Transistor acting as a switch in closed circuit (with a thermistor)

In summary, the conversation discusses a circuit diagram that is being used as a switch to turn on a heater (or air conditioner) based on temperature. The confusion lies in whether the circuit is using electrical or conventional current and the role of the thermistor in the circuit. The expert advises to focus on the functionality of the circuit rather than the type of current. They also explain how the transistor acts as a switch and how the voltage across R2 determines the turn on point. The conversation ends with a discussion about the characteristics of neon lamps and their role in the circuit.
  • #1
DJ-Smiles
47
0

Homework Statement



Ok so I have been given a diagram which if it worked should be attached. I am having a fair bit of trouble understanding it. Now I am aware that there is both electrical and conventional currents but I am confused as to whether this is electrical or conventional. I have been told that the circuit is acting as a switch which when reaches a low temperature (resistance is high) will turn the heater on (in schematic it is a lamp). I have looked at the schematic and if it was conventional current the thermistor would not play a very big role would it? It appears to make more sense if the circuit was electrical current but then again the transistor seems to be collecting at the emitter and emitting at the collector as the transistor is an NPN. If I am correct in assuming it is an electrical current, what happens along the way. I know the capacitor is stored but as you move along and get to the junction where it goes up to the transistor and keeps going straight to the thermistor, what is the voltage that the transistor is exposed to at the emitter?

However if it is a conventional current, what purpose does the thermistor have? To me, it looks like to should be electrical but I am having trouble understanding if it is or if it's conventional. To follow up, where would I move the transistor if I wanted the heater (well when I move on it will be an air conditioner) to turn on when the temperature is high (resistance is low). If it is is electrical, I am thinking you swap the normal resistor with the thermistor.

Thanks to anyone who can help me!.

P.S. Sorry if the format isn't perfect, I am stressing and it may seem a bit jumbled.
 

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  • #2
Also don't take note of the number of each resistor (R1 or R2) I just drew that up and put the normal resistor first.
 
  • #3
No worries for those who see this in the future, crisis averted!
 
  • #4
I had to look it up to make sure I understood what you meant by conventional current.
http://www.mi.mun.ca/users/cchaulk/eltk1100/ivse/ivse.htm

First it should be said all circuit analysis is done with 'conventional current' or at least I am not aware of anyone who does it the other way around. Second, charge carriers in a circuit do not have to be electrons. It can be holes in semiconductors (which are positively charged and flow in the direction of conventional current), it can be ions in a solution like a battery, etc.

In summary, I find that webpage above very misleading.

Conventional current or not, it doesn't matter at all. A voltage source establishes a field orientation in the wires and with conventional current we assume the carriers are positive so that current flows away from the +ve terminal. Later on, if it matters what the actual charge carriers were doing, we reconcile conventional current with the actual carriers.

For example, if a part of the circuit is made of metal then the charge carriers are electrons. In this case, if conventional current points to the right, then electrons in the wire will actually move left. This is equivalent.

If the current is passing through a doped semiconductor like a transistor then we have holes and electrons moving. Holes are positively charged. If our circuit analysis finds conventional current flowing to the right, then electrons in the semiconductor will flow left and holes will flow right. We have both directions going. This is equivalent to conventional current to the right.

So having said that, get rid of your hangups with conventional current and from now on we can just speak of current.To your circuit: first assume Q1 acts as a switch. In reality it will act as open switch (no current flowing), then it will be partly on (in active mode where it acts like an amplifier) and then it will be all the way on like a closed switch (the voltage across Q1's collector and emitter will be about 0.3V and not change much from there). How "on" the transistor is is determined by voltage across Q1's base and emitter. Because the current through the transistor's collector is exponentially related to the voltage across its base and emitter, the base/emitter voltage of Q1, once on, will not change much from about 0.7 to 0.8 volts.

So now you will see that Q1 will turn on (in active mode or switch closed) when the voltage across R2 is at least 0.7 volts. Calculate the corresponding value of R2 and at what temperature that is achieved. Will the transistor turn on for higher or lower temperatures than this point?

The lamp is probably neon or some such thing and has specific characteristics which maybe is provided? For example, it may not conduct at all unless Q1 is able to pull a certain minimum current through it. So Q1 being a little on (active mode) may not be enough to start the lamp. It may have to be a lot on (closed switch, called saturated). This happens when the 0.7 volt drop rises a little to maybe 0.8 volts. Notice I am saying a lot of about and maybes here. As you can see, the difference is tiny and probably won't make much difference to the value of resistance R2 you find. But you may want to calculate the sensitivity of the turn on point as a function of the base emitter voltage to see if it will matter at all.

Another is neon lamps show some hysteresis. Once it fires, the current to keep it going is smaller. This is very useful. Suppose the circuit turns on at 18C. What if the temp drops to 17.9C in 30 seconds and then rises to 18.1C a minute later? Maybe this goes on for a while. The circuit will turn on and off fluctuating around the set point. The bit of hysteresis the neon lamp provides means the current must be high to start it but there is a gap to a lower current that must be reached to stop it. So this fluctuation will not occur. If the set point to start heating is 18C, maybe the setpoint to stop heating is 21C.

You will need the characteristics of the lamp to determine that.
 
  • #5


I can provide some clarification on the use of a transistor as a switch in a closed circuit with a thermistor.

Firstly, the type of current (electrical or conventional) does not affect the function of the circuit. Both types of current describe the flow of charged particles in a closed circuit, and the behavior of the circuit will be the same regardless of which type of current is used to describe it.

In this circuit, the transistor is acting as a switch, controlling the flow of current to the heater (or lamp). When the thermistor senses a low temperature (high resistance), it allows a small current to flow through the base of the transistor, which then amplifies and allows a larger current to flow from the collector to the emitter. This turns on the heater or lamp.

The capacitor in the circuit acts as a temporary energy storage device, providing a small initial current to the base of the transistor when the circuit is first turned on. This ensures that the circuit can turn on quickly when the thermistor senses a low temperature.

If you were to switch the thermistor with a normal resistor, the circuit would still function in the same way, but the temperature at which the switch turns on would be different. In this case, the switch would turn on at a high temperature (low resistance) instead of a low temperature (high resistance).

I hope this helps to clarify the function of the circuit. If you have any further questions, please do not hesitate to ask.
 

1. How does a transistor act as a switch in a closed circuit?

A transistor can act as a switch in a closed circuit by controlling the flow of current through the circuit. When a small current is applied to the base of the transistor, it allows a larger current to flow between the collector and emitter, effectively "switching" on the circuit.

2. What is the purpose of using a thermistor in conjunction with a transistor switch?

A thermistor is a type of resistor that changes its resistance with temperature. By placing a thermistor in the circuit with a transistor switch, the resistance of the thermistor can be used to control the amount of current flowing through the circuit. This allows for more precise control and can help prevent overheating of the circuit.

3. What types of circuits typically use a transistor as a switch?

Transistors are commonly used as switches in electronic circuits that require fast switching speeds, such as computer processors, power supplies, and amplifiers. They are also used in circuits that require precise control over the amount of current flowing through the circuit, such as in LED lights and motor control systems.

4. How is a transistor switch different from a mechanical switch?

A transistor switch operates using electronic signals, while a mechanical switch is manually operated. Additionally, a transistor switch can handle much higher currents and switching speeds than a mechanical switch. However, a mechanical switch is typically more reliable and has a longer lifespan than a transistor switch.

5. Can a transistor switch be used in both AC and DC circuits?

Yes, a transistor switch can be used in both AC and DC circuits. However, the circuit design and components may differ slightly depending on the type of current being used. In AC circuits, the transistor may need to be coupled with a diode to prevent current from flowing in the reverse direction.

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