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Патент USA US3070688

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Dec. 25, 1962
Filed July 24, 1959
2 Sheets-Sheet 1
Dec. 25, 1962
Filed July 24, 1959
2 Sheets-Sheet 2
7/715 M/ f/awrs
United grates Fatent O "”ice
Patented Dec. 25, 1962
John R. Gomez-sail, Elgin, lit, assignor to MeGraw-Edi
son Company, Eigin, ill-1., a corporation of Delaware
Filed .luly 24, 19%, Ser. No. 829,322
5 ‘Chums. (4C5. MiG-122)
tion of one embodiment of the invention, taken in connec
tion with the accompanying drawing wherein:
FIGURE 1 is a perspective view of a complete thermal
timer embodying my invention;
FIG. 2 is a sectional elevation taken substantially along
the plane indicated as 2—2 in FIGS. 1 and 3;
PEG. 3 is a sectional elevation taken substantially along
the line 3-3 in FIG. 2;
The present invention relates to thermal timers.
FIG. 4 is an exploded perspective view of some of the
A commonly-used type of thermal timer includes a strip
of thermostatic bimetal and an electric heater for it oper 10 parts of the device of FIGS. 1 to 3;
FIG. 5 is a partial view similar to FIG. 2 showing the
ating at a constant rate of heat input. Electric power
device in a di?erent operated position, and
may be applied to the heater during the timing period
H63. 6 and 7 are graphs depicting certain operations
for heating the bimetal, and the bimetal, when it has de
of a device embodying my invention.
?ected to a pre-selected position in response to tempera
The device shown in FIGS. 1 to 5 includes two body
ture rise, actuates a control device such as an electric
members lit and 12 of thermally-conducting material
switch. Typically, the maximum reliable operating time
such as steel, arranged to be held together by screws 11
for such a device having a bimetal strip an inch or
and 13 for clamping and partly enclosing a heating ele—
two long, is a few minutes.
ment 14 and a pair of electric insulating sheets 16 and
At the start of such a timing operation the temperature
of the device rises at a rate determined by the rate of 20 328. The clamping arrangement insures good thermal con
ductivity between the heating element and the two body
heat input and the thermal capacity (heat-absorbing ca
members ‘ill and 12. The heating element 14 consists of
pacity) of the device. As the temperature of the device
a block of refractory material having a negative tempera
rises above the temperature of its surroundings, the de
ture coefficient of electric resistance, such as carbon,
vice loses heat to its surroundings. This loss reduces the
alumina or silicon carbide. Metal wires 22 and 24
rate at which heat goes into storage and consequently
affixed to the resistor block 14 serve as electric terminals
reduces the rate of temperature rise. As the tempera
ture continues to rise the loss of heat increases corre
and lie outside the clamp provided by the body members
it} and 12.
Mounted on the body member 12 and in good thermal
and smaller. As operation is continued, an equilibrium
temperature is approached at which the heat loss balances 80 contact therewith is a bimetal strip 26 arranged to bend
from a cold curved position as shown in FIG. 2, to a hot,
the heat input. High reliability of the timing function is
spondingly and the rate-of-temperature-rise gets smaller
obtained by having the bimetal perform the control func
tion while the temperature is still changing rapidly, that
is, considerably before the temperature reaches its equi~
approximately straight position as shown in FIG. 5. In
the position of FIG. 5, an insulating button 28 carried by
the bimetal, engages a spring switch blade 32 for moving
librium value. The device operates similarly on cooling 35 it toward a second blade 34 and closing a circuit be
tween them. Insulated wires 36 provide connections to
with similar timing characteristics.
rate of temperature change at the end of the operation,
whereas the length of the interval that can be measured
is severely limited by the high rate at the beginning.
In accordance with the present invention 1 supply heat
said switch blades for controlling a circuit.
As may be seen best in FIG. 3, the upper portions of
body members it} and i2 partially enclose the bimetal
26 and switch 32—34-. An electric insulating sheet 38
lies over the top of the members 1% and 12, and a metal
cover 42 completes the enclosure. A wrapping of as
bestos cord provides thermal insulation.
The device of FIGS. 1 to 5 is a thermal timer and may
to a thermal timer at a controlled rate that constantly
be used in any known manner.
varies with the temperature of the device for thereby
adjusting the input to the changing rates of heat loss, all
for providing a better pattern of temperature change,
preferably a low rate at the beginning of the operation
current is applied to the heating element 14 and heat is
generated therein. initially this heat causes a tempera
ture rise of the heating element itself and of the rest of
the structure, particularly the body members it? and 12.
As the temperature of the device rises it begins to lose
heat to the surroundings and the rate of such loss in
creases with the difference of temperature between the
timer and its surroundings. If the rate of heat genera
tion in the heating element 14 is high enough, the tem
perature will in time become high enough that the switch
One shortcoming of such a device is that the rate of
change of temperature is highest at the beginning and
continually slows down during the timing operation. The
accuracy of the timing function is limited by the low
and a high rate at the end.
For a given accuracy of
time measurement, such operation provides a time~control
interval at least three times as long as does operation at
constant heat input as in prior devices. Preferably I
employ a heater with a high negative-temperature coeffi
cient of resistance, energized from a constant-voltage
Such a heater may be constructed of carbon,
silicon carbide, alumina or similar materials. The heater
itself responds to the temperature of the device for regu
lating the rate of power input.
The thermal timer of my invention is sensitive to 60
changes of heating voltage and consequently variation of
the supply voltage provi es a convenient method of
changing the time setting. Furthermore, on temperature
decreasing operation, when the input voltage is reduced
for shortening the time interval, the operation can ap
proach a condition of constant-heat input and conse
quently provide a wide range of time variation.
Objects of the invention include the provision of a low
cost, simple, compact, thermal timer of small mass and
Typically, an electric
32-34‘ will be closed for causing operation of apparatus
connected to the circuit wires 36. The device is capable
of performing a timing function because time elapses
while the device is heating up to the temperature at which
switch 32—34 will close.
The rate of motion of bimetal 23 is substantially pro<
portional to the rate of temperature rise of the device,
which is substantially proportional to the rate at which
stored heat accumulates in the device, and that in turn is
the excess of the rate of heat generation in element 14
over the rate of heat loss from the device to its surround
ings. Since the rate of loss increases with temperature,
if the rate of heat generation is constant, as in prior de
vices, the rate at which heat accumulates and therefore
small power consumption capable of operating reliably 70 the rate of temperature rise will decrease as the tem
and accurately for measuring long time periods. These
and other objects will appear from the following descrip~
perature rises.
Maximum precision, ,or complete accuracy of opera~
tion, would be achieved if, for example, the device closed
the switch 3;.——34 after the same interval of energization
of heater 14, every time it was so energized. The time
interval is affected by the voltage applied to the heater,
and by environmental temperature, and accordingly that 5
voltage and temperature are controlled. For example,
hey may be held substantially constant. Of course it is
of heater M decreases (conductivity increases) so that
the electric current and rate of heat generation increase.
For example, heating element 1d may be constructed
of carbon, alumina or silicon carbide.
FIG. 6 shows time and temperature curves depicting
operation on rising temperature of one actual device con
structed according to my invention.
7 shows simi
lar curves for operation on decreasing temperature. Each
curve depicts operation with a constant voltage applied
sary to control other conditions for minimizing the con 10 to the negative-temperature-coef?cient heating element.
basis for comparison with prior art devices, one
sequent impairment of accuracy. Uncontrolled vari—
constant-heat-input curve is included. This is the zero
ations of supply voltage and ambient temperature affect,
voltage curve in PEG. 7. At zcro voltage the heat input
respectively, the rate of heat input and the rate of heat
was constant at zero». -sxcept
loss. Since the difference between those two rates ai—
for its inversion, this curve
is substantially the same as would be obtained from a
fects the rate of tem erature rise and t ereby affects the
economically impractical in a low-priced timer to hold
these things precisely constant, and it then becomes neces
measured time-interval, accuracy is improved by keeping
that dfference as large as possible compared to the two
rates themselves. This relationship lies behind the em
purposes the maximum usable time interval at constant
heat input would be taken as one hour for the device
pirical rule of the prior art devices, that the best accuracy
is obtained by terminating the operation while the tem
perature is still changing rapidly.
One desirable mode of operation would consist in
keeping the heat input a constant percentage above the
heat loss, as for example 10%. The rate at which heat
then would accumulate in the device for raising its tem 25
whose performance is depicted by the curves of FIGS. 6
and 7. in contrast, much greater usable time intervals
are shown by the other curves, where the rate of heat
input increased with temperature. in particular the 210
volt curve in
6 shows excellent performance for tim
ing intervals to 31/; hours. The 200 volt curve provides
even longer times but with a lower rate of temperature
As depicted in HS. 7, a thermal timer can time by cool
ing even though some heat is supplied to it by its heater.
it the heat is generated in the heating element at a rate
less than the rate of loss of heat to the surroundings, the
, erature would be 10% of the loss rate.
Since the loss
increases as the temperature rises, this mode of opera
tion would cause the rate of temperature rise to increase
with temperature, so that the temperature plotted against
time would describe part of a parabola.
Actually a de
vice operating strictly according to that mode appears
incapable of getting started because it requires zero heat
input at the beginning. So a practical device would need
a higher initial heat input than called for by the parabolic
While such a nearly~parabolic curve represents good
operation, much of the advantage of my invention can
be realized with a device whose operation only approxi
ut with rising temperature.
ure of the device will fall.
For many
Here, as in the
rismg-temperatar operation, if the heater generates heat
at a constant rate, the temperature will change fast initial
ly, but more slowly as the operation proceeds. -lere again,
my invention, by causing the heater to generate heat at a
higher rate at high temperatures than at low temperatures,
reduces the rate of temperature-change at the beginning
of the operation for increasing the time during which the
operation can run and still provide reliable timing. For
mates it. Thus in FIG. 6 the 220 volt and 210 volt curves 4.0
example, the 170 volt curve of FIG. 7 shows excellent
show such approximations in that they show a low rate
for timing periods of 21/2 to 3 hours.
of temperature rise at the beginning and a high rate at
With my present invention I obtain reliable long oper
the end. In the curves of FIGS. 6 and 7 the rate of
ating times in a small low-cost device. I obtain this im
temperature change at any instant is proportional to the
slope of the curve at the point representing that instant. 45 proved pertormance at no loss of flexibility in design be
cause my device responds in much the same Way as do
The steepest part of the curve shows the greatest rate of
prior devices to changes in such design parameters as the
temperature change.
heat storage mass, the thermal insulation, and the voltage
A further advantage of the high rate of temperature
applied to the heating element. My invention also pro
change at the end of the operation results from the re
vides a similar, reliable, long, timing period on cooling
quirement that the device do mechanical work to per
it a low voltage is applied to its heater. And it has the
form a control function at the end of the measured time
further advantage that when the heater is deenergized
interval. For example, work is required even to operate
entirely my device will cool in a fraction of its heat-up
electric contacts. In order for contacts to carry substan
tial current they cannot just touch; they must be pushed
My invention, though illustrated here by a speci?c em
against each other with some force. Because of the re—
bodiment, includes all modi?cations and variations With
silience in the system as, for example, the resilience of
in the scope of the appended claims.
bimetal 26 itself, as well as that of the contact springs
I claim:
32 and 34, some work is required for developing that
1. A thermal timer comprising, in combination, a heat
‘force. Finally, because the value of the required force
storing body exposed to the atmosphere ambient of said
is, in any practical device, somewhat erratic, the neces~ 60 timer whereby heat loss occurs between said body and
sity for developing that force introduces an element of
said atmosphere with the rate of said heat loss increasing
uncertainty into the operation. Because the temperature
as said temperature of said body increases, temperature
rises rapidly at this end point of the operation, the tim
sensitive switch means adjacent to and controlled by the
ing irregularities due to mechanical loads, including the
heat of said body, said switch means adapted to operate
necessity for operating contacts, are kept small. I cause
at a pre-set temperature of said body, and an electric re
the heat to be generated in heating element 14, not at a
sistor having a negative temperature coetlicient 0f resist
constant rate as in prior devices, but rather at a rate that
ance supplying heat to said body whereby said increase
increases with temperature, and preferably at a rate that
in heat loss is compensated for by said electric resistor
increases sharply at the end of the operation.
and said thermal timer has a wider range of control.
In order to automatically increase the rate of heat gen
2. A thermal timer comprising, in combination, a heat
eration as the temperature of the timing device rises, I
storing body exposed to the atmosphere ambient whereby
preferably make the heater 14 ofa material having a
heat loss occurs between said body and said atmosphere
large negative temperaturecoe?icient of resistance and
with the rate of said heat loss increasing as the tempera
energize it from a substantially constant-potential electric
ture of said body increases, a thermostat adjacent to and
supply. As the temperature rises, the electric resistance 75 controlled by the heat of said body, said thermostat
adapted to operate at a pre-set temperature of said body,
and an electric resistor having a negative temperature co
e?icient of resistance and connected to a constant voltage
source of electrical energy, said resistor supplying heat
to said body whereby said increase in heat loss
pensated for by said electric resistor and said
timer has a wider range of control.
3. The combination of claim 2 wherein said
includes silicon carbide.
4. The combination of claim 2 wherein said
includes alumina.
5. The combination of claim 2 wherein said
includes carbon.
References Cited in the ?le of this patent
resistor 10 2,758,175
is com
Hall ________________ __ Mar. 9, 1926
Dubilier _____________ __ Feb. 21, 1933
Bruce _______________ __ Dec. 24, 1940
Hand ________________ __ July 8, 1941
Crise ________________ __ June 18, 1946
Buske _______________ __ Apr. 12, 1955
Hotchkiss ____________ .__ Aug. 7, 1956
Wallace et a1 __________ __ Sept. 18, 1956
Pricer _______________ __ Apr. 23, 1957
De Lancey ___________ __ Sept. 16, 1958
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