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.

Description

BACKGROUND OF THE INVENTION[0001]This present invention relates to control devices for controlling gas fired appliances, such as furnaces and water heaters, and is more particularly concerned with a flame-sense arrangement that operates according to the principle of flame rectification.[0002]The present invention is also concerned with a gas appliance control that employs an intermittent ignition system for the pilot light flame of the appliance in which the igniter or spark feature can also serve as a flame rod of the flame-sensor arrangement.[0003]Flame rectification refers to a method of flame detection, using the property of a flame in which a plasma or ionized region within the flame serves as a unidirectional conductor, so that a current can flow from an interior of the flame towards the outside of the flame. Typically, a flame rod or conductive probe penetrates the flame envelope, and serves as anode, while a grounded pilot gas jet on the burner chassis serves as cathode. The...

AI Technical Summary

Benefits of technology

[0009]Accordingly, it is an object of the present invention to provide a flame rectification flame-sense circuit that overcomes the above-mentioned drawback(s) of the prior art, and that is capable of providing outputs corresponding to flame presence, flame absence, flame rod grounding to chassis, and flame strength or quality.

[0010]It is another object to provide a flame rectification flame sense circuit that achieves these objects using a simple and straightforward circuit arrangement, and which does not require a large number of components to do so.

[0011]In accordance with one aspect of the present invention, A flame sense circuit arrangement is provided for detecting and evaluating quality of a flame, e.g., the pilot burner flame, in a burner of a fuel-fired appliance, in which the gas burner has a chassis in electrical contact with a point of reference voltage, i.e., which can be considered “ground” or zero volts. A source of AC voltage is connected between such point of reference voltage (e.g., ground) and one electrode of a capacitor, the capacitor having a second electrode coupled to a first electrical node, i.e., node “A” in FIG. 1. A resistive voltage divider arrangement having a first resistance is connected between said first node “A” and a second node “B”. A second resistance is connected between the second node “B” and a source of DC voltage, e.g., +5 VDC. The flame rod has a conductor connected with the first node “A” and has one end thereof positioned to be in contact with the flame in the burner when flame is present. A voltage follower amplifier has a high impedance input coupled to the second node and is capable of receiving a net DC voltage component. The amplifier has a low impedance output to supply an output voltage level to the input of the processor arrangement from the low impedance output. The processor arrangement is capable of providing indications corresponding to respective flame conditions including flame absence, flame rod in contact with ground, and a plurality of flame quality levels. These can be different series of coded pulses, each of which corresponds to a flame condition, and the technician or installer can look those up in an installation guide. An indicator device (e.g., an LED or a cluster of LEDs) is or are coupled to the processor arrangement and can be adapted to provide perceptible indicia corresponding to said conditions of flame absence, flame rod in contact with ground, and the plurality of flame quality levels. A low-value protective resistor in series between the flame rod and the capacitor serves to prevent excessive currents in the event of a grounded flame rod. This can be a resistance of about 10 kilohms, and can be between the capacitor and the first node, or between the first node and the flame rod.

Problems solved by technology

A voltage that is within one given voltage range or another can mean the presence of flame at the pilot gas burner, or can also mean that the flame is of marginal, poor, or good quality.

A voltage that is outside that range, i.e., above it at a first or second high voltage level, can mean that flame is absent or that there is an electrical problem with the flame rod.

Up until now, these arrangements could only detect the presence or absence of flame, but have not been able to quantify or evaluate the condition of flame when it is present, even though that information would be useful during control installation or for troubleshooting.

Examples of flame quality issues that would be useful, if they could be distinguished and detected, include a coated or contaminated flame rod; small pilot flame; or a waving pilot flame (i.e., not enveloping the flame rod), each of which can result in a weak flame current.

Other installation or trouble shooting conditions include grounding due to mis-wiring during installation, ceramic insulator body breakdown, i.e., the insulator holding the igniter / flame rod becoming cracked or contaminated with the rod then coming into electrical contact with the chassis.

Other quality issues on installation can occur if there is lead wire damage due to exposure to excessive temperatures, or contamination forming a conductive link to the chassis, i.e., build-up or coating on the ceramic body resulting in leakage current leakage from flame rod to ground.

To date, this has not been achieved.

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