltage var-iable with air temperature, first replace the upper resistor of the divider with a 4. 7k © resistor in series with a 4. 7k © trimpot. The lower half of the divider is then replaced with a 470 © resistor in series with a 500 © NTC thermistor. These values are only a guide and can be varied to suit different thermistor and fan types. Note that component lead length should be minimised to avoid introducing noise into the feedback circuitry. Getting the correct fan starting voltage is a matter of trial and error. The values shown on the circuit give a starting voltage of about 6. 8V at room temperature but trimpot VR1 can be used to raise this voltage as necessary. The output can then rise to about 10V if the interior temperature rises sufficiently. The 4. 7k © resistor could be reduced to 3. 9k © and VR1 adjusted to give a lower starting voltage if the fan speed is still too high at 7V. After running for one hour or so,...

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