Download Thermostat Pattern drawing

Thermostat
I introduce the temperature regulator(Thermostat) which used a thermistor for the temperature sensor.
This circuit can control a outside system by driving a relay when the temperature of the thermistor
becomes a setting temperature. The strict temperature setting can not be done with this circuit like the
thermometer. The resistance value of the thermistor changes by the ambient temperature. This circuit
changes the change of the resistance value into the change of the voltage using the transistor. Then, it
compares that voltage and the voltage of the setting temperature using the voltage comparator and it
drives a relay. It is a relatively simple circuit. With this circuit, the difficult point is to handle an
analog signal (the temperature change). The characteristic of the thermistor, the transistor and so on
has an influence on the performance of the equipment just as it is. Even if the thermistor or the
transistor of the same name is used, the same performance (the setting temperature range and so on)
isn't sometimes gotten. The design to have considered the ruggedness of the part is possible. This
time, I made a circuit based on the result which measured the characteristic of each part. I think that
this way becomes reference as the case when there is not data of the part.
Pattern drawing
of the thermostat
(Wiring side
The one which is shown in the pink shows the parts of the external.
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Circuit explanation
of the thermostat
The characteristic measurement of the thermistor
The thermistor which was used this time is D-53 which is made by NEC. D-53 is NTC(Negative
Temperature Coefficient Thermistor)-type thermistor. It has the characteristic that the resistance
value decreases when the temperature rises. I used this thermistor because it was possible to be
obtained by chance by me. Because I didn't understand detailed data, first, I measured the change of
the resistance value by the temperature change. As the heat source, I used the wirewound resistor of
100 W. This resistor was used as the loading resistor in case of the stabilised power unit making.
When applying 65-W electric power to this resistor, the surface temperature becomes over 150°C.
In the resistance value measurement this time, I raised a temperature in 150°C but there are many
thermistors which make +100°C or +120°C the upper limit temperature in usual use. The one which
avoided using continuously at more than +150°C is safer.
Generally, the practicable temperature of the thermistor is referred to as follows.
150°C for short-term exposures (1-24 hours)
105°C for long-term exposures (1-12 months)
Because a practicable range is decided by the thermistor, for the details, it had better confirm data.
There is self generation of heat in one of the consideration points when using a thermistor. When
applying an electric current to the thermistor, the heat occurs. In case of the NTC-type thermistor,
when many electric currents occur, the generation of heat decreases a resistance value. The vicious
circle of the increase of the electric current value and the decrease of the resistance value occurs.
Because it is, the electric current to apply to the thermistor must be made necessary and minimum.
The thermistor which was used this time has the resistance value of about 5 Kohm at 25°C. The
electric current which flows through the thermistor at this time is about 0.55
mA(12V/(16Kohm+5Kohm+1Kohm)) and the consumption electric power becomes 1.5 mW((0.55mA)2 x
5Kohm). If being this value, there is no problem. The resistance value falls when the temperature rises
and the electric current increases. However, because the resistor of 16K ohm is included, when the
resistance value of the thermistor falls, the consumption electric power of the thermistor decreases.
The voltage converter
The figure on the left is the circuit which changes the change of the resistance value of the thermistor
into the change of the voltage. It is the basic amplification circuit which used a transistor. The electric
current which flows through emitter(E) from collector(C) changes when the electric current which
flows through base(B) changes. A changing rate is shown at the current amplification
factor(hFE=IC/IB). The hFE depends on the kind of the transistor and a class is sometimes divided in
the transistor of the same name.
This time, the transistor which I used is 2SC1815(Y). At the data sheet, the hFE of the Y class is 120240. As shown in this data, the hFE is different every transistor.
At the circuit this time, the collector voltage (Vc) doesn't make be saturated in the change range with
the resistance value (Rt) of the thermistor. I make the change range of Rt 5k ohm and I decide the
proper value of R1.
R2 is 1k ohm. (For the stabilization of IB, it is as stable as low. As for being too low, it is hopeless.)
R3 is 2k ohm. (When TR1 is in the ON condition, about 6-mA electric current is passed.)
The one which was measured in above condition is the following graph.
At the circuit this time, because I wanted to use the collector voltage without making it be saturated
when Rt is less than 1k ohm, I decided the value of R1 on 16k ohm.
I measured with the circuit which was experimentally assembled to decide the rough value of R1.
After that, I measure once again after putting together a circuit finally and I do the adjustment which
included the characteristic of the related parts, too.
Because the voltage converter is an amplifier, the signal which was applied to the base is amplified
and appears at the collector. The bypass capacitors are put in each part to make an influence by the
unnecessary signal from outside little. Also, when the wire which connects with the thermistor and
the voltage converter is long, there is possibility that an alternating current signal is led to the
connection wire and that the converter malfunctions. Therefore, the shielded wire is used for the wire
which connects the thermistor.
The voltage comparator
At the circuit this time, the voltage comparator is used to detect whether or not the temperature which
was detected with the thermistor becomes the setting temperature. It compares the output voltage of
the thermistor voltage converter and the temperature setting voltage. When there is a voltage
difference, by the condition of the difference, it does ON/OFF of the relay drive circuit.
When the voltage of the voltage converter(the negative terminal) is high(the temperature is low), the
output of the comparator becomes ON condition. Because it is, the relay doesn't work(S and B are
connected).
The output of the comparator becomes OFF condition when the temperature rises and the voltage
from the voltage converter(the negative terminal) becomes lower than the temperature setting
voltage(the positive terminal). With it, the relay becomes an operating state(S and M are connected).
There are following purpose in the resistors to put to the both edges of the variable resistor for the
temperature setting.
The resistor on the side of +12 V
This is the resistor to make temperature setting voltage smaller than the maximum output voltage
of the voltage converter.
When making temperature setting maximum when there is not this resistor, the positive terminal of
the comparator becomes 0 V. Therefore, until the output voltage of the voltage converter becomes 0
V, the relay doesn't work. At the circuit this time, the output voltage of the voltage converter doesn't
become 0 V. Because it is, after never, the relay works.
When a maximum temperature is guarded with the other equipment, this resistor isn't needed. When
there is not a limitation receptacle, a resistor is needed so as not to become above the upper limit
temperature. You consider a maximum temperature and must fix an appropriate resistance value.
The relay drive circuit
The output of the comparator is an open collector(Type which supplies a power supply to the
collector from outside).
In case of LM319, the maximum permission loss electric power of the built-in transistor is 500 mW.
Because it is, the small relay can be directly driven.
The circuit this time isn't doing to the such circuit. However, I changed into the drive circuit which
used a transistor because it was unstable that the relay works. It wasn't worked when it should work.
When the output of the comparator is ON(It detects below the setting temperature), the base of the
drive transistor becomes grounding voltage approximately. Therefore, the electric current doesn't
flow through the base of the drive transistor and the transistor becomes OFF condition. The relay
doesn't work.
When the output of the comparator is OFF(It detects above the setting temperature), the electric
current flows through R7 and R8 into the base of the drive transistor and the transistor becomes ON
condition. The relay works.
When not using an drive transistor, the operating state of the relay becomes opposite.
Parts explanation
of the thermostat
The thermistor (D-53)
This is the thermistor which was used this time. It is doing a disk type in D-53 which is made by
NEC.
The lead line which isn't insulated is put to the body of the thermistor. Because it is, when using, the
lead wire must be insulated with the endured insulating material to the high temperatures such as the
glass fiber tube.
As for the size of the thermistor, the diameter of the disk is 7 mm and the thickness is 2 mm.
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The transistor (2SC1815
This transistor is used with the thermistor voltage converter and the relay drive circuit.
In case of the thermistor voltage converter, hFE becomes an important factor. I used Y type as the
class of hFE.
The voltage comparator (LM319)
I used LM319 as the voltage comparator. The other comparator is OK.
I think that the comparator which has a hysteresis characteristic suits more.
As for the way of making have the hysteresis characteristic using the resistor, refer to "Circuit
explanation of the temperature controller of the bending apparatus".
Data sheet for LM319