Non-continuous sensing apparatus for a temperature control

A non-continuous sensing device for a temperature control including A.C. power switching element, a zero-crossing detector, a temperature sensor and a trigger pulse generator for generating a trigger pulse used to trigger the A.C. power switching element according to an output of the temperature sensor. The zero-crossing detector generates a pulse signal each time the instantaneous absolute value of the A.C. power voltage decreases to a value equal to or smaller than a predetermined value. The temperature sensor and the trigger pulse generator are kept active only for the duration of the pulse signal generated by the zero-crossing detector, thereby reducing the energy consumption and physical size of the entire device.

BACKGROUND OF THE INVENTION 
The present invention relates to a non-continuous sensing apparatus for a 
temperature control, and more particularly to an improved electronic 
control device having reduced size and energy consumption. 
There is shown in FIG. 1 a circuit diagram of a conventional heater drive 
controller by way of example. A heater 1p is power-supplied from an A.C. 
power source 18p through a bidirectional thyristor 2p connected in series 
with the heater 1p. The thyristor 2p is gated by a control signal input to 
its gate terminal from a comparator 4p which is devised so as to generate 
the control signal according to an output of a resistance bridge 5p in 
which a temperature sensor 6p is contained as a bridge component. Both the 
comparator 4p and the bridge 5p are always supplied with a D.C. power from 
a D.C. voltage source 13p consisting of a rectifying diode 17p, a resistor 
16p, a Zener diode 15p and a smoothing capacitor 14p. Comparing a sensor 
output with a reference voltage which corresponds to a predetermined 
control value of temperature, the comparator 4p outputs a switching-on 
trigger signal at the time of the zero-crossings of the A.C. power source 
18 p. The trigger signal is fed to the thyristor gate 2p so as to energize 
the heater 1p while the bridge output continues to indicate that the 
temperature measured by the sensor 6p is lower than the predetermined 
control value of temperature. 
In such a conventional device, the heat dissipation or the energy 
consumption in the control system is help being large, because the 
temperature sensor 6p and the comparator 4p are kept supplied with a 
current at all times during the operation of the device. In addition, in 
case the temperature sensor 6p, as is often the case, can not be chosen so 
as to have a large resistance value, the current to be supplied to the 
sensor must be increased to obtain a necessary sensitivity, and the 
problem of heat dissipation becomes more serious. Large dissipation of 
heat not only makes it difficult to assemble the whole device compactly 
but also causes the device to be damaged due to overheating. 
OBJECTS AND SUMMARY OF THE INVENTION 
An object of the present invention is to provide an improved heater control 
device in which the heat dissipation from the control circuit including a 
D.C. power supply is largely reduced so as to not only make the device, if 
assembled compactly, free from troubles due to over-heating but also to 
save energy. Another object of the present invention is to provide a 
heater control device which works sensitively without using an amplifier 
even if the resistance of the temperature sensor is small. 
A heater control device based on the present invention comprises a 
zero-crossing detector which generates a pulse signal each time the 
instantaneous absolute voltage of the A.C. power source becomes lower than 
a certain definite small value. The pulse signal controls a D.C. switching 
element to supply a current to a temperature sensor only for a short 
duration of the pulse signal. The temperature measured by the sensor is 
compared by a comparator with a predetermined temperature control value, 
and if the temperature is lower than the predetermined value, the 
comparator outputs a trigger pulse to an A.C. power switching element 
which is connected in series with the heater to be controlled. For the 
A.C. power switching element, a uni- or bi-directional thyristor is used. 
The thyristor, is made conducting by the trigger pulse as long as the 
temperature measured by the sensor is lower than the predetermined value. 
With the temperature increased up to the predetermined value, the 
comparator ceases outputting the trigger pulse and the thyristor is then 
turned off. As briefly described above, the sensor is kept active only for 
a short duration of time equal to the width of the pulse signal generated 
by the zero-crossing detector, so that the comparator also need not always 
be kept active except for the same duration of time. 
Accordingly, the power consumption in the control system is greatly 
reduced. In addition, according to the present invention, the sensitivity 
of the temperature sensor can easily be improved by increasing a current 
supplied thereto, because the current is pulse-shaped in accordance with 
the pulse signal of the zero-crossing detector. 
Other objects and advantages of the present invention will become apparent 
from the following detail description when taken in conjunction with the 
appended claims.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 2, a heater 1 is power-supplied from an A.C. power source 
18 through a bi-directional thyristor 2 connected in series with the 
heater 1. The thyristor 2 is controlled by a trigger signal which is 
output from a comparator 4 and then input to the gate of the thyristor 2 
through a resistor 3. A temperature sensor 6, which is, for example, made 
of a platinum resistor with a positive temperature coefficient, constitute 
a resistance bridge 5 together with resistors 7, 8, 9, and 10. The 
resistor 9 is of a variable type and gives a point Q a standard potential 
so selected as to correspond to a predetermined control value of 
temperature. A potential there appears at a point R where the sensor 6 and 
the resistor 7 are connected together which varies in accordance with the 
resistance variation of the sensor 6 subject to the temperature to be 
measured. The standard potential and the varying potential are 
respectively input to the non-inverting and the inverting input terminals 
of the comparator 4. The current to be supplied to the resistance bridge 5 
is switched on or off by a transistor 11, whose base is fed a switching 
pulse from a zero-crossing signal detector 12 which generates a pulse 
signal each time the instantaneous absolute voltage of the A.C. power 
source 18 becomes lower than a certain definite small value. The D.C. 
power to operate the comparator 4, the resistance bridge 5 and the 
zero-crossing detector 12 is supplied by a D.C. voltage source 13 
consisting of a rectifying diode 17, a resistor 16, a Zener diode 15 and a 
smoothing capacitor 14. In this embodiment, the D.C. voltage source 13 has 
the positive side of its output connected to a common line 19 of the whole 
circuit. 
In such a circuit construction, each time the voltage of the A.C power 
source 18 is varying in the vicinity of the zero level with that zero 
level included, the zero-crossing detector 12 outputs a pulse signal at a 
point S. The relationship between the A.C voltage and pulse signal is 
shown by the time-charts A and B in FIG. 3, where A and B show the A.C 
voltage and the pulse signal, respectively. Supplied with the pulse signal 
at its base, the transistor 11 is made conducting so as to allow a D.C. 
current to flow in the bridge circuit 5 only for a duration of the pulse 
signal, and at the same time, the potential at a point T, from which a 
negative voltage is supplied to the comparator 4, turns to a value 
substantially equal to the potential at the output point U of the D.C 
power source 13. Otherwise, the potential at the point T is kept at zero 
volts or the potential of the common line 19 of the whole circuit, and 
therefore, the comparator 4 is also kept active only for the same duration 
of the pulse signal. The potential at the point T is shown by a time chart 
C in FIG. 3. Under such a performance of the circuit, if the temperature 
is lower than the predetermined value, the potential at the point R is 
higher than that at the point Q when the current is supplied to the bridge 
5 due to the transistor 11 conducting, and the comparator 4 then outputs a 
negative voltage signal which is used to trigger the thyristor 2. On the 
other hand, if the temperature increases up to the predetermined value, 
the comparator output turns to zero. Even if the temperature increases 
over the predetermined value so as to invert the above potential 
difference between the points R and Q, the comparator output still remains 
constant at zero, because it is supplied only with a negative voltage 
source. Therefore, only while the temperature is under the predetermined 
control value of temperature, will the comparator 4 continue to output the 
trigger signal to the thyristor 2 each time the A.C. voltage crosses the 
zero level. The heater 1 is thus kept energized until the temperature 
rises up to the predetermined value. The A.C current through the heater 1 
and the time-variation of the potential at the output point of the 
comparator 4 are shown, respectively, in the time charts E and D in FIG. 
3. After the temperature has increased to the predetermined value, the 
potential at the point P remains zero without the generation of the 
trigger pulses, whereas the heater current is kept cut off except for half 
a cycle after the final trigger pulse. In addition, it will be understood 
from the circuit shown in FIG. 2 that the D.C. power source 13 does not 
constitute a constant-voltage source, although a Zener diode 15 is used. 
The Zener diode 15 is only for keeping the upper limit of the D.C. output 
at the Zener voltage. When the instantaneous voltage rectified by the 
diode 17 is lower than the Zener voltage, the capacitor 14 supplies the 
necessary currents. The variation of the D.C. voltage does not exert any 
bad influence on the temperature measurement, because the sensor 6 
together with the other resistors constitute a bridge circuit. Therefore, 
the resistor 16 is chosen to have a high resistance and a low wattage in 
accordance with a very small average value of the pulse-shaped output 
current supplied through the transistor 11, even if the pulsed current 
through the bridge 5 is made relatively large so as to increase the 
sensitivity of the bridge 5. 
In the following, there is shown in FIG. 4 an example of the circuit 
construction of the comparator 4, for which a commercially available IC 
operational amplifier may of course be used. Referring to FIG. 4, 
transistors 33 and 34 constitute a differential amplifier together with a 
resistor 40. The collector output current of the transistor 34, being 
amplified by a transistor 35, is supplied through a resistor 37 to a 
transistor 39 whose collector forms the final output terminal 38 
corresponding to the point P in FIG. 2. A terminal 36 which is connected 
to the base of the transistor 34 is an inverting input terminal, while a 
terminal 32 which is connected to the base of transistor 33 is the 
non-inverting terminal. The emitter of the transistor 35 is connected 
through a diode 31 to the collector of the transistor 33 and constitutes a 
terminal 30 which is to be connected to the common line 19 in FIG. 2. From 
the common line, the positive source voltage is supplied. On the other 
hand, the emitter of the transistor 39 and the other side of the resistor 
40 are supplied with the negative source voltage through a terminal 41 
which is to be connected to the point T in FIG. 2. In such a circuit 
construction, if the potential at the inverting terminal 36 is lower than 
that at the non-inverting terminal 32, the transistor 34 is kept 
non-conducting. On the contrary, if the potential at the inverting 
terminal 36 is higher, then the transistor 34 is made conducting. With the 
transistor 34 turned on so as to be conducting, the transistors 35 and 39 
are also made conducting to "pull in" a current from the terminal 38, 
which corresponds to the point P in FIG. 2. The diode 31 inserted in the 
emitter line of the transistor 35 is provided so as to make it impossible 
for this comparator to generate an output unless the negative potential at 
the inverting terminal 36 is more deeply negative than a predetermined 
negative potential. This is to prevent trigger pulse generation when the 
inverting terminal potential rises abnormally, for example, due to an 
accident short of the temperature sensor 6 (FIG. 2), which is connected 
between the inverting terminal 36 and the common line 19 (FIG. 2). As long 
as the short-circuit between the inverting terminal 36 and the common line 
is "perfect", neither the transistor 34 nor 35 is made conducting, in 
principle, due to the emitter-base resistance of the transistor 35, even 
if the diode 31 is not inserted. But, if the short-circuit has a small 
value of resistance, the potential at the base of the transistor 34 may 
possibly be kept at a enough low level so as to make the transistors 34 
and 35 conducting. By inserting the diode 31, both of the transistors are 
kept inactive, unless the potential at the base of the transistor 34 is 
kept lower than the common line potential at least by the potential drop 
in the diode 31. 
In addition, there is shown in FIG. 5 a preferable circuit example of the 
zero-crossing detector 12, for which a commercially available IC unit may 
be used. In FIG. 5, terminals 50 and 57 are D.C. power input terminals, 
and a terminal 59 is a signal output terminal, which corresponds to the 
point S in FIG. 2. An A.C. voltage from the A.C. power source 18 in FIG. 2 
is input between a terminal 51 and the terminal 50 which is connected to 
the common line 19 in FIG. 2. While the absolute value of the A.C. 
instantaneous voltage is sufficiently high, either a transistor 53 or 54 
is alternately made conducting, and a transistor 55 is kept 
non-conducting, because the potential of its base rises substantially up 
to the common line potential due to the voltage developed in a resistor 
56. The transistor 55 is made conducting only for a short time-width in 
which both of the transistors 53 and 54 are kept non-conducting due to the 
falling level of the A.C. absolute voltage down to a level equal to or 
lower than the emitter-base cut-off voltage of both transistors. The width 
of the pulse signal obtained by this zero-crossing detector circuit is 
about 100 .mu.sec., while that of a conventional zero-crossing detector is 
about 300 to 400 .mu.sec. According to this zero-crossing detector 
circuit, the power dissipation of the heater control device is reduced to 
1/3 or less. 
FIG. 6 shows an another embodiment of the heater control device based on 
the present invention. This embodiment, in which a uni-directional 
thyristor 2a is used as an A.C. power switching element, is suitable to 
control a heater of relatively low power, since the heater is supplied 
with only half cycles of the A.C power. In this embodiment, a 
zero-crossing detector 12a, a bridge circuit 5a and a comparator 4a are 
operated with a positive D.C voltage supplied. Therefore, a PNP transistor 
is used as a D.C switching transistor 11a in this circuit. The negative 
output of a D.C power source 13a is connected to the common line 19a of 
the whole circuit. In FIG. 6 all the elements which correspond to those in 
FIG. 2 are given the same reference numeral with a symbol "a" suffixed. 
While a temperature detected by a sensing bridge 5a remains lower than a 
predetermined value of temperature, a transistor provided at the output 
stage within the comparator 4a is kept non-conducting, so that a pulse 
signal from the transistor 11a is input, as a trigger pulse, to the gate 
of the thyristor 2a through a resistor 21 and a diode 20. If the 
temperature rises up to the predetermined value, said transistor within 
the comparator 4a is made conducting, and the pulse current from the 
transistor 11a flows into the comparator 4a without triggering the 
thyristor 2a. 
The present invention is not limited to the embodiments described above, 
but it is possible to make various modifications without departing from 
the spirit of the invention. Modifications, changes and equivalents to the 
appended claims should be considered as within the scope of the invention.