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.