one of the
most common types of temperature sensors, the thermistor. Thermistors are used
widely as self-resetting fuses, self-regulating heating elements, and
temperature sensors.
what is a
thermistor?
Well, it's
short for thermal resisters, a thermistor is a very simple, discrete
two-terminal device whose effective resistance values change with temperature.
Let's take, for example, a thermistor that is in a 25 degrees Celsius
environment..Using an ADC, we can measure the voltage drop, V temp, across a thermistor
and find that it is 2.256 volts. From there, you can use any temperature
conversion method and software to calculate what temperature the 2.256 volts
across a thermistor corresponds to.
What does
thermistor mean?
A resistor whose resistance varies rapidly and
predictably with temperature and as a result can be used to measure
temperature.
Thermistor
work
Thermistors are
temperature sensors. They are semiconductor devices whose resistance varies
with temperature, and the ones we'll be looking atnare
negative-thermal-coefficient sensors. There also exists
positive-thermal-coefficient thermistors, but those tend not to be used as much
for sensors.
Two
thermistor
Let's take a
look at a couple of thermistors. I have here two thermistors. This is a very common
sort of packaging for a thermistor, where it's basicallybjust a little bead of
semiconductor in some sort of sealing package and two wires coming out the end.
This is a slightly different version. It's again got a bead of semiconductor,
but here the wires and the thermistor are covered with an epoxy coating.
What
is epoxy coating?
That epoxy
coating is basically waterproof. They don't guarantee its waterproofness, but
for our purposes it's waterproof enough. You can dip this thermistor into
liquid and not short things out.
Use
in lab
These are the
onesthat we'll be using for our labs, because that waterproof property means
that we can stick them into a water bath and measure the temperature of a water
bath with them. Water baths are a good way to keep a fairly constant
temperature that we can measure with the thermometer and with the thermistorand
so calibrate the thermistors. The particular device that we're using there,
this black one with the epoxy, has a part number that's quite long. It starts with
NTC,which stands for negative thermal coefficient.
Thermistor,
PTC or NTC Used
The terms
thermistor, PTC or NTC are often used for temperature sensors. But what is
hidden behind them and what distinguishes these sensors? Let's look at that
now. Thermistors are electrical resistors based on semiconductors. They change
their resistance depending on the temperature.
What
is PTC thermistors
PTC thermistors
have high conductivity at low temperatures. The resistance value increases as
the temperature rises. Therefore, we speak of a positive temperature
coefficient, the English acronym for this is PTC.
NTCs
NTCs have a high
conductivity at high temperatures. The resistance value decreases with
increasing temperature. Therefore, we speak of a negative temperature
coefficient, the English acronym for this is NTC.
Thermistors,
i.e. NTC and PTC, have a strong non-linearity. This means that the graph of the
electrical resistance is linear to the temperature only in a very limited
range. Thermistors are therefore well suited for temperature measurement in
limited measuring ranges or as a limiter for inrush or continuous currents.
Apart from what I mentioned earlier: from a physical point of view, platinum
resistance sensors are also PTC elements, because their resistance increases
with temperature. But compared to semiconductor elements, they have a much
better linearity and can therefore be used in very wide ranges for accurate
temperature measurement.
Light dependent resistors and Thermiator
Light dependent
resistors (or LDRs) and thermistors are resistors that have
varying resistance, depending on the light intensity or
temperature, respectively. This is the symbol for an LDR. as a
symbol equation, V = IR. There is a proportional relationship
between resistance and potential difference, so if resistance of a
component This is the symbol for a thermistor. This graph shows the
relationship between temperature and resistance of a thermistor.
Let’s look at
how light dependent resistors can be used in a circuit. This circuit
contains a battery, a light dependent resistor and a bulb. It
also has 2 voltmeters, connected in parallel with each component,
to measure the potential difference across each component. The question
we want to answer is: How does changing light intensity affect the
brightness of this bulb?
How does changing light intensity affect the brightness
of this bulb?
Before we look
at each condition, we need to remember that potential difference
is shared between the components in a series circuit. That
means if the battery has a potential difference of 6 V, and the
LDR has a resistance of, for example, 2 V, then the bulb must
have a potential difference of 4 V. That’s because the potential
difference of the LDR and the bulb must add up to the
potential difference of the battery (6V). If potential difference
across the LDR increased, for example to 4 V, then the
potential difference across the bulb would decrease to 2 V. Let’s first think
about what would happen during the daytime, when light intensity is
high. We know that as light intensity increases, resistance of
an LDR decreases, so when light intensity is high, resistance
will be low. Because potential difference is proportional to
resistance, if resistance is low, potential difference will also be
low. This means there will be a low potential
difference across the LDR - let’s say 1 V. Because the
potential difference of both components must add up to 6 V, the bulb
must have a potential difference of 5 V. This high potential
difference would cause the bulb to be brightly lit. So in high
light intensity, the bulb turns on.
what happens at night,
Now let’s
consider what happens at night, when the light intensity is
low. At low light intensity, resistance of an LDR is high. A
high resistance across the LDR means that the potential difference
across it will be high - let’s say 5 V. That means the potential
difference across the bulb would be 1 V. This low potential
difference would cause the bulb to be dimly lit. That means the bulb
is dim or turns off when it’s dark.This is useful for getting your
mobile phone screen to go dark when you put it to your ear - the low
light intensity causes the screen to go dark.
How
thermistors are used to control appliances as temperature changes.?
Now let’s look
at how thermistors are used to control appliances as temperature
changes. This circuit is very similar to the previous example,
but in this case there is a fan instead of a bulb - this could
be the cooling fan inside a computer, which needs to turn on or
speed up when the components inside the computer get too hot.As the
computer is used, the temperature of the components increase.
The resistance of a thermistor decreases as temperature increases,
so when the thermistor gets hot, there will be a low resistance. The low
resistance of the thermistor means the potential difference
across the thermistor will be low - let’s say
1 V. This means
the potential difference across the cooling fan will be 5 V.
This high potential difference causes the fan to turn on or
speed up, which cools down the computer.As the computer cools down, the
temperature of the thermistor decreases. When temperature is
low, the resistance of the thermistor is high. This means the
potential difference across the thermistor will also be high - let’s
say 5 V. The potential difference across the fan decreases to
1 V, and the low potential difference causes the fan to slow down or even
turn off. This means the computer’s cooling fans turn off when
the temperature is low and they are no longer needed.
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