Valleylab Force FX-8C, Service Manual

Page: 83 / 218

Power Supply/RF Board
Principles of Operation
Force FX-8C Service Manual 4-27
Thermal Sensing (Fan Control)
A reference voltage is applied to the noninverting input of comparator
U2A. Resistors R15 and R16 determine the reference voltage. When the
temperature of NTC thermistor R19 is 40° C, the reference voltage is set to
be equal to the voltage at the inverting input of U2A. Resistor R10
provides positive feedback causing approximately 5° C of hysteresis.
At temperatures below 40° C, the output of U2A is low. When the
thermistor exceeds the threshold, the voltage comparator changes state
causing the output at U2A to go high, turning on FET Q2. This applies
12 V to the cooling fan. When the temperature drops to 35° C, the circuit
reverts to its low temperature state, and the cooling fan switches off.
Thermal Sensing (High Temperature Limit)
A reference voltage is applied to the non-inverting input of comparator
U26A. Resistors R111 and R110 determine the reference voltage. When the
temperature of NTC thermistor R109 is 50° C, the reference voltage is set
to be equal to the voltage at the inverting input of U26A. Resistor R94
provides positive feedback causing approximately 10° C of hysteresis.
When the thermistor exceeds the threshold, the voltage comparator
changes state, causing the output at U26A to go high. The main
microcontroller reads this signal (TEMP_HI) and flashes number 451
alternately with the power settings. When the temperature drops to 40° C,
the circuit reverts to its low temperature state, and the generator allows
activation for an unlimited duration.