Electrical transmission or interconnection systems – Switching systems – Condition responsive
Reexamination Certificate
2002-06-20
2004-09-21
Tibbits, Pia (Department: 2838)
Electrical transmission or interconnection systems
Switching systems
Condition responsive
C431S018000, C431S042000
Reexamination Certificate
active
06794771
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to burner flame sense circuitry, and more particularly to electronic flame sense circuitry having multiple flame sense electrodes for sensing multiple burners.
BACKGROUND OF THE INVENTION
Advances in the sophistication and reliability of control electronics have long made their incorporation in consumer appliances desirable. However, only recently has the cost of such electronics been compatible with the extremely competitive marketplace for these appliances.
One such commercial and consumer market into which control electronics have now been widely incorporated is that for consumer and commercial cooking appliances such as ovens. The control electronics for such modern ovens provide programmable cooking cycles and control each aspect of the flame control system, primarily safety control. Many such modern ovens incorporate a gas distribution system (GDS) that includes an ignition module, solenoid valves, burners, and hot surface igniter or spark electrodes. The ignition module dispenses with the necessity of continually having a pilot flame burning in the appliance to reliably ignite the gas burners when called for by the thermostat. The electronically controlled solenoid valve controls the gas flow for each cooking cycle, and allows for proper purging and gas shutoff during fault conditions. Such gas distribution systems typically include an electronic flame sense circuit to sense when the burners are ignited. This flame sense is used to control the direct spark ignition of the gas and to sense failure or flameout conditions. These conditions may necessitate reactivating an ignition sequence in an attempt to relight the burners or shutting off of the gas solenoid valve to allow for oven cavity purging before re-ignition is attempted. Electronic flame sense circuits typically rely on a physical phenomena of flame known as current rectification within a flame. According to this principle, a flame will conduct electricity in one direction. As such, the flame may be modeled as a resistor diode combination that allows current flow only in a single direction therethrough. These circuits are, of course, designed such that they are fail safe. That is, the typical failure mode of these circuits is such to indicate to the electronic controller that no flame is sensed. In this way, the electronic controller will shut off the gas solenoid valve to the oven burners.
In typical consumer ovens, at least two burner elements are included within the oven cavity. Typically, a bottom burner is used during bake cycles, while an upper burner is used to allow broiling. In such applications, a need exists for flame sensing of both the upper and lower burners. While separate flame sense circuits could be utilized, such would serve to simply increase the cost of the sensing circuitry required by a factor of two. Indeed, in applications where multiple burners are used, the provision of multiple flame sense circuits increases the cost of the circuitry accordingly.
Recognizing that the two-burner configuration in a consumer oven allows operations of only one burner at a time, i.e., either baking or broiling, a single dual flame sense circuit integrating two flame sensors has been developed as illustrated in FIG.
1
. Under typical operating conditions, only one of the two flame sense electrodes
100
,
102
would be required to sense flame at any given point in time based on the alternate controlled operation of the bake and broil burners. The flame-sensing portion of this circuit is powered from the line voltage L
1
through a capacitor
104
. Each flame sense electrode
100
,
102
also includes a current limiting resistor
106
,
108
. A voltage divider network including resistor
110
, and the RC combination of resistor
112
and capacitor
114
is also included. The midpoint between this resistor
110
and the RC combination
112
,
114
is coupled through resistor
115
to the gate of a
116
of a junction field effect transistor (JFET)
118
, whose drain is coupled through resistor
120
to a 5 volt DC input and whose source
122
is coupled to ground.
With no flame present at either burner being sensed by sensing electrodes
100
,
102
, operation of the flame sense circuit of
FIG. 1
generates an output voltage level equal to the drain to source voltage which is sensed by the electronic controller (not shown) as a no flame condition. That is, current flow during the positive half cycle of source L
1
flows through capacitor
104
resistor
110
and the RC network
112
,
114
. This generates a positive gate source voltage VDS. With such a positive voltage at gate
116
, the JFET
118
remains in a conducting state allowing current flow therethrough. During the negative half cycle of source L
1
, current flows from ground through the RC network,
112
,
114
, through resistor
110
and capacitor
104
to the source L
1
. During this negative half cycle, the voltage developed at the gate
116
across the RC network
112
,
114
is negative. This negative voltage, however, is not sufficient to pinch off the JFET
118
to halt current flow therethrough. As a result, the JFET
118
will remain on, and the controller will continue to sense a very small voltage v
DS
.
If a flame is present at either burner as sensed by electrodes
100
,
102
, the flame sense circuit may be represented as illustrated in FIG.
2
. As may be seen from an analysis of this
FIG. 2
, a flame may be represented as a series combination of a resistor
124
and a diode
126
. As will be understood by those skilled in the art, the flame provides rectification whereby current flow is allowed only in a single direction therethrough. During this flame sense condition, current flow will be from source L
1
through capacitor
104
to a current divider network comprised of resistor
106
and flame (resistor
124
and diode
126
), and the voltage divider network of resistor
110
and RC network
112
,
114
. However, the resistor
106
is sized in relation to resistor
110
to allow a majority of the current flow from source L
1
during this positive half cycle through its branch of the circuit.
During the negative half cycle, however, the rectification action of the flame prevents any reverse current flow through resistor
106
of the circuit. Instead, all of the current flow during the negative half cycle flows from ground through the RC network
112
,
114
through resistor
110
and capacitor
104
to source L
1
. As a result of the unequal current flow through the RC network
112
,
114
during the positive and negative half cycles of source L
1
, an accumulation of negative charge is developed across capacitor
114
. This negative charge is coupled to gate
116
of JFET
118
, which pinches off the JFET
118
halting current flow therethrough. Because this negative charge is not drained away during the positive half cycle, the JFET
118
remains in an off condition during the entire period of flame presence. This will be sensed as a constant 5 voltage level by the electronic controller, which will be read as a flame present condition. As soon as the flame (resistor
124
and diode
126
) disappears, operation of the circuit will return to that illustrated and described above with reference to
FIG. 1
, allowing the JFET
118
to turn on and dropping the sensed voltage flow a high level (e.g. 5 v) to a low level (e.g. V
DS
).
While the circuit of
FIG. 1
provides a significant cost savings over the usage of two separate flame sense circuits, a passive failure at one of the flame sense electrodes may go undetected and result in a failure to sense flame when actually present. Such a condition is illustrated in FIG.
3
. If one of the flame sense electrodes
102
is shorted
128
to ground, the circuit will no longer sense flame at either of the flame sense electrodes
100
,
102
. When neither the oven nor the broiler is turned on, the circuit appears to operate normally with the JFET
118
remaining in its conducting mode allowing current to flow therethrough. As a result,
Leydig , Voit & Mayer, Ltd.
Ranco Incorporated of Delaware
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