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Circuit explanation
of the thermostat
I mention the point when I design a circuit this time below.
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.
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 120-240. 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 6mA 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.
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
positive terminal is
higher than the voltage of
the negative terminal, the
inner transistor of the
comparator becomes OFF
condition (the H level
condition). Then, when
the voltage of the positive
terminal is lower than the
voltage of the negative
terminal, it becomes ON
condition (the L level condition).
At the circuit this time, the voltage of the voltage converter is connected with the negative
terminal of the comparator and temperature setting voltage is connected with the positive
terminal. I am doing like this in the relation of the pattern of the printed board. This is OK even if
it is opposite but the output of the comparator becomes opposite.
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
i
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.
The output voltage of the voltage converter
doesn't become a power supply voltage even if it
maximum. A few electric currents flow through
the input of the comparator. With this electric
current, the voltage drop occurs with the resistor
which connects with the collector of the voltage
converter. When making temperature setting
voltage a power supply voltage, the temperature
setting voltage always becomes higher and it becomes not possible to do a temperature
adjustment. The relay always becomes an operating state.
If careful when using, it is OK even if it doesn't put a resistor.
The resistor on the side of the grounding
This is the resistor which decides the upper limit with setting temperature.
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.
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
th
tti t
t ) th b
f th d i t
it
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.