Flame rectification circuit using operational amplifier

1. A flame sense circuit configured to receive a first voltage produced by an AC signal source and a flame rectification probe, the flame sense circuit comprising:

an operation amplifier having an inverting input adapted to receive the first voltage, a non-inverting input, and an output;

a first resistor (R 2 ) connected on a negative feedback loop between the output and the inverting input of the operational amplifier;

a second resistor (R 1 ), (R 1 ) being connected to the inverting input;

a microcontroller configured to detect a second voltage from the output of the operational amplifier to determine whether a flame is present at the flame rectification probe; and

a flame check reset circuit, the flame check reset circuit comprising:

a transistor having a base, a collector, and an emitter, wherein the collector is operably connected to the inverting input of the operational amplifier; and

a flame check reset output that provides a third voltage to the base of the transistor;

wherein a fourth voltage is supplied to the emitter;

wherein the transistor operates in the cut-off mode when the third voltage is higher than the fourth voltage and, while the transistor is in the cut-off mode, the second voltage of the operational amplifier that is output to the microcontroller depends on the presence and quality of the flame; and

wherein the transistor operates in the forward-active mode when the third voltage is less than the fourth voltage and, while the transistor is in the forward-active mode, the second voltage of the operational amplifier is compared to a predetermined reset check expected voltage in the microcontroller to determine if a component has failed.

2. The flame sense circuit of claim 1 , further comprising a flame check gain circuit, the flame check gain circuit comprising:

a flame check gain output that provides a fifth voltage to the non-inverting input of the operational amplifier while the transistor is in the forward-active mode;

wherein the second voltage of the operational amplifier is compared to an expected gain check voltage in the microcontroller to determine if a component has failed.

3. A method for detecting a flame, the method comprising the steps of:

providing alternating current to a flame rectification probe to produce a first voltage as an input for a sense circuit, wherein the flame rectification probe is placed in proximity to the flame;

conditioning the first voltage using the sense circuit to produce a second voltage;

outputting the second voltage to a microcontroller, and

determining with the microcontroller whether the flame is present based on a magnitude of the second voltage;

wherein the step of conditioning the first voltage is performed using an operational amplifier;

wherein the method further comprises the step of performing a component fault check to determine if any of the components of the sense circuit have failed;

wherein the sense circuit comprises an operational amplifier and wherein step of performing a component fault check comprises:

providing a flame check reset circuit; and

providing a flame check gain circuit;

wherein the flame check reset circuit is operably connected to an inverting input of the operational amplifier and wherein the flame check gain circuit is operably connected to a non-inverting input of the operational amplifier.

4. The method of claim 3 , further comprising the steps of:

sending a first signal from the flame check reset circuit such that the inverting input of the operational amplifier senses a positive voltage; and

outputting a zero voltage from the sense circuit to the microcontroller,

wherein them microcontroller detects a component fault if an output voltage other than zero is provided by the sense circuit.

5. The method of claim 4 , further comprising, the steps of:

sending a first signal from the flame check reset circuit such that the inverting input of the operational amplifier senses a positive voltage;

sending a second signal from the flame check gain circuit such that the non-inverting input of the operational amplifier senses an input voltage; and

outputting a differential voltage from the sense circuit to the microcontroller,

wherein the microcontroller detects a component fault if the differential voltage does not match an expected differential voltage.

6. The method of claim 5 , wherein the step of providing a flame check reset circuit further comprises providing a flame check reset circuit including a PNP transistor.

7. The method of claim 6 , further comprising the steps of:

connecting an emitter of the PNP transistor to a circuit rail voltage;

connecting a base of the PNP transistor to a reset input, the reset input capable of providing a reset signal; and

connecting a collector of the PNP transistor to the inverting input of the operational amplifier.

8. The method of claim 7 , wherein during the step of determining whether a flame is present, the reset signal is provided to the base of the PNP transistor such that the PNP transistor enters a cutoff mode.