Note: Descriptions are shown in the official language in which they were submitted.
?~
The present inven-tion ~elates -to an automatic
temperatu~e control device ~or use in an electric appliance
such ~s an elec-tric hlanke-t or electric carpet
Th~s application is a divisional application of
copending application No. 378,514, filed May 28, 1981.
The present invention will he described with
reference to the aCcomp~n~ing drawings, in which:-
lQ
Flg. 1 is a circuit diagram of a conventional
automa-tic temperature control device for an elecric blanket;
F~g. 2 shows t~le construction of a wire used ~
-the conventional electric blanket and having double func-
tions of heating and temperature sensingi
Fig. 3 is a graph showing -the relation between the
:impedance and temperature of a sensor used in the wire
Fig. 4 is a bloc]c diagram showing an embodiment
of an automatic temperature control device for an electric
blanket according to the invention;
Fiys. 5, 6 and 7 are circuit diagrams showing
details of the respective blocks in Fig. 4; and
Figs. 8 and 9 are time~ charts showing voltage and
current waveorms in the circuits of Figs. 4 to 7.
~ conventional automatlc temperature control device
for an electr~c ~lanket has been constructed as shown in Fig.
1. In Fig. 1, reference numeral 1 denotes an ac source,
2 a switch, 2 a ~ire having double functions of heating and
-temperature sensing and comprising a heater element 4, a
sensor 5 a,nd a~ control element 6.
~ .
The wire 3 has ~ construction such ~s shown in
Fi~o 2 in ~ich the control ele~ent 6 is wound helic~
~ t~
1 on an insulating core 7. Also, the sensor 5 separates the
heater element 4 from the control element 6. Reference
numeral 8 denotes an insulating coating~ Practically, the
wire 3 is arranged in a serpentine fashion inside an
electric blan~et. The sensor 5 has a negative temperature
coefficient of resistance and is usually formed by a
plastic thermistor made of a thermosensiti~e material.
This sensor 5 has an impedance Z related to the -temperature
T as shown in Fig. 3, wherein the impedance Z is a resultant
value o a capacitive impedance ZC and a resistive
impedance ~R~ and the temperature sensing property of the
sensor 5 is greatly influenced by the capacitive impedance
ZC at low temperat~lres and by the resistive impedance ZR
at hish temperatures. When a DC voltage is applied to a
plastic thermistor, the plastic thermistor is polarized and
deteriorated to increase its impedance, so that it is neces-
sary to use a plastic thermistor under the application of
an ac voltage which is as uniform as possible in its
positive and negative polarities.
~hen, the conventional example sho~ in Fig. 1
will be explained in greater detail. In Fig. 1, a
swi.ching element 11 (in this example, a semiconductor
control device ~enerally known as an SCR is used) is
connected in series with the heater element 4 between
25 lines 9 and 10 Similarly, resistors 12 and 13 are con-
nec-ted in series therebetween. Between the junction of
the resistors 12 and 13 and a gate of the SCR 11, a
variable resistor 14, a resistor 15, the control element 6,
1 a diode 17 and a triggering elernent 1~ are connected in
series. Arranged between the control element 6 and the
heater element 4 is a capacitor 19~ In this automatic
temperature control device, during positive half cycles
of ~he ac source voltage in which the line 9 is at a posi~
tive po-tential relative to the line 10, the capacitor 19
is charged at a potential which is determined by the
resistance values of the resistors 12 and 13, variable
resistor 14) resistor 15 and sensor 5, and when the
potential on the capacitor 19 reaches a breakdown voltage
o:E the triggering element 18, a trigger pulse is applied
to the gate of the SCR 11 so that the SCR 11 is turned
on to supply electric power to the heater element 4.
The conduction phase angle of the SCR 11 becomes approxi-
mately 0 when the heater element 4 is at low -temperature
and hence the sensor 5 has a high impedance, thereby
supplying maximum electric power to the heater element 4.
While, when the temperature of the heater element 4 is
high and the impedance of the sensor 5 is low, the con-
duction phase angle becomes approximately 90Q to decreasethe supply of electric power to the heater element 4.
In this way, the automatic temperature control is carried
out. Since the phase angle is changed by the variable
resistor 1~, the user can obtain a desired blanket
temperature by selecting a suitable resistance value for
the variable resistor 14.
However, the aforementioned automatic temper-
ature control device has the following disadvantages.
1 Namely, since the clrcuit comprisiny the diode 17 is con-
nected in parallel with the sensor 5, the ac voltage
applied to the sensor 5 at positive half cycles is different
from the ac voltage applied thereto at negative half cycles.
Consequently, the sensor 5 is polarized and deteriorated to
increase its impedance-, which results in a danger such that
the controlled temperature shifts to higher temperatures.
Further, in the event of a failure of the components used
such as short-circuiting of the triggering element 18,
the supply of electric power to the heater element 4
becomes uncontrollable still maintaining a maximum amount
o:E electric power supply, thereby causing a danger such
tha-t the heater element 4 is overheated. Additionally, a
similar dznger takes place when the SCR 11 is short-
circuited, or a self-triggering failure occurs in the SCR
11 in which the SCR 11 is turned on without being triggered
by triggering pulses. Thus, it is possible that these
dangerous conditions cause a human body injuring accident
in the worst case, since an electric blanket is used by
babies or aged persons who cannot push aside an overheated
blanket by themselves. Further, the automatic temperature
control device employing the phase angle control of an
SCR has brought about an unfavourable condition to
generate noises which cause interference, in particular,
~5 with a radio receiver while it is used by a user lying in
his bed.
It is a first main object of this inven-tion to
provide an automatic temperature control device which
l comprises a heater elernent connected to an ac source, a
senso.r for detecting -the temperature of the heate.r element,
a teMperature responsive circuit including a grounded-base
transistor with its base-emitter circuit connected in series
wi-th the sensor and a diode connected across the base-emitter
junction of the transistor with the PN junction of the diode
and the base-emitter junction o:E the transis-tor connected
inversely in parallel with each other, thereby using a
collector current of the grounded-base transistor as a
temperature signal, an electric potential setting circuit
for producing an electric potential thereby to preset a
desired temperature, a comparison circuit Eor comparing an
output of the temperature responsive circuit with that of
the electric potential setting circuit, ancl swltching means
d:riven by an output of the comparison circuit to regulate
electric power supplied -to the heater element, and which is
featured to supply a uniform ac voltage to -the sensor so
as to prevent the sensor from being polarized and assure
its accurate temperature detection.
An object subsidiary to the first main object is
to provide an automatic temperature control device which
further comprises an integrating circuit for integrating
the output of the temperature responsive circuit (the col-
lector current of the grounded-base transistor) to produce
an in-tegration output ~o be supplied to the comparison cir-
cuit and stopping its integrating oper`ation during positive
half cycles of the ac source voltage in which electric
power is supplied to the heater element, thereby affording
5~
1 a s-table and accurate temperature signal.
Another object subsidiary to the first main object
is to provide a safe automatic temperatuxe control device
which further comprises a power supply interrupting circuit
including a heating resistor connected in series with the
series circuit of the sensor and the base-emit~er circuit
of the transistor and a thermal fuse connected to the ac
source which can be fused by the heat generated in the heat-
lng resistor thereby to interrup-t electric power supply to
the heater element, whereby, when the heater element is
overhea-ted to cause the sensor to be melted ancl the heater
element and the control element to be short--circuited to
each other, the heating resistor is supplied with an over~
load which is 17 times as large as its rated load thereby
to be heated to lnterrup-t electric power supply to the
heater element.
It is a second main object of this invention to
provide a highly safe automatic temperature control device
which comprises a heater element connected -to an ac
source9 a sensor for detecting the temperature oE the
heater element, a temperature responsive circuit for
detecting the temperature of the heater element by detect-
ing an impedance of the sensor by means of the control
element which is in contact with the sensor, an electric
potential setting circuit for producing an electric
- potential thereby to preset a desired temperature, a pulse
supplying circuit. for producing a zero-crossing pulse, a
pulse discriminating circuit for determining whe-tller
... .
1 an input signal thereto is a pulse and, as a result of the
determination of the input pulse, producing an output
pulse in phase with the input pulse, a comparison circuit
for comparing an output of the temperature responsive
circuit with that of the electric potential setting circuit
in synchronism with the zero-crossing pulse and supplying
a zero-crossing pulse to the pulse discriminating circuit
when the temperature of the heater element is below a
preset temperature level t ancl switching means triggered by
an output of the pulse discriminating circuit to supply
electric power to the heater element, whereby the switching
means is triggered by the zero-crossing pulse to enable
automatic temperature control with reduced noise genera-
tion and the occurrence of any failure in the respective
constituent circuits are checked in synchronism with the
zero-crossing pulse so as to interrupt the supply of
electric power to the heater element when the occurrence
of a failure has been detected.
An object subsidiary to the second main object
is to provide a highly safe automatic temperature control
device wherein the comparison circuit comprises an electric
circuit for fixing the output of the electric potential
setting circuit to an electric potential level which is
lower than a circuit dc power supply voltage by a pre-
~5 determined magnitude whe:n a zero-crossing pulse does not
occur so that the outputs of the temperature responsive
circuit and the electrical potential setting circuit are
compared with each other in synchronism wi-th input
.. . .. ~ .
~ Q ~
1 zero-crossing pulses thereby to produce output zero-crossing
pulses, and wherein, even when any failure occurs in the
Olltput of the temperature responsive circuit and a false
output signal appears representing that the temperature of
the heater element is very low, it is possible to inter-
rupt the supply of electric power to the heater element.
Another object subsidiary to the second main
object is to provide a highly safe automatic temperature
control device wherein an SCR is used as the switching
means and there is provided an SCR failure sensing circuit
for detecting a non-triggering conduction failure (self-
-triggering) of the SCR to interrupt the supply oE electric
power to the heater element.
A further object subsidiary to the second main
object is to provide an inexpensive and hiyhly safe auto-
matic temperature control device wherein the pulse
discriminating circuit is constituted by resistors, a
capacitor and switching elements, and in the absence of
input zero-crossing pulses the capacitor stores electric
charge supplied from the circuit dc power supply through the
resistors, and upon receipt of input zero-crossing pulses,
the switching elements are turned on to discharge the
stored electric charge to the switching means, thus
preventing the application of triggering pulses -to the
~5 switching means when a failure occurs in the electric
circuits of the device.
A still further object subsidiary to the second
main object is to provide a highly safe automatic temperature
1 con-trol device which further compri.ses a disconnection
sensing circuit for preventing zero-crossing pulses from
being applied to the pulse discriminating circuit, when
breakage of the control element occurs, by causing the
output of the temperature responsive circui-t -to be shifted
apparently to the side of a lower temperature s-tate and
to stay there upon occurrence of such breakac3e of the
control element, thereby preventing the temperature under
control from becoming high.
A yet further object subsidiary to the second
main object is to provide a highly safe automatic temper-
ature control. device wherein a plurality of electric
circui-ts such as the disconnection sensing circuit, the
comparison circuit, etc. are connected so tha-t zero-
crossing pulses are successively transmitted through the
circuits and output pulses from the last one o:E the
circui-ts are applied to the pulse discriminating circuit
as inpu-t signals thereto whereby a failure of circuit
components in the transmission path of zero-crossing
pulses can be checked without requiring additional
components for use in the checking and the supply of
electric power to the heater component can be in-terrupted
when a failure occurs in the circuits.
Reference is firstly made to a block diagram of
Fig. 4. For a better understanding of the present
inventionl reference should also be ma,de to Figs. 8 and
9. In Fig. 4, a voltage VAc of an ac source 1 is applied
to a circuit of a diode 20, a resistor 21 and a capacitor
,. " . ?~ q ~
1 22 by closi.ng a switch 2 to produce a dc vol-tage Vcc to be
used as a dc power supply for the circuits A temper-
ature responsive circuit as designated by a block 23
detects a current IS resulting from the amplification of
an electric current which flows through a ground line of
the ac source 1, the temperature responsive circuit 23, a
heating resistor 33, the control element 6, the sensor 5,
the heater element 4 and the other supply line of the ac
source 1 with its magnitude varying with the change of the
impedance of the sensor 5, during negative half cycles of
-the voltage VAc of the ac source 1, and gives rise to the
simultaneous flow of a current IT which is substantially
identical with the current IS. Since the sensor 5 is
capacitive, this current IS leads in phase by about 90 as
shown in Fig. 8 and overlaps, in phase, positive half
cycles of the ac source voltage in which an SCR 11 is
turned on, so that the waveforrn of the current IS is
distorted by the influence of a potential gradient generated
by -the heater element 4. Maturally, the current I5 is not
distorted when the SCR 11 has been turned offO Therefore,
the magnitude of the current IS has great dependency upon
the presence or absence of the distortion/ resulting in
undesirable variations in the relation between the temper-
ature of the heater element 4 and the maynitude of the
~5 current I5. An integrating circuit, such as designated by
a block ~4, is adapted to integrate the current IT and
produce an output voltage VI in inverse proportion to the
temperature of the heater element 4. F'or the reasons set
~ 3~ 3
1 forth above, the integra-ting circuit 24 integrates the
current IT which has excluded a region of the current IS
where the distor-tion appears. As the temperature of the
heater element 4 rises, the impedance of the sensor 5
decreases, resulting in an increased currerlt IT and a
decreased voltage VI. An electric potential setting
ci.rcuit as designated by a block 25 is adapted to produce
a potential Vs The potential Vs is manually variable
and may be adjusted by the user to obtain an optimum con-
trolled temperature. A block 26 denotes a pulse supplyingcircuit which produces various pulses V0, Vzl, Vp and VN
.in synchronlsm with the ac source voltage VAc The pulse
Vzl stands for zero-crossing pulses which are generated
at zero-crossing points through which the waveform of the
ac source voltage VAc transfers from negative halE cycles
to positive half cycles.
A block 27 denotes a disconnection sensing circuit
adapted to detect disconnection or breakage o~ a control
element 6 and which produces a voltage V~ to be applied to
the control element 6 in synchronism with the zero-crossing
pulses Vzl and further produces other zero-crossing
pulses Vz2 in synchronism with the zero-crossing pulses
Vzl if the control element Ç is not disconnected nor
broken, the.reby allowing an electric current to flow there-
through. Wit~out the disconnection sensing circuit 27,a decrease in the current IS flowing through the sensor S
upon occurrence of disconnection or breakage of the
control elemen-t 6 would give rise to a signal indicative
.. --~ 1~ - -
P
1 of a decreased temperature of the heater element 4, there-
by causing -the temperature under control to rise danger-
ously in excess of a preset temperature~
A btock 28 denotes ~ comparison circuit which~
in the absence of the zero-crossing pulses Vz~ app:Lied
thereto, fixes the output voltage Vs from the electric
potential setting circuit 25 at a level of (V~c ~ VF = VsF,
where VF is a constant forward voltage drop of a diode),
and which, in the presence of the zero-crossing pulses
V~2 from the disconnection sensing circuit 27 r releases
the Eixed level in synchronism with the reception of the
zero-crossing pulses and compares the output vo]tage V
from the i.ntegrating circuit 24, which is in inverse
proportiorl to the temperature of the heater element 4, with
a voltaye VST which is the value of the output voltage Vs
from the electric potential settiny circuit ~5, at this
ti~e, thus producing the other zero-crossing pulses Vz3 in
phase with the zero-crossing pulses Vz2 when the detected
temperature is below a preset temperature level.
A block 29 denotes a pulse discriminating
circuit which, upon receipt of the zero-crossing pulses
Vz3 fxom the comparison circuit 28 I produces other zero-
crossing pulses Vz4 in phase with the zero-crossing pulses
Vz3 which pulses Vz4 are applied to the gate of the SCR 11
to trigger it and supply electric power to the heater
element 4. When the detected tempera~ure exceeds a preset
temperature level or when a failure occurs in the pulse
supplying circuit 26, disconnection sensing circ~1it 27 or
. . . . ~, ~,. _ , ... .
cormparison clrcul-t 28, no input pulse Vz3 is app:Lied to the
pulse cliscrim:inat:ing circul-t 29, and hence there is no out-
put pulse Vz,l from the pulse discrimina-ting c.ircult 29,
t~-lereby maintainirlg the SCR 11 untr~ggered. Wherl a failure
occurs in Ihe pulse supplying clrcuit 26, disconnection sen-
sing circuit 27 or comparison circuit 28, so thdt, a con-
tinuous input signal Vz3 is appli,ed to the pulse discriminat-
ting circuit 29, the ou-tput signcll Vz4 of t~le pulse ~,scri-
mlnating circuit 29 is reduced -to a level (practically not
more than 0.2 volts) which is so low as to fail to -trigger
the SCR 11. Thus, the SCR 11 is not. triggered also in this
case. F'urther, the pulse discriminating circuit 29 itself
is so designed that i-ts output signal Vz4 disappears or is
reducec1 to a low level which is insu:Eficient to tr.igger the
SCR 11 when a failure occurs in the pulse discr,imirlating
circuit 29 itself.
With the above-descr:ibed construction, the zero-
crossing pulses delivered from the pulse supplylng circuit
26 a.re transmltted successively through the disconnection
sensing circuit 27 and comparison circui-t 28, whose sae
operation has to be assured, and the final output signal
causes the pulse discriminating circuit 29 to tri.gger the
SCR :L1. In this manner, the respec-tive circuits confirm
the occurrence of no failure -therein along with the accom-
plishment of their proper functions while the ze:ro-crossing
pulses are generated. Thus, it is possible to accomplish
-the two functions simultaneously and instantaneously to
ensure that tlle circuits can be kept
~ .... . . . ~n
1 in highly safe conditions, yet without requi.ring any great
increase o~ circuits or componen-ts for assuring a safe
operation of the device. Even when there are involved
o-ther circuits, whose safe operatlon has to be assured, in
5 acldition to the two electric circuits of -the disconnection
sensing circuit 27 and comparison circuit 28 as e~emplified
in the above construction, the aforernentioned safety
system can readily assure a highly safe operation of the
device~ Therefore, it can be said that this safety system
has a wide field for its application.
A block 30 denotes an SCR failure sensing
circuit~ When it is detected that the SCR 11 is turned on
in spite of the absence of the zero-crossing pulses Vz3
from the comparison circuit 28~ the SCR failure sensing
ci.rcuit 30 applies a heavy load to a hea-ting resistor 31
whlch load is 17 times as high as a rated load of the
heating resistor 31 by making use of outpu-t pulses VN and
VD from the pulse supplying circuit 26, so that the
heating resistor 31 is heated to cause a thermal. fuse 32
to be fused, thereby interrupting the supply of electric
power to the heater element 4. Accordingly, even if the
SCR 11 has been short-circuited or has brought about
self-triggering thereof, the supply of electric power to
-the heater element 4 can be also interruptedO
A wixe 3 has a function -to detect abnormal
heating when abnormal heat is generated accidentally in
the wire 3. Namely, when the heater elemen-t 4 is
abnormally heated until the temperature therearound reaches
.. .. . . . . ... ... . . .. . .
t~ jt-`~
1 the rnelting point (167C) of the sensor 5 made oF a plastic
thermistor~ the con-trol element 6 is brough-t into short-
cirrui-ting contact with the heater elemen-t 4 due to a
-tensile s-tress in -the control element 6 remaining from the
time o~ lts coiling. Then, the heating resistor 33 is
supplied with the ac source voltage 'JAC thereby to be
heated by a heavy load current which is 17 times as large
as its rated load current. As a result, the thermal fuse
32 is fused to interrupt the supply of electric power to
the heater element 4.
In -the foregoing, a description has been made
of the overall construc-tion of the safety system of the
automatic temperature control devlce according to the
invention. The construction of each of the specific
circuits employed in the system will then be described
in greater detail.
Fig. 5 shows details of the pulse supplying
circuit 26 in which the ac source voltage VAc is con-
ver-ted into an ac voltage VAc' whose phase is slightly
delayed~ as shown in Fig. 8, through a fil-ter constituted
by a resistor 34 and a capacitor 35, and the ac voltage
VAc' is supplied to a point 36. When the ac voltage
VAc' exceeds a reference voltage VF determined by a
constant current source 37 and a diode 38 during a rising
positive half cycle, a comparator 39 is inverted to
produce an output voltage Vp. Thereafter, when the ac
vol-tage V~c' continues to increase and reaches a level
of the sum of the reference voltage VF and a base-emitter
1 forward drop VB~ o~ a transistor 40, transistors 40 and
41 are tur~ed on to generate an output pulse VD which falls
~o gl-ound potential. According:Ly, the falling point of
VD is necessarily delayed from the rising point of Vp,
whereas -the rising point of VD :Ls necessarily in advance
oE the falling point of Vp. As the ac voltage VAc'
enters a negative half cycle, a transistor 43 is turned
on, thereby turning off transistors 46 and 47, which have
been conductive by being fed from constant current
sources 44 and 45, and simultaneously generating an output
pulse VN which falls to ground potentialO Reference
numeral 49 denotes a base-emitter resistor Eor the tran-
sistor 47.
By applying the thus produced pulses Vp and VN
to set and reset terminals of a flip-flop 50, respectively,
and then a Q output of the flip-flop 50 and the pulse VN
to a NOR circuit 51, the zero-crossing pulse Vzl is
produced from the output terminal of the NOR circuit 51.
A description has hereinabove been made of the
ac voltage VAc' whose phase is slightly delayed from that
of the ac source voltage VAc ~ the pulse VN which rises
during negative half cycles of the ac voltage VAc', the
pulse Vp which rises during positive half cycles of the
ac voltage VAc', and the pulse VD which falls during
2~ positi.ve hal~ cycles of the ac voltage VAc' and rlses
during the positive peri.od of the pulse Vp.
ReEerring to Fig. 6 which shows i.n detail the
essential struc-ture of the present invention, details of
3f~D l~
1 the temperature xesponsive circuit ~3, integrating circuit
24, electric potential setting circuit 25, disconnection
sensiny circuit 27, comparison circuit 28~ and pulse dis-
criminating circuit ~9 will be describedO For giving a
better understanding of the present invention, reference
should also be made to Fig. 8.
The temperature responsive circuit 23 comprises
a grounded base transistor 52 and a diode 53 which are
connected in antiparallel with the base-emitter circui-t of
the transistor 52 such that the PN junction of the diode
53 and the base-emitter junc-tion of the transistor 52 are
connected inversely in parallel with each other. During
positive half cycles o~ the ac source voltage V~c ~ the
supply of electric power to the heater element 4 is
controlled, and during negative half cycles of -the ac
source voltage VAc , the impedance of the sensor S is
detected as an equivalent of the temperature of -the heater
element 4. The current IS flowing through the sensor S
during negative half cycles of the ac source vol-tage VAc
is equal to the emitter current of the transistor 52.
In Fig. 8, a period designated by x is one in
which electric power is supplied to the heater element 4
and a period designated by y is one in which the supply
of electric power to the heater element 4 s-tops. Since
~5 the sensor 5 has a capacitive impedance as shown in
Fig. 3/ the phase of the sensor current IS leacls that of
the ac source voltage VAc by about 90, so that a
portion of the sensor current IS overlaps the period for
17 ~
q~
1 controlling the supply o~ electric power to the heater
element 4. As a result, an electric potential. gradient
caused in the heater element 4 when electric power is
supplied to the heater element 4 affects the sensor
S curren-t IS to distor-t the waveform of the overlapping
portion of the sensor current IS as shown .in FigO 8.
Naturally, the distortion disappears within the period y
in Fig. 8. During positive half cycles of the ac source
voltage V~ , the current IS flowing through the sensor 5
passes the diode 53. Since the magnitude of the forward
voltage of the diode 53 and the base-emitter ~ol-tage of
the transistor 52 are substantially equal -to each other
and further either one of the above-mentioned voltages is
~ar smaller than the voltage VAc , the sensor S is fed
wi-th a symmetrical ac voltage, whereby the promotion of
the deterioration of the sensor 5 by the polarization
effect is suppressed. The heating resistor 33 is impressed
wi-th -the ac source voltage VAc when the heater element 4
is heated -to reach an abnormally high temperature at which
20 the sensor 5 is melted and the heater element 4 is brought
into short-circuiting contact with the control element 6,
whereby the hea-ting resistor 33 is heated to fuse the
thermal fuse 32~ The resistance (390 Q in this example)
of the heating resistor 33 is much smaller than the
resistance of the sensor S, so that the insertion oX the
resistor 33 does not cause an error in`the sensor current
S'
The collector current IT, whose magnitude is
1 substantially equal to that of the sensor currert Is) flows
through -the collector of the transistor 52.. However, a
distortion would appear or disappear in the sensor current
IS depending on the supply or the stoppage of supply of
electric power to -the heater element 4, thus .resulting in
-the corresponding fluctuation of the magnitude of the
sensor current I~ or the collector current IT. In order
to rernove the above-described disadvantage, the integrat-
ing circuit 24 is provided to cut the distortion component
of tne collector current IT and to produce a dc signal
indicative of the temperature under control by integrating
the collector current IT whose distortion component has
heen removed.
The integrating circuit 24 comprises a capacitor
~/L5 54 for integrating the col~ctor current IT and a discharge
resisto.r 55 for gradually discharging electric charge on
the capacitor 54. A transistor.56 is provided to cut the
distortion component. This transistor 56 staying in the
conductive state under the control of a constant current
source 58 is turned off to cut the distortion component
when a transistor 57 connected in the base circuit of the
transistor 56 is turned on by the pulse Vp which rises
during positive half cycles of the ac source voltage VAc.
A diode 59 is provided to assure the turn-ofE oE the
transistor 56 when the transistor 57 has been turned on.
A description will be given~of the relation
between the ternperature of the heater element 4 and the
output voltage VI of the integrating circuit 24 brought
... . - ~ q ~ ' ' '
1 by the above-mentioned construction. As the temperature of
tne heater element 4 increases, the impedance of the
sensor 5 decreases and the sensor current IS and the col-
lector currenc IT increase. Consequently, the lntegration
voltage of the capacitor 54 increases and the output
voltage VI of the integrating circuit 24 decreases. On the
other hand, as the temperature oE the heater element 4
decreases, the above-mentioned situation is reversed and
the outpu-t voltage VI of the integrating circuit 24
increases. Thus, the output voltage VI oE the integrating
circuit 24 varies in inverse proportion to the temperature
o-f the heater element 4.
Nex~, the electric potential setting circuit 25
has a series circuit of the resistor 60, variable
resi.stor 61 and resistor 62 connected across the circuit
dc power supply Vcc ~ and its output potential Vs is taken
out :Erom a junction of the resistor 60 and varia~le
resistor 61. The resistance of the variable resistor 61
may be varied by the user to adjust the outpu-t potential
Vs o-E the electric potential setting circui-t 25.
An explanation will be given hereunder of the
disconnection sensing circuit 27. The disconnection
sensing circuit 27 comprises a transistor 68 which is
turned on by being fed with a constant current :Erom a
2S constant current source 63 via diodes 64, 65, 66 and 67
when the zero-crossing pulse Vzl is not supplied to the
base o~ a transistor 70. A resistor 69 is a base-emitter
resistor .for the transistor 68. When the zero-crossi.ng
1 pulse Vzl is applied to the base of the transistor ~0, the
transistor 70 ~s turned on, which causes a transistor 74
t.o be turned cn through a resistor 71 and a current
m~rror circuit constituted by a diode 72 and a transistox
73 to produce an output potential Vx at the emi-tter output
o:E the transistor 74. When the control element 6 is not
disconnected llor broken, a current is supplied from the
constant current sOUrce 63 to flow through the control
element 6 and simultaneously to cause the transistor 68
to be turned off, -thus producing the zero-crossing pulse
Vz2 a-t the co:Llector of the -transistor 68. If the control
e~ement 6 is disconnected or brolcen, a current cannot flow
through the control element 6 even though the transistor
7~ is turned on, so that -the transistor 68 continues to
lS be conductive and hence no zero-crossing pu:Lse Vz2 is
genexatedO Reference numeral 75 designates a base~
emitter resistor for the transistor 7~. Diodes 76~ 77
and 78 are provided to protect the transistor 74 from a
surge voltage. The diodes 54, 65, 66 and 67 are provided
20 to assure the turn-off of the transistor 68 when the
t.ransis-tor 74 is turned on.
As will be seen from the above description,
when the control element 6 is not disconnected nor broken,
-the zero-crossing pulse ~Z2 is produced in phase with the
zero-crossing pulse Vzl when the zero-crossing pul.se V
i5 applied to t:he base of the transistor 7a.
The comparison circuit 28 will now be explainedO
The comparison circuit 28 is adapted to compare -the output
.. .. ~ . , . ... . , .. , . . .... . . ~, ~ .
1 poten-tial VI from the integrating circuit 24 with the output
poten-tlal Vs from the electric potential setting circuit
250 In the absence of the zero-crossing pu ! se Vz2 from the
disconnection sensing circuit ~7, the transi.stor b8 remains
conductive, and a txansistor 80 also stays in the con-
ductivP state with its base current drawn through a
resistor 79, thereby ensuring that the output potential Vs
from the electric potential setting circuit 25 is fixed
to a level VsF which is lower than the circuit dc power
supply voltage Vcc by the forward voltage drop VF of a
diode 81. Accordingly, the relation of Vs > Vl holds
between the output potential V~ of the electric potential
se-tting circuit 25 and the output potential VI o~ the
integrating cirGuit 24, which renders tne output level of
the comparator 82 low. When the zero-crossing pulse Vz2
is produced by the turn-off of the transistor 68, the
~ransis~or 80 is turned off to release the ou-tput potential
VS of the electric potential setting circuit 25 from its
fixed potential level to compare the output potential V
of the integrating circuit 24 with the preset value VsT
of Vs provided by the electric potential setting circuit 250
In this comparison, during the period x sho~l in Fig. 8
where the temperature of the heater element 4 remains lower
than the preset temperature, every time when -the relation
~ Vs ~ VI becomes satisfied, the output of the comparator
82 turns to a high level to thereby produce the ~ero
crossing pulse Vz3~ On the other hand, during the period
y during which the temperature of the heater element 4
.... ~ . . .. ., .. .. . ....... ., . .. , .. ~ . . . . ... . .. . . . .
P ~
1 remains higher than the preset temperature, the relation
o Vs ~ VI holds to maintain the ou-tput of the comparator
82 at a low level, thereby preventing the generation of
the zero-crossing pulse Vz3~ Rererence numeral 83 denotes
a base-emitter resistor for the transistor 80~ The diode
81 fulfils a useful purpose as described helowO When a
failure occurs in the tempera-ture responsive circuit 23
or -the integrating circuit 24, namely, when an open-
circuit failure occurs in the transistor 52, for example,
the ou-tput potential VI of the integrating circuit 24
becomes equal to the circuit dc power supply voltage V
Assuming that the diode 81 is not provided, the output
po-tential Vs of the potential setti.ng circuit 25, which
has been .Eixed to a level VsF lower than the circuit dc
lS power supply voltase Vcc by the emitter-coll.ector satura-
tion voltage VcE5a~ of the transistor 80 when the
transistor 80 is conductive, would be released to its non-
fixed potential level VsT in synchronism with the generation
of the zero~crossing pulse Vz2. Therefore, normally the
relation of VI > Vs is maintainecl rendering the output
le~rel of the comparator 82 high, thus preventing the output
zero~crossing pulse Vz3 from being generated by the
comparison cixcuit 28. It should be noted that the
comparator 82 has an input offset voltage VI0. Accordi~gly,
when the relat:ion of VIo > (VI - Vs) holds, the output level
of -the comparator 82 becomes low when the transistor 80
is turned on~ As a result the zero-crossing pulse Vz3 is
continuously produced from the comparison circuit 28,
... .. ~ 1~
1 resulting in the dangerous overheating of the heater
el.ement ~. However, owing to the provision of the diode 81,
it is possible to have the potential Vs lowexed to the
level VsT further by the magnitude of the forward voltage
drop V~ acros.s the diode 81 even when the transistor 80
is conducti~e, thereby eliminating the adverse affect by
the input offset voltage of the comparator 82.
Next, an explanation will be given o the pulse
discriminating circu~t 29. When the pulse dlscriminating
circui~ 29 does not receive the zero-crossiny pulse Vz3
from the comparison circuit 28 (i.e., when the output
level of the comparator 82 is low), transistors ~4 and 85
are nonconductive and a capacitor 86 is gradually charged
from -the circuit dc power supply voltage Vcc through a
resistor 87. Upon receipt of the zero-crossing pulse Vz3
from the comparison circuit 28 by the base of the trans
i.stor 84, the transistor 84 is rendered conduc-tive
-through a resistor 88 and the transistor 85 is also
rendered conduc-tive through a resistor 89. As a result,
electric charge stored in the capacitor 86 discharges
forming the zero-crossing pulse Vz4 through the translstor
85 and resistor 90 to the gate of the SCR 11. Reference
numeral 91 denotes a base-emitter resistor for the
transistor 85, and reference numeral 92 a gate resistor
~5 for the SCR 11~
The pulse discriminating circuit 29 makes use
of a specific property of the instantaneously rising
pulse waveform of the zero~crOssing pulses at the
1 zero-crossing points thereby to effect pulse discrimination.
- ~ In this example, since the ratio of an occurrence period
to a non-occurrence perlod of the zero-crossing pulses is
taken to b~ 2 or more, even when the resistor 87 has
a considerably large resistance value, it is possible to
produce at æero-crossing points the pulse Vz4 of a magnitude
sufficient to trigger the SCR 11. As an example in this
casPj when th~ circuit dc power supply voltage Vcc is
5 volts and the resistance of the gate resis-tor 92 of the
SCR 11 is 1 K~, the resistox 87 may have a resistance value
of 33 KQ. Accordingly, even if there occurs a failure to
render the transistor 85 continuously conductive (for
exanlpler in the cases o~F an open-circuitiny failure of
each of the transistor 52, the diode 64 and the resistor
79~ and a short-circuiting failure of the transistor 85),
the gate of the SCR 11 is supplied only with a voltage
not more than a division of the circuit dc power supply
vol-tage Vcc by the resistors 87, 90 and 92, for example~
0.15 volt in this case. Since the appli~d voltaye does
not exceed a non-triggering voltage VGD of the SCR 11,
the SCR 11 is not triggered, and the supply oF electric
power to the heater element 4 stops. This state is
indicated by a period z in Fig. 8. The period z in
FigO 8 is illustrated under the assumption that an open-
circuiting failure has occurred in the transistor 52.
Fig. 7 shows a concrete construction of theSCR failure sensing ciruci~ 30. This circuit will be
described making reference also to Fig. 9.
=aS~ -
1 In Fig. 9, the period "a" deno-tes a state where
the SCR 11 is normai but nonconductive, the per.iod "b"
denotes a state where the SCR 11 is normal and conductive,
the period "c" denotes a state where a self~triggering
failure has occurred in the SCR 11, and the period "d"
denotes a state ~here a short-circuiting failu.re has occur-
- red in the SCR 11. In Fig. 7~ resistors 93 and 94, a
transistor 95, diodes 96 and 97, a transistor 98, a constant
current source 99, a resistor 100 and a transistor 101
constitute an electric circuit for confirming the turn-off
of the SCR 11. During negative half cycles o the ac
source voltage VAc , the transistors 95 and 98 are turned
off and the txansistor 101 is turned on, so -that the
collector voltage waveform VO~ of the transi.s~or 100 is at
a low level. In the period "a" in Fig. 9 where the SCR
11 is nonconductive, during positive half cycles of the
ac power source voltage VAc an input ac voltage VO rises
as the ac power source voltage VAc rises, so -that the
transistors 95 and 98 are turned on and the transistor
101 is turned off to raise its collector voltage VOO to a
high level. Numeral 102 designates a flip-flop circuit,
which is provided for storing the~ presence or absence o-f
the zero~crossing pulse Vz3 delivered from the comparison
circuit 28.
In the period "a" where the temperature of the
heater element 4 is higher than the pr.eset temperature,
the ~.ero~cross.iny pulse Vz3 is not produced and the
flip-flop circuit 102 is kept reset by the inpu-t reset
q,;~jv~
1 pu:Lse VN to maintain its output voltage VO at a low level.
Numeral 103 designates a NOR circuit, which produces an
ou-tput voltage Vy of a high level when all of its input
voltages are of a low level, but which does not deliver
any output voltage of a high level during the period ~a~7 t
since the pulse5 VD and/or VOo of a ~igh level are
supplied to the ~OR circuit 103 during the pericd l'a"~
In the period "D" tr.e pulse Vz3 is produced to
turn on the SC~ 11 when the temperature of the heater
element 4 Ealls below the preset temperatureO The input
voltage VO does not rise during positive halE cycles of
the ac power source voltaye VAc , so that it follows that
the trans:istors 95 and 98 remain nonconductive and the
transistor 101 remain conductive to maintain its collector
vol-tage VOO at a low level. On the other hand, the flip~
flop circuit ]02 is sel by the input set pulse Vz3 and
maintains its output voltage VQ at a high level during
positive half cycles of the ac voltage VAc. As a result
high level inputs to the ~OR circuit 103 are maintair.ed
by the pulses VD and/or VO to prevent a high level output
voltage from being generated by the NOR circuit 103.
In this manner, in the normal operation, the generation
of a high level output voltage by the NOR circuit 103
is prevented.
~5 In the period "c" where the SCR 11 conducts due
-to lts self-triggering in spite of the absence of the
pulse Vz3 , the Elip-flop circuit 102 main-tains its output
voltage VQ at a low level because of the absence of an
.... .. ... , a~l~ .
~ o
l input set pulse thereto. As a result~ there occurs a
period in positive half cycles of the ac voltage VA~ where
all the inputs to the NOR circuit 103 take a low level
to render the output voltage Vy o tle NOR circult 103
hiyh. Consequently, an SCR 104 is triggered to supply the
heating resistor 31 with a heavy load which is 17 times
as large as its rated load. Thus, the heating resistor 31
is heated to fuse the thermal fuse 32, thereby interrupt~
ing the supply of electric power to the heater element 4.
lQ In the period "d" where the SCR 11 conducts due
to a short-circuiting failure in spite of the absence of
the pulse Vz3 , the SCR 104 is triggered in -the same manner
as ~he period "c", and the h~ating resistor 31 is heated
to fuse the thermal fuse 32 so that the supply o~ electric
power to the heater element 4 may be interrup-ted.
In Fig. 7, a diode 10S is provided to protect
the transistor 95 and the diodes 96 and 97 against a high
backward voltage which appears during negative half
cycles of the ac power source voltage VAc. Numeral 106
designates a gate resistor for t'sle SCR 104.
