Programmable

A programmable fan allows you to control the speed of the fan to optimise cooling performance. This option can create new ideas in thermal management by developing simple circuits that can either pulse width modulate, vary the voltage, or vary the resistance. The amount of flexibility this option can provide is extensive.

Because brushless DC fans rely on an electronic circuit to rapidly switch the magnetic fields, circuitry can be added to enhance the fans ability and add additional value to your system. The programmable option uses an internal voltage regulator to set the speed of the fan by means of a third lead wire tied to the adjust leg of the regulator. See Figure 2. The speed can be controlled two ways - controlling the voltage to the adjust lead or controlling the resistance between the adjust lead and the return. Either pulse width modulation or a variable voltage/resistance can accomplish either method.

For pulse width modulation (PWM), The pulse signal should be applied to the program lead (yellow wire) and referenced to the return lead. For PWM, by means of applying voltage on and off, the amplitude should be equal to the nominal voltage of the fan, the frequency should be held constant (20kHz recommended) and the duty cycle allowed to vary between 0 and 100%. The voltage must be current limited so that there is no more than 5mA to the program lead. For PWM by means of controlling resistance, the program lead should by cycled between an open and a short to the circuit return, with the frequency held constant. Controlling the speed by varying the resistance or voltage is accomplished the same way as PWM. The difference is that the voltage/resistance is varied linearly instead of varying the duty cycle. Applying a direct voltage to the adjust leg will achieve the same results as varying the resistance. As you will notice Figure 2, the resistors tied to the adjust leg creates a voltage divider. By varying the external resistor, a voltage divider supplies the adjust leg with a varying voltage.

Each fan is different in the range of input voltage to fan speed. The rule of thumb is a zero voltage input will result in a minimum speed condition and an input equal to 50% of the fans nominal voltage will result in a maximum speed condition. For instance, a 24vdc fan will operate at full speed when 12vdc is applied to the program lead.

Unfortunately, the voltage range is preset. If your requirements call for linear change from 0 to 10 volts, an op-amp can be used to customise the programmability input. This option is not available on all models and minimums do apply. For controlling the speed by resistance, a closed circuit between the program lead and the return will result the fan operating at half speed. An open circuit will result in a full speed condition. Different resistance values will yield different fan speeds.

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