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Chapter 5 Theory of Operation
On-Board Power Supplies
4
5
The variable-speed fan is driven by a temperature-controlled switching
regulator which is powered by +12V_ER. Comparator U2002-A is
configured as an oscillator whose output (at C2013) is a triangle wave.
Thermistor R2016 senses the incoming air temperature and U2001-A
converts it to a voltage. U2002-B compares this voltage to the triangle
wave and outputs a square wave whose duty cycle varies with temperature.
The square wave is buffered by U104-C and Q2001 and then filtered by
L2003 and C2004 to create a dc voltage that varies with the temperature
and is used to power the fan. Below 30 °C, the fan voltage is set to
approximately 7 volts by R2003 and R2015 (since CR2004 is reverse-
biased). Above 50 °C, U2001-A’s output voltage is below the minimum
voltage of the triangle wave, keeping Q2001 on constantly and applying
full voltage to the fan.
The PWR_FAIL* line is provided by the main supply to indicate brown-
out or sagging line input condition. The microprocessor uses this line to
initiate saving the current state of the instrument in non-volatile memory.
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