Burner flame detection circuit

Claim 2

In a burner control system, an a.c. power source, a burner ignition means, a biased closed solenoid fuel valve having a winding, a normally closed bimetal safety switch, a space thermostat, and a relay having a winding and normally open contacts; circuit connections connecting said solenoid winding in series with said safety switch across said power source through said relay contacts and said thermostat when they are closed, and circuits connections connecting said solenoid winding and said safety switch across said power source through said thermostat independent of said relay contacts; a resistance heater operative when energized to heat said bimEtal safety switch and open it in a predetermined short period of time, and unidirectional, signalcontrolled, solid state switching means; circuit connections connecting said relay winding, said resistance heater, and said solid state switching means across said power source; gating means operative to apply a firing signal to said solid state switching means thereby to effect half-wave energization of said relay winding and said resistance heater; and means including transistor amplifying means responsive to burner flame conduction to shunt said SCR gating means when combustion occurs.

Claim 3

The system claimed in claim 1 in which said circuit connections connecting said solenoid winding and said safety switch across said power source independently of said relay contacts includes a current limiting resistor, which current limiting resistor has a value which permits sufficient energization of said solenoid winding to hold said solenoid valve open but prevents sufficient energization thereof to pull said valve open against its closing bias.

Claim 4

A burner control system operating on an a. c. power source comprising; a burner, a spark igniter, a normally closed solenoid valve having a winding, a current limiting resistor, a normally closed bimetal safety switch, a space thermostat, and a relay having a winding and normally open contacts; circuit connections connecting said spark igniter in series with said safety switch across said power source through said relay contacts and said thermostat when they are closed, circuit connections connecting said solenoid winding in parallel with said spark igniter and in series with said safety switch across said power source through said relay contacts and said thermostat when they are closed, and circuit connections connecting said solenoid winding, said current limiting resistor, and said safety switch across said power source and through said thermostat independently of said relay contacts; a resistance heater operative when energized for a predetermined short period to open said safety switch, and signal controlled solid state switching means; circuit connections connecting said relay winding, said switching means, and said resistance heater across said power source through said thermostat, gating means operative to apply a firing signal to said switching means to effect conduction thereof, and controlled solid state switching means responsive to signal conduction through burner flame, when it appears, to shunt said gating means, whereby energization of said resistance heater and said igniter is cut off substantially instantaneously upon the appearance of burner flame and whereby said igniter is rendered operative substantially instantaneously upon a subsequent loss of burner flame so as to be operative during the predetermined period required to open said safety switch following loss of flame.

Claim 5

A burner control system operating on an a. c. power source comprising; a burner, a spark igniter, a normally closed solenoid valve having a winding, a current limiting resistor, a normally closed bimetal safety switch, a space thermostat, and a relay having a winding and normally open contacts; circuit connections connecting said spark igniter in series with said safety switch across said power source through said relay contacts and said thermostat when they are closed, circuit connections connecting said solenoid winding in parallel with said spark igniter and in series with said safety switch across said power source through said relay contacts and said thermostat when they are closed, and circuit connections connecting said solenoid winding, said current limiting resistor, and said safety switch across said power source and through said thermostat independently of said relay contacts; a resistance heater operative when energized for a predetermined short period to open said safety switch, and signal controlled solid state switching means; circuit connections connecting said relay winding, said switching means, and said resistance heater across said power source through said thermostat, gating means operative to apply a firing signal to said switching means to effect conduction thereof, and controlled solid state switching means responsive to signal conduction through burner flame, when it appears, to shunt said gating means, whereby energization of said resistance heater and said igniter is cut off substantially instantaneously upon the appearance of burner flame and whereby said igniter is rendered operative substantially instantaneously upon a subsequent loss of burner flame so as to be operative during the predetermined period required to open said safety switch following loss of flame.

Claim 6

In a burner control system operating on an a.c. power source, a burner, electrically operated fuel supply means, ignition means, a space thermostat, and a normally closed thermal time switch; first circuit means controlled by said thermostat series connecting said fuel supply means and said thermal time switch across said power source; a resistance heater operative when energized for a short period to open said thermal time switch, and an SCR; second circuit means controlled by said thermostat series connecting said SCR and said resistance heater across said power source, gating means for said SCR including relatively low resistance means connecting the control electrode of said SCR with the anode side thereof; PNP transistor means connected between said SCR control electrode and the cathode side thereof and being so arranged and operative when conducting to shunt said SCR gating means and cut off conduction; means forward biasing said transistor means comprising relatively high resistance means connecting the base of said transistor means to the anode side of said SCR and a capacitor connected between said base and the cathode sIde of said SCR; said forward biasing means including diode means of SCR polarity in series with said high resistance means and operative to effect the full charging of said capacitor during the conductive half cycles thereof thereby to reverse bias and cut off conduction of said transistor means; and circuit means including an air gap arranged to be bridged by burner flame connecting said transistor base to ground thereby to provide a discharge path for said capacitor during the non-conductive half cycles of said diode when burner flame is present.

Claim 7

In a burner control system operating on an a.c. power source, a burner, electrically operated fuel supply means, ignition means, a space thermostat, and a normally closed thermal time switch; first circuit means controlled by said thermostat series connecting said fuel supply means and said thermal time switch across said power source; a resistance heater operative when energized for a short period to open said thermal time switch, and an SCR; second circuit means controlled by said thermostat series connecting said SCR and said resistance heater across said power source, gating means for said SCR including relatively low resistance means connecting the control electrode of said SCR with the anode side thereof; PNP transistor means connected between said SCR control electrode and the cathode side thereof and being so arranged and operative when conducting to shunt said SCR gating means and cut off conduction; means forward biasing said transistor means comprising relatively high resistance means connecting the base of said transistor means to the anode side of said SCR and a capacitor connected between said base and the cathode sIde of said SCR; said forward biasing means including diode means of SCR polarity in series with said high resistance means and operative to effect the full charging of said capacitor during the conductive half cycles thereof thereby to reverse bias and cut off conduction of said transistor means; and circuit means including an air gap arranged to be bridged by burner flame connecting said transistor base to ground thereby to provide a discharge path for said capacitor during the non-conductive half cycles of said diode when burner flame is present.

Claim 8

The control system claimed in claim 4 which further includes relay means operative to connect said ignition means across said power source when said SCR is conducting in the absence of burner flame.

Claim 9

The control system claimed in claim 4 in which the relative values of said capacitor and said high resistance means are such as to require a plurality of conductive half cycles to fully charge said capacitor.

Claim 10

The control system claimed in claim 4 which includes relay means in said second circuit means operative to complete said first circuit means when said SCR is conducting.

Claim 11

The control system claimed in claim 4 which includes a small capacitor connected between said SCR control electrode and the cathode side thereof operative to prevent the application of a firing pulse to said SCR upon the closure or the opening of said thermostat.