Kyocera FS-1050 Service Manual - Page 81

Heater lamp control circuit Activation of the heater lamp is dominated by the HEAT signal which is derived by the engine CPU (U01) at its pin #1. When its level is high, transistor QA03-a turns on, photo-triac PC2 and triac TRC1 turn on simultaneously, and the heater lamp is applied with the primary AC voltage in turn. Switching of triac TRC1, as affected by the HEAT signal is made in synchronization with the zerocross signal ZCROSS which is generated by the power supply unit. The zero-cross signal detector watches the transition of alternating plus and negative current and detects the zero crosses. This detector derives the resultant ZCROSS signal at its pin #43 of the engine CPU (U01). Since abrupt change in the current flow can be significantly avoided by synchronizing triac TRC1 with the zerocross signal, the possibility of noise due to the primary AC supply is greatly reduced. 33 THERMA P10/ANI0 CPU U01 GND P50/A8 1 HEAT P00/INTP0 43 ZCROSS Engine board Heater lamp control circuit +5V R52 R56 +5V U04-1 2− 3 +393 1 +5V Q11 QA03-b GND GND QA03-a GND AC input Thermal cutout Heat roller Heater lamp Thermistor Power supply unit Zero cross signal detection +24 V circuit Triac (TRC1) (PC2) Photo-triac Bias board High voltage board FS-1050 Figure 4-3-5 heater lamp control circuit 4-22