Optimizing Temperature Alarm Circuit with Thermistor and Resistor

[Stripe]

Homework Statement

Ok so for Diagram A:
It is desired to set up an alarm to indicate when it is too cold. For this purpose the circuit shown is to be used and it is intended that the globe will go on only when the temperature falls below 18C. The positions A and B in the circuit are to be occupied by a resistor and a thermistor (20 ohms resistance at 100C and 500 ohms resistance 0C.)
Which of the following combinations would be successful?

A is thermistor B is 1 ohm resistor

A is thermistor B is 100 ohm resistor

A is a 1 ohm resistor B is a thermistor

A is a 100 ohm resistor B is a thermistor

AND then diagram B:

It was imagined that the globe should glow but did not. If all components were correctly soldered, which of the following could best explain the fault?

The thermistor is too hot.

The base voltage needs to be lowered

Resistance of the transistor is too low

The 100 Ohm resistance should be bigger.

Homework Equations

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The Attempt at a Solution

Ok so part a, i thought that the answer was one of the first 2, because i thought that because the electricity would reach a first, it would have to be changed there before it reaches the base.

I thought that if A was a normal resistor, say 100 ohm, regardless of what happens at B, the base will always receive the same voltage because if B was the thermistor in this case, it would only affect the voltage after it?

Along with this could someone explain the importance of A and B? i thought the base would still work if b was not present? Then i thought maybe B was there to create a parallel circuit so A could work?

For part B, the answer was that the 100 ohm resistor should be bigger. BUT WHY? isn't the 100 ohm resistor located AFTER the transistor so it makes no difference?

and i was told that voltage never changes but current does? GOD I AM CONFUSED

:| please help

 

[Delphi51]

Well, the circuit is too complicated to understand at a glance, but if you realize that the current through the base of a transistor is "very small", and ignore it - then you can understand it. Just ignore the transistor for a moment. Then the resistors a and b form a voltage divider; they pick out part of the battery voltage. If a and b have equal resistance, then the voltage where they join will be half the battery V. If b is bigger than a, it picks off more of the battery V. In fact the voltage is Rb/(Ra +Rb)*battery V.

V must increase to turn the transistor on so it will turn on the light. So b has to increase (or a has to decrease) when it gets cold. This determines whether you put the thermistor at a or at b.

What base voltage do you need to turn on a transistor? I think it is 0.7 but I'm not sure (it has been a while for me). Anyway, if you know the turn-on V, and can estimate the resistance of the thermistor at 18 degrees, you can calculate the R for the fixed resistor.

 

[tomcue]

I would like to clarify some concepts and provide a clear explanation for the questions posed. Firstly, the purpose of the circuit is to detect when the temperature falls below 18C and trigger the globe to turn on. The thermistor, a type of resistor whose resistance changes with temperature, is used to sense the temperature and the resistor is used to control the flow of current in the circuit.

In terms of the combinations, either A or B can be the thermistor, as long as the other component is a resistor. The key is that the thermistor should have a lower resistance at lower temperatures, so that when the temperature drops below 18C, the overall resistance of the circuit decreases and the globe turns on. Therefore, options A is thermistor B is 1 ohm resistor and A is a 100 ohm resistor B is a thermistor would be successful.

Now for the importance of A and B in the circuit, both components play a crucial role. A, which can either be the thermistor or a resistor, controls the amount of current that flows through the circuit. B, which is the other component, acts as a voltage divider and helps to regulate the voltage that reaches the base of the transistor. The base voltage needs to be at a certain level for the transistor to turn on and allow current to flow to the globe. Therefore, both A and B are necessary for the circuit to function properly.

For part B, the 100 ohm resistor should be bigger because it is used as part of the voltage divider and affects the voltage that reaches the base of the transistor. If the resistor is too small, it will allow too much current to flow and the base voltage will be too high, preventing the transistor from turning on. Therefore, a larger resistor is needed to regulate the voltage and ensure the circuit works properly.

In terms of voltage and current, it is important to understand that they are related but different quantities. Voltage is the potential difference between two points in a circuit, while current is the flow of electric charge. In a series circuit, the voltage is shared across all components, while the current remains constant. In a parallel circuit, the voltage is the same across all components, while the current is divided among them. Therefore, in this circuit, the voltage at the base of the transistor will change depending on the resistance of A and B, but the overall voltage in the circuit will remain the same.

I hope this explanation helps