Temperature compensated timing circuit for use in heating...

Details Temperature compensated timing circuit for use in heating... Temperature compensated timing circuit for use in heating...

1999-05-07

2001-03-06

Paschall, Mark (Department: 3742)

Electric heating

Heating devices

With power supply and voltage or current regulation or...

C219S497000, C219S499000, C219S505000, C099S3290RT, C099S328000

Reexamination Certificate

active

06198077

ABSTRACT:

TECHNICAL FIELD
The present invention relates generally to heating appliances such as toasters or toaster ovens, and more specifically to a temperature compensated timing circuit that controls the duration of a heating cycle of the appliance to compensate for the increasing internal temperature of the appliance on successive heating cycles.
BACKGROUND OF THE INVENTION
In heating appliances, such as toasters and toaster ovens, a food item is placed in a bread cavity of the appliance and toasted for a desired time, which is known as the heating cycle of the appliance. The duration of the heating cycle determines the extent to which the food item is cooked or toasted. For example, in a conventional toaster, the time required to toast successive food items to the same extent decreases for each successive heating cycle due to the bread cavity transmitting a certain amount of heat to the food item once the bread cavity has been warmed during previous heating cycles. In other words, the already warm bread cavity transmits a certain amount of heat to the food item in addition to the heat generated by the appliance during the heating cycle, resulting in less time being required to toast the food item. As a result, if the duration of the heating cycle is constant, food items placed in the bread cavity during subsequent heating cycles will be toasted more than those in previous heating cycles due to the additional heat transmitted from the bread cavity. To compensate for heat transmitted from the bread cavity to the food item during the second and subsequent heating cycles, conventional toasters include a timing circuit that compensates for this heat by reducing the duration of successive heating cycles.
FIG. 1
is a schematic of a conventional timer
10
that compensates for heating of the bread cavity by reducing the duration of successive heating cycles, as will now be described in more detail. In the timer
10
, an external force is applied to close a switch
12
thereby applying an AC voltage from a voltage generator
14
to an input node
16
of the timer
10
. An external lever (not shown) of the toaster containing the timer
10
is typically pushed down to apply the external force to close the switch
12
and to lower a food item into the bread cavity. The external lever is typically maintained in a down position by a mechanical latching mechanism (not shown) thereby maintaining the switch
12
closed. A coil
23
generates an electromagnetic force when energized to release the mechanical latching mechanism, thereby allowing the food item to be raised from the bread cavity and allowing the switch
12
to open, as indicated by the dotted line
25
, as will be described in more detail below. However, when the switch
12
is closed, the AC voltage from the voltage generator
14
on the node
16
is rectified by a diode
18
, and the magnitude of this rectified voltage is reduced by a voltage divider formed by series-connected resistors
20
and
22
.
A capacitor
26
is coupled to a node
24
defined between the resistors
20
and
22
, and filters the rectified voltage to provide approximately a DC supply voltage on the node
24
. As explained below, a timing circuit
28
receives the supply voltage on node
24
and generates a first trigger signal V
t1
, on a node
29
a delayed time after the switch
12
is closed to apply the supply voltage to the circuit
28
. The timing circuit
28
includes a resistor
30
and a variable resistor
32
connected in parallel with a resistor
34
and a thermistor
36
. The thermistor
36
presents a resistance having a value that is a function of the temperature of the thermistor, as understood by those skilled in the art. The thermistor
36
has a negative temperature coefficient so that as the temperature of the thermistor increases, the value of the resistance presented by the thermistor decreases. Typically, the thermistor
36
is mounted near the bread cavity, of the toaster and thus presents a resistance having a value that is a function of the temperature within the bread cavity. The resistor
34
and thermistor
36
in parallel with the resistor
30
and the variable resistor
32
present an equivalent resistance R
T
between the node
24
and a capacitor
38
coupled between the node
29
and ground. The capacitor
38
and equivalent resistance R
T
together form an RC circuit with the voltage across the capacitor
38
having a value that varies as a function of time. The time dependence of the voltage across the capacitor
38
is determined by the values of the equivalent resistance R
T
presented by the resistors
30
-
34
and thermistor
36
and the capacitor
38
, as well understood by those skilled in the art. In operation of the timing circuit
28
, the voltage on the node
24
is applied through the equivalent resistance R
T
to charge the capacitor
38
and thereby develop first trigger signal V
t1
. The rate at which the capacitor
38
charges and thus the rate at which the magnitude of the first threshold signal V
t1
increases is a function of the resistance presented by resistors
30
-
34
and thermistor
36
, as previously described. A diode
52
and resistor
54
discharge the capacitor
38
when switch
12
is open.
A diac
40
receives the first trigger signal V
t1
on a first terminal and has a second terminal coupled through series connected resistors
42
and
44
to ground. When the first trigger signal V
t1
has a magnitude less than a predetermined breakdown voltage, the diac
40
presents a high impedance and no current flows through the diac. When the first trigger signal V
t1
exceeds the breakdown voltage, the diac
40
turns ON and current flows from the node
29
through the diac
40
and series-connected resistors
42
and
44
. The resistors
42
and
44
operate as a voltage divider, with the voltage across the resistor
44
being applied as a second trigger signal V
t2
to a silicon controlled rectifier (SCR)
46
, which is connected in series with the coil
23
and a resistor
50
. When the second trigger signal V
t2
exceeds a second breakdown voltage, the SCR
46
turns ON causing current to flow from the node
24
through the resistor
50
and coil
23
, thereby energizing the coil. The resistor
50
reduces the magnitude of the voltage applied across the coil
23
when the SCR
46
is turned ON. As mentioned above, energizing the coil
23
releases a mechanical latching mechanism (not shown) to allow the switch
12
to open and the food article to be raised from the bread cavity.
The overall operation of the timer
10
during a heating cycle of a conventional appliance containing the timer will now be described in more detail. Initially, assume the switch
12
is open, isolating the voltage generator
14
from the node
16
. To initiate a heating cycle, an external force is applied to close the switch
12
thereby applying the voltage from the generator
14
to the input node
16
. When the voltage from the generator
14
is applied on the input node
16
, the diode
18
rectifies this voltage and the supply voltage on node
24
is developed, as previously described. In response to the voltage on the node
24
, the capacitor
38
begins charging at a rate determined by the value of the equivalent resistance R
T
presented by resistors
30
-
34
and thermistor
36
. The variable resistor
32
is adjusted in relation to a “toast darkness” scale to control the duration of the heating cycle. As previously described, the thermistor
36
has a negative temperature coefficient so that as the temperature in the bread cavity increases the value of the resistance presented by the thermistor
36
decreases. Thus, as the temperature of the bread cavity increases, the equivalent resistance R
T
presented by the resistors
30
-
34
and the thermistor
36
decreases, causing the capacitor
38
to charge at a faster rate. The voltage across the capacitor
38
corresponds to the first trigger signal V
t1
, and as the capacitor
38
charges the magnitude of the first threshold voltage V
t1
increases at a rate determ

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