RA201 guards against a failure of the +24 volt power supply. When +24 volts is applied at CN201-2 and CN102-A15 drops to LOW, RA201 is energized and its ac contacts stay open. If the +24 volts power from CN102-A20 stops, the ac contacts of RA201 close, stopping the drive circuit and turning off the fusing lamp. 8.3.2 Overheat Protection The overheat protection circuit on the main board turns off the fusing lamp if the temperature goes too high. The input from TH1 (Hot Roller Thermistor) goes to pin 3 of IC17 as well as to the CPU. As the temperature of TH1 increases, its resistance decreases and therefore the voltage at pin 3 of IC17 increases. If this voltage becomes greater than the voltage at pin 2, the output at pin 1 becomes HIGH. Q1 then turns off and cuts the +24 volt supply to ac drive board. The fusing lamp then turns off. The overheat protection circuit becomes effective at about 215°C. TF1 provides overheat protection in case a short bypasses the control and drive circuits. This fuse will open if it reaches approximately 192°C.
