The tachometer output of the fan is an open collector, so to electrically connect the
tachometer output to T1G will require a pull up to 5.0V. Connecting the fan speed
control to the PWM will require a similar interface; the PWM output is connected to a
ZVN2110 N channel MOSFET to create an open collector output, and then a pull up to 12V
provides the high side output for the fan.
Note:
Because the MOSFET will invert the output of the PWM peripheral, the 0-99.9% duty
cycle range of the PWM will be converted into a 100-0.1% duty cycle. The
firmware must account for this to provide the correct control range for the
fan.
To implement the system for this example, a prototype Click board was
modified with the MOSFET output transistor and the two pull-up resistors. The schematic
of the board is shown below.Figure 2-2. Prototype Click Board with Fan
Interface
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