username2816144
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Hey guys,

I was doing a quesiton in the book and it asked, what is the purpose of a variable resistor and why is used in almost every circuit. Is it because it maintains a constant current as without a resistor, the current would gradually increase or something, Im probably wrong though.
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The Good Doctor
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I don't know exactly what the model answer for your exam would be, but this is how I would answer it. A variable resistor has 3 terminals and basically makes a potential divider circuit (look that up), varies the proportion of one resistance compared to the other so can be used to output a controllable DC voltage, from zero to the DC voltage that is applied across the variable resistor.

To my knowledge, they are usually used to provide an easily adjustable reference voltage that is read by some sort of voltage sensor in order to control some sort of corresponding continuous output quantity - which is handled by other circuitry. This could be anything from the adjustable brightness of a light, to the speed of an electric motor.

I hope that wasn't too confusing. Tell me if it was and I will try to explain it another way.
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uberteknik
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(Original post by The Good Doctor)
I don't know exactly what the model answer for your exam would be, but this is how I would answer it. A variable resistor has 3 terminals and basically makes a potential divider circuit (look that up), varies the proportion of one resistance compared to the other so can be used to output a controllable DC voltage, from zero to the DC voltage that is applied across the variable resistor.

To my knowledge, they are usually used to provide an easily adjustable reference voltage that is read by some sort of voltage sensor in order to control some sort of corresponding continuous output quantity - which is handled by other circuitry. This could be anything from the adjustable brightness of a light, to the speed of an electric motor.

I hope that wasn't too confusing. Tell me if it was and I will try to explain it another way.
Be careful with your terminology.

What you have described is a potentiometer.

A variable resistor is a slightly different configuration. i.e. a potentiometer with one of the track ends shorted to the wiper arm. This provides a continuously varying resistance from 0 to the maximum value of the resistance track.

It may appear to be nit-picking, however the distinction is critical when describing the function and action of a specific application.
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uberteknik
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(Original post by Hgdfu)
Hey guys,

I was doing a quesiton in the book and it asked, what is the purpose of a variable resistor and why is used in almost every circuit. Is it because it maintains a constant current as without a resistor, the current would gradually increase or something, Im probably wrong though.
When designing a circuit, the calculated values of required resistances often do not tally with available values of fixed resistors. In this instance, if the required designed value of resistance needed is critical for the operation of the circuit, then a variable resistor can be used to overcome that inadequacy and trimmed to an exact match.

Variable resistors are also used in sensor applications such as requiring a Wheatstone bridge to balance a circuit and find the value of an unknown variable.

They are also used in analogue circuits like amplifiers to control the gain of the circuit or to adjust the break frequency of filters. Extending that application, they are used to control things like oscillation frequency, Q-factors and phase shifting in resonant circuits, offset nulling compensation for operational amplifiers, semiconductor d.c. biasing and the list goes on.
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username2816144
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Ok thanks, but in a standard circuit where we are attempting to find the resistance of a resistor, we would add a variable resistor correct as well. Does the Current increase over time, so wouldnt the variable resistor be used to maintain a constant current in this circuit?
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uberteknik
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(Original post by Hgdfu)
Ok thanks, but in a standard circuit where we are attempting to find the resistance of a resistor, we would add a variable resistor correct as well. Does the Current increase over time, so wouldnt the variable resistor be used to maintain a constant current in this circuit?
What do you mean by 'standard circuit'?

Without knowing the context of your experiment I cannot provide an answer.

It is true that in a series circuit where the unknown component changes resistance over time, then a variable resistor can be used to compensate and keep a constant current drawn from the supply.

i.e.

Rtotal = Runknown + Rvariable = constant.
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