Temperature control device for fixing heat source of copying machine

Claim 1

A temperature control device for a fixing heat source of a copy machine including: an a.c. power source (2); a heat source (4) generating heat by being energized by power supplied from said a.c. power source thereby fixing an image transferred onto a copying sheet; at least one temperature sensor (14) for sensing the temperature of said heat source; and control means (20) for controlling the power supplied from said a.c. power source to said heat source in response to the temperature-indicative signal applied from said temperature sensor, said control means (20) comprising: a trigger pulse generating circuit (22, 24) detecting the zero-cross point of the a.c. waveform supplied from said a.c. power source thereby generating a trigger pulse signal in synchronism with the zero-cross point; a power supply circuit (6) permitting supply of the power from said a.c. power source to said heat source in response to said trigger pulse signal; and a control signal generating circuit .(28, 30, 32, 64) generating a control signal for controlling said trigger pulse generating circuit, said control signal generating circuit functioning to supply the full-wave current to said heat source (4) until a first predetermined temperature (Tl) is sensed by said temperature sensor (14), to supply the half-wave current to said heat source until a second predetermined temperature (T2) is then sensed by said temperature sensor, and to suitably change the duty factor (Dl, D2) of said half-wave current for maintaining constant the temperature of said heat source.