Flame Sense Circuit for Gas Pilot Control

Abstract

A flame sense circuit for a gas burner control employs a flame rod for flame rectification. An AC source applies an AC wave through a capacitor to the flame rod, and the flame rod is also connected via a resistive voltage divider to a DC voltage source. A voltage divider node is connected to a voltage follower, which impedance-matches an ADC input of a microprocessor controller. The flame sense circuit senses conditions of no-flame, and flame rod in contact with or leaking to ground, and when flame is present can quantify sensed flame as good, weak and marginal.

1 . A flame sense circuit arrangement for detecting and quantifying a sensed flame in a burner of a fuel-fired appliance, comprising:
 a gas burner having a chassis in electrical contact with a point of reference voltage;   a source of AC voltage connected between said point of reference voltage and one electrode of a capacitor, the capacitor having a second electrode coupled to a first electrical node A;   a resistive voltage divider arrangement having a first resistance connected between said first node A and a second node B, and a second resistance connected between said second node B and a source of DC voltage;   a flame rod having a conductor connected with said first node A and having one end thereof positioned to be in contact with the flame in said burner when flame is present;   a voltage follower amplifier having a high impedance input coupled to said second node B and capable of receiving a net DC voltage component, and having a low impedance output;   a processor arrangement coupled to said low impedance output and capable of providing indications corresponding to respective flame conditions including flame absence, flame rod in contact with ground, and a plurality of sensed flame quantity levels; and   an indicator device coupled to said processor arrangement and adapted to provide perceptible indicia corresponding to said conditions of flame absence, flame rod in contact with ground, and said plurality of flame quality levels.   
     
     
         2 . The flame sense circuit arrangement according to  claim 1 , comprising a low-value protective resistor in series between said flame rod and said capacitor. 
     
     
         3 . The flame sense circuit arrangement according to  claim 1 , wherein said first resistance has a value on the order of about 20 megohms to 50 megohms, and said second resistance has a value on the order of about 10 megohms. 
     
     
         4 . The flame sense circuit arrangement according to  claim 1 , wherein said capacitor has a capacitance value on the order of about 2 nf. 
     
     
         5 . The flame sense circuit arrangement according to  claim 1 , wherein said voltage follower amplifier is formed of an operational amplifier having a (+) input terminal coupled to said second node B, a (−) input terminal, and an output terminal that is coupled to said (−) input terminal and to an input of said processor arrangement. 
     
     
         6 . The flame sense circuit arrangement according to  claim 5 , said voltage follower amplifier having an output impedance on the order of 10 Kilohms. 
     
     
         7 . The flame sense circuit arrangement according to  claim 5 , comprising a second capacitor coupled between the second node B and said point of reference voltage and adapted to filter net DC voltage to the (+) input terminal of said operational amplifier. 
     
     
         8 . The flame sense circuit arrangement according to  claim 1 , wherein said processor arrangement includes an analog-to-digital converter having an input coupled to said indicator device. 
     
     
         9 . The flame sense circuit arrangement according to  claim 8 , wherein said microprocessor is programmed to give a flame-absent indication when the voltage value is at a first predetermined voltage above a reference level, a grounded flame rod indication when said voltage value is at another predetermined voltage below said first predetermined voltage, and to provide one or more flame present indications when said voltage value is between said reference level and said second predetermined voltage. 
     
     
         10 . The flame sense circuit arrangement according to  claim 9 , wherein said microprocessor is programmed to provide a plurality of sensed flame quantity indications when said voltage value is within each of a plurality of zones between said second predetermined voltage and said reference level. 
     
     
         11 . The flame sense circuit arrangement according to  claim 1 , wherein said flame rod is a pilot flame igniter electrode, and said circuit arrangement further includes a spark coil secondary connected in series with said flame rod and said first node A, and a threshold breakdown switch device connected between said first node A and a chassis ground. 
     
     
         12 . The flame sense circuit arrangement according to  claim 10 , wherein said indicator device includes one or more LEDs which provide coded visual signals corresponding respectively to each of said conditions.