Thermistors in Temperature Indicator Circuits

To measure temperatures precisely, you might need a thermometer. In many instances, though, a relative estimate will do and an absolute value is not necessary. An LED may change colour or a basic LED illumination may be used to notify the user that, for instance, an electric drill or hoover cleaner is getting heated.

If there was a green light on these monitors to show that the temperature was okay, that would be much better. To alert the user when the equipment gets too hot, the light must progressively change colour as the temperature rises.

In this article, we'll design a basic temperature indicator circuit by utilising a PTC thermistor and an NTC.

when is common knowledge, an NTC thermistor is a temperature-dependent resistor, meaning that when the temperature rises, the resistor's resistance falls. Because of this, it responds to rising temperatures with a negative resistance, earning it the moniker of negative temperature coefficient thermistor (NTC).

Similar to a temperature-dependent resistor, a PTC thermistor has the opposite purpose.

A PTC thermistor reacts to a rise in surrounding temperature by increasing its resistance. Because of this, it responds positively to temperature increases and is hence known as a positive temperature coefficient thermistor (PTC).

The next straightforward temperature indicator circuit makes use of the aforementioned features of both NTC and PTC thermostors to provide a rapid temperature reading via lit LEDs.

Circuit Overview

This is precisely what this straightforward temperature indicator with a thermistor base achieves.

At a relatively low temperature, the value of R3 is low and the value of R4 is high. There will be a voltage across R3-D3 that is high enough to light the green LED during the positive half cycle of the mains voltage.

R3's value is chosen to guarantee that there is just enough voltage across it during the mains voltage's negative half cycle to light the red LED.

As the temperature rises, PTC thermistor R3's value rises and NTC thermistor R4's value falls. Over time, the red LED will begin to light up with ever-increasing brightness as the green LED gradually loses brightness, until only the red LED is completely lighted.

Capacitor C3 and resistor R2 regulate the amount of current that the LEDs draw. Dissipation is kept to a minimum with this method.

With a diameter of at least 6 mm and no less, R3 and R4 should both be of a good size. The minimum values required for the NTC and PTC thermistor at 25 °C and 25 to 33 ohms, respectively, are specified. It is important to handle the circuit carefully because it conducts the entire mains voltage.

One NTC thermocouple powers a three-LED temperature indicator.

By using this straightforward circuit, you may quickly determine whether the temperature at a specific area is within the predicted range.

A green LED shows compliance, a blue LED indicates a temperature below the acceptable value, and a red LED indicates a temperature above the required value. This makes the device's interpretation simple and easy to understand. The gadget is simple to use and portable.

State of equilibrium

Take the scenario of a reference temperature of 22.5°C, for instance. The ohmic resistance of the NTC (Negative Temperature Coefficient Resistance) for this value is equal to its indicated value, which in this case is 47 K Ohm.

Now let us compute the potential at the NTC's positive terminal.

U1 is equal to RNTC / (R2 + R4 + RNTC). nine V

We find that the potential is 4.478 V.

In an equilibrium state, 47 K Ohm is also the value of the adjustable resistor placed in the circuit's other branch, which is, incidentally, fully symmetrical.

Now let us compute the potential available at the non-inverting direct input of the operational amplifier IC1.

U2 equals [(R1 + R3 + A) / R3 + A] nine V

4.522V is the resultant value. We can see that the potential available at the direct input is more than the potential available at the inverting input when we look at the operational amplifier IC II scenario.

The operational amplifier's output 7 enters a high state as a result.

Operational amplifier I experiences the exact identical circumstances, with output 1 likewise showing a high state. The following are the effects of this:

The NAND gate III's output is in a low state, turning off the red LED L1.

The blue LED L3 is off because the NAND gate IV's output is in a low condition.

The green LED L2 is switched on and the NAND gate I's output is in a low state, resulting in a high state on the NAND gate II's output.

The outside air cools off. The ohmic resistance of an NTC increases when it is placed in a thermal environment with decreasing temperature. This temperature drop causes potential to rise in relation to the previous equilibrium state:

directly into Operational Amp I's input.

in operational amplifier II's inverting input. A new equilibrium state is created if the temperature differential is large enough (we shall revisit this).

Nothing changes for operational amplifier I: the inverting input's potential is still lower (if not higher) than the direct input's.

As before, output 1 is still in a high state.

On the other hand, with regard to operational amplifier II, the potential of the inverting input is higher than that of the direct input. Output 7 transitions to a low voltage.

It can be confirmed that only the blue LED L3 is on in this new scenario, with LEDs L1 and L2 being off.

The ohmic resistance of the NTC reduces with temperature in comparison to the original equilibrium state, and it is evident that in this instance:

amplifier I's output 1 changes to a low state.

Amplifier II's output 7 is still in the high state. LEDs L2 and L3 extinction is the consequence of this. All that's still lit is the red LED L1.

Reliability of the apparatus

As long as the temperature difference is less than 1°C, the green LED stays lighted when resistance values are utilised for a certain temperature and verified by the appropriate position of the adjustable A cursor.

Reducing the values of resistors R3 and R4 will provide a higher sensitivity. By selecting 220 ohms, for instance, the circuit responds to a variation of roughly 0.5 degrees already.

On the other hand, the values of the same resistors should be raised if a lesser sensitivity is desired. The device only responds to a fluctuation of approximately 1.5°C for 750 ohms.

Establishment

This circuit's extremely basic printed circuit board is depicted in the first figure below. There are no specific remarks that are needed. A plan of the component layout is shown in the second adjacent picture below. Keep in mind that the battery and module were adhered to immediately. Take note of the polarity.

There is nothing special that has to be adjusted with the circuit. Just positioning the movable slider to the desired location will do. The technique to be applied is actually quite easy.

To get the stable position that corresponds to the green LED lights, slowly move the adjustable slider in one direction or the other whenever the room temperature reaches the chosen reference temperature (which can be verified with a thermometer).

Circuit Layout

Extremely Basic Meter-Based Temperature Indicator

Using a 1 mA moving coil metre, a few transistors, and resistors, the following post explains how to construct a basic temperature indicator. The diode 1N4148 serves as the project's sensor device.

How It Operates

A 1N4148 diode is used to sense the temperature in the temperature metre circuit shown above. The negative temperature coefficient of the 1N4148 diode is measured by differential amplifiers Q1 and Q2 in our system. This voltage corresponds to a scale of 2 mV/°C.

An offset voltage of about 0.55 V may be visible on the diode. By modifying the RV1 setting, the offset voltage reading on the metre is adjusted.

A differential amplifier is used to ensure that temperature-related variations in transistors don't impact the calibration.

As there is no device warm-up period in this circuit, it is recommended that a push-to-ON button be used to turn on power in order to preserve battery life.

The battery drains at around 10 mA while the metre is operating, therefore if a continuous reading is required, a small power supply needs to be included.

Technical details

Range of Temperature: 0 to 100°C

Sensor: 1N4148 silicon diode or a like one

Scaling: Mostly linear from 0 to 100 degrees Celsius

Methods for Accuracy

The temperature of boiling water and melting ice serve as trustworthy reference points for the two-point calibration process of the metre.

First, fine-tune RV1 until the metre reads 0°. Then, submerge the thermometer and the diode sensor in melted ice.

Once the water is boiling, place the sensor in it (make sure to use a kettle or other container that can hold some steam) and adjust RV2 until the temperature reaches 100°.

It is imperative that you do not use an electric jug with an exposed heating element when doing this.

If you would rather use a known ambient temperature as the low reference point, you may also start by setting RV1 to have the metre read 0° when the sensor is at that temperature.

Next, make sure the metre accurately displays the temperature difference by adjusting RV2 while the sensor is submerged in hot or boiling water.

Readjust RV1 to display the true ambient temperature after the sensor reaches ambient temperature again (giving cooling time).