IMPROVEMENTS IN OR RELATING TO CONTROLS FOR ELECTRICAL HEATING DEVICES

摘要

1293961 Control of temperature, electrically NORTHERN ELECTRIC CO 13 Jan 1970 [12 Feb 1969] 1614/70 Heading G3R A heating element R L (e.g. electric blanket element) is supplied with half wave D.C. via an SCR 1 (operated in a zero voltage switching mode) from an A.C. source 10, 11. The element also acts as a temperature sensor and is connected in abridge circuit comprising R 2 , R 3 , R 4 , R 5 , R 6 and R L supplied with half wave D.C. via a diode CR1 and having the base/emitter circuit of a gate signal control transistor Q 1 across its output diagonal. The desired temperature is pre-set by sliders 14, 17. In operation, when R L is cool, transistors Q 1 , Q 2 are non-conducting and a gate signal is applied to SCR 1 via C 1 , R 8 , CR 4 . The capacitor C 1 advances the phase of the trigger signal (e.g. by 80 degrees) with respect to the A.C. source so that SCR 1 fires substantially on the zero going positive crossing of the A.C. supply. As R L becomes hotter its resistance increases and the resulting bridge output causes Q 1 to conduct which in turn causes Q 2 to conduct thereby reducing the gate voltage at SCR 1 and preventing firing. Current supply to R L is thus interrupted. A diode CR 5 across R L causes a fuse F 1 to blow if SCR 1 should develop a short circuit. Since diode CR 2 in the same arm of the bridge as R L , is ambient temperature sensitive, a diode CR 6 is provided in another arm for temperature compensation. CR 2 and CR 4 prevent the load voltage from affecting the control circuit when SCR 1 is conducting. Diode CR 3 and R 7 pass a signal in phase with the A.C. voltage which signal holds Q 2 "ON" when this voltage goes positive and prevents the SCR from firing when the A.C. voltage is greater than 10 volts.