# Why Does a Lamp Light Up Slowly When Connected with a NTC Thermistor?

• moenste
In summary, in this conversation, a student is provided with a 2 V cell, a lamp, a switch, and a thermistor with a negative temperature coefficient of resistance. When the thermistor is connected in series with the lamp and the switch is turned on, the lamp lights up slowly due to the thermistor slowly passing the current from the 2 V cell. If the lamp is omitted and the switch is turned on, the cell is soon destroyed by overheating because the thermistor loses its resistance with the increase in temperature, subsequently becoming more like a wire or small value resistor. This leads to a decrease in the load resistance and an increase in the current, causing the battery to overheat and destroy itself.
moenste

## Homework Statement

A student is provided with a 2 V cell, a lamp, a switch and a thermistor with a negative temperature coefficient of resistance. The lamp, which is in series with the cell as in Figure 1, lights immediately the switch is turned on.

Explain why (a) when the thermistor is connected in series with the lamp, as in Figure 2, and the switch is turned on, the lamp lights up slowly, and (b) if the lamp is omitted, as in Figure 3, and the switch is turned on, the cell is soon destroyed by overheating.

2. The attempt at a solution
We have a NTC thermistor, whose resistance decreases with the increase in temperature. I'm still not sure whether I got the usage of this apparatus. It is used in order not to create large voltage rises for the devices like lamps. Is it correct?

(a) If my understanding of this divide is corrrect, than the lamp lights slowly because the thermistor slowly passes the current from the 2 V cell.

(b) If the resistance of the thermistor decreases with the increase in its temperature, then over time it will have no resistance and the battery will ovearheat itself?

Where am I wrong / correct?

moenste said:
If the resistance of the thermistor decreases with the increase in its temperature, then over time it will have no resistance and the battery will ovearheat itself?
Before the battery gets overheated, what will happen to the thermistor itself?
moenste said:
If my understanding of this divide is corrrect, than the lamp lights slowly because the thermistor slowly passes the current from the 2 V cell.
Right. Can you say why?

moenste
cnh1995 said:
Before the battery gets overheated, what will happen to the thermistor itself?
Shouldn't it become an ammeter? An ammeter has no resistance, and also will be a thermistor. No?

cnh1995 said:
Right. Can you say why?
Because the thermistor is designed to keep other devices from getting too much voltage?

moenste said:
Shouldn't it become an ammeter? An ammeter has no resistance, and also will be a thermistor. No?
Yes. That is correct. I actually misread the OP and thought the thermistor itself will get burned before the battery because of increased temperature, but we can't say that without knowing more about the thermistor.
moenste said:
Because the thermistor is designed to keep other devices from getting too much voltage
Initially, thermistor resistance will be in series with the lamp, which will create a voltage divider and the bulb will get a reduced voltage. So it will light up slowly.

moenste
(a) question:
cnh1995 said:
Initially, thermistor resistance will be in series with the lamp, which will create a voltage divider and the bulb will get a reduced voltage. So it will light up slowly.
Alright, so for the first question let's sum up. The resistance of the thermistor will be in series with the lamp and will divide the voltage between the thermistor and the lamp and therefore the lamp will get a reduced voltage. And since the thermistor loses it's resistance over the temperature, the lamp will get more and more voltage until it will have a 100 % light.

(b) question:
cnh1995 said:
Yes. That is correct. I actually misread the OP and thought the thermistor itself will get burned before the battery because of increased temperature, but we can't say that without knowing more about the thermistor.
The cell will be destroyed by overheating because the thermistor will loose its resistance with the increase in temperature and therefore it will become an ammeter subsequently. That's the answer?

And also is something like this correct: since the current will only go through the battery so this is the reason why the cell will be destroyed?

moenste said:
The cell will be destroyed by overheating because the thermistor will loose its resistance with the increase in temperature and therefore it will become an ammeter subsequently. That's the answer?
It can't actually "become an ammeter". It won't suddenly grow a meter movement and test leads. It may share an important characteristic with ammeters, a low resistance, but that's as far as its "ammeterness" goes. A better comparison would be that it becomes more like a wire or small value resistor.

moenste
gneill said:
It can't actually "become an ammeter". It won't suddenly grow a meter movement and test leads. It may share an important characteristic with ammeters, a low resistance, but that's as far as its "ammeterness" goes. A better comparison would be that it becomes more like a wire or small value resistor.
And the battery will be destroyed by overheating because it has no resistance in the circuit?

moenste said:
And the battery will be destroyed by overheating because it has no resistance in the circuit?
Yes. As the load resistance gets smaller the current in the circuit rises, and so too must the power dissipated by the battery's internal resistance. This heats the battery. If the load resistance were to go to zero, the only limiting factor for the current would be battery's own internal resistance. As the battery gets hotter it's chemical reactions quicken, causing more current, more heat, and so on until the battery destroys itself.

moenste

## 1. What is an NTC thermistor?

An NTC thermistor, or Negative Temperature Coefficient thermistor, is a type of temperature sensor that exhibits a decrease in resistance as temperature increases. It is made of a semiconductor material and is commonly used in electronic circuits for temperature sensing and temperature compensation.

## 2. How does an NTC thermistor work in a circuit?

When an NTC thermistor is connected in series with a power source and a load, the current passing through the circuit causes the thermistor to heat up. As the temperature increases, the resistance of the thermistor decreases, causing a decrease in the overall circuit resistance. This change in resistance can be measured and used to determine the temperature.

## 3. What are the advantages of using an NTC thermistor in a circuit?

An NTC thermistor is a cost-effective and accurate temperature sensor that can be easily integrated into electronic circuits. It has a wide temperature range and a fast response time, making it suitable for a variety of applications. Additionally, NTC thermistors are highly stable and have a long lifespan.

## 4. What are some common applications of NTC thermistors in circuits?

NTC thermistors are commonly used in temperature monitoring and control systems for devices such as ovens, refrigerators, and air conditioners. They are also used in battery chargers, power supplies, and motor control circuits to provide temperature compensation and protection against overheating.

## 5. How do you choose the right NTC thermistor for a circuit?

The key factors to consider when choosing an NTC thermistor for a circuit include the temperature range, accuracy, response time, and stability requirements of the application. Additionally, the thermistor's resistance-temperature curve and thermal time constant should be matched to the circuit's design and operating conditions. Consulting with a thermistor manufacturer or using online selection tools can help in finding the most suitable NTC thermistor for a specific circuit.

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