Patent GB-1270629-A (GB)
Title: Electrothermal heater control
Inventor: HIRSBRUNNER HANS G
IPC: F23N5/20
Filed: 1970-06-25
Abstract: 1,270,629. Burner ignition and control apparatus. TEXAS INSTRUMENTS Inc. 25 June, 1970 [30 June, 1969], No. 30867/70. Heading F4T. [Also in Division H3] A burner ignition and control apparatus for a domestic heating system comprises a solenoid actuated gas valve 13, spark ignition means energizable when thermostat 15 demands heat, to cause fuel ignition, energization of the ignition means ceasing thereafter, a triggerable semiconductor current-switching device in the form of triac Q1 connected to energize the valve 13, triggering means operable to trigger the triac initially only if the ignition means generates sparking and thereafter for as long as the thermostat demands heat, a control thermistor TH1 connected to prevent triggering of the triac Q1 when the thermistor temperature is above a predetermined threshold temperature, and heating means in the form of thermistor H 1 operable to heat thermistor TH1 when the ignition means generates sparking, thermistor TH1 requiring a predetermined heating time interval to reach its threshold temperature whereby, if the gas is not ignited within this time, triggering of the triac Q1 is terminated to prevent further fuel flow to burner 11. As in Fig. 3 (not shown) a second heating thermistor (CH) may be provided to continuously heat thermistor TH1 to compensate for changes in ambient temperature. Also a second triac Q2 is provided to be triggered on demand from thermostat 15, positive temperature coefficient thermistors TH1, TH2, TH3 being connected to the gate such that if the ignition circuit 17 functions for too long a time without ignition, or the furnace overheats, or the forced-air draught supply to the burner becomes insufficient, respectively, the trigger to triac Q2 will be removed and the apparatus will shut down. The triacs Q1 and Q2 are latched after initial triggering by the reactances of inductive winding 13W and of capacitor C2 respectively. In normal operation the switch of thermostat 15 is closed and triac Q2 triggered into conduction to charge capacitors C3 and C4 and start heating thermistor H1, the charge on capacitor C4 eventually causing breakdown in neon bulb NE1 to trigger SCR Q3 to discharge capacitor C3 through winding W1 of transformer T1 and thus cause sparking at electrodes 19. The potential dropped across resistor R3 triggers triac Q1, and valve 13 is opened to supply gas to the burner for ignition; but if ignition does not occur within 4-10 sec., thermistor H1 heats thermistor TH1 above its threshold temperature to shut off triac Q2 and shut down the apparatus. On ignition however a conduction path is established between electrodes 19 and, via lead 21, capacitor C4 is prevented from charging, and thus the ignition circuit 17 shuts down to prevent thermistor H1 from further heating. After shut down caused by overheating of any of thermistors TH1, TH2, TH3, followed by subsequent cooling thereof, a new ignition sequence is automatically established. In a modification of the apparatus as in Fig. 2 (not shown) a further pair of thermistors (H4), (TH4) acts as a timer to purge the apparatus of unburnt gas before ignition, and the thermistor TH1 is positioned in the triggering circuit of triac Q1. Also in this modification the ignition circuit 17 is supplied with power at all times to provide for burning of residual gas after valve 13 has been shut, and a lock out, necessitating manual reset, occurs after unsuccessful ignition.