Power Supply/RF Board
4-30 Force FX Electrosurgical Generator C Service Manual
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.
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