Current Voltage Dividers2

8/19/2019 Current Voltage Dividers2

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Voltage dividersVariable resistors

ElectronicsSimple voltage dividers

Terry Sturtevant

Wilfrid Laurier University

October 1, 2012

Terry Sturtevant Electronics Simple voltage dividers

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Voltage divider (no load)Voltage divider (with load RL )

Applications of Kirchhoff’s Voltage Law


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V s R 1

R 2


V l di id V l di id ( l d)

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V s R 1

R 2

V s = V R 1 + V R 2


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Voltage dividers Voltage divider (no load)


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V s R 1

R 2

A voltage divider “divides” the supply voltage

It’s useful when you need a different voltage than the supply



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V s R 1

R 2

Part of V s across R 1





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V s R 1

R 2

Part of V s across R 1

V R 2 = V s − V R 1





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Since the current is the same in both resistors, the voltage isdivided between the two; thus it is a voltage divider.



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gVariable resistors

g ( )Voltage divider (with load RL )


Voltage divider circuits are very common, even when one or both circuit elements aren’t resistors.



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gVariable resistors



Voltage divider circuits are very common, even when one or both circuit elements aren’t resistors.



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gVariable resistors


V s

R 1

R 2

V out


Voltage dividers Voltage divider (no load)( )

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Variable resistors Voltage divider (with load RL )

V s

R 1

R 2

V out

Usually a voltage divider is drawn like this so it looks like a

ladder.


Voltage dividersV i bl i

Voltage divider (no load)V l di id ( i h l d RL )

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V s

R 1

R 2

V out

Usually a voltage divider is drawn like this so it looks like a

ladder.As you climb the ladder, the voltage increases.


Voltage dividersV i bl sist s

Voltage divider (no load)V lt di id ( ith l d RL )

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V s

R 1

R 2

V out

Since I 1 = I 2,




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V s

R 1

R 2

V out

Since I 1 = I 2, (by Kirchhoff’s current law,)




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V s

R 1

R 2

V out

Since I 1 = I 2, (by Kirchhoff’s current law),

and V s = V R 1 + V R 2 ,




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V s

R 1

R 2

V out


and V s = V R 1 + V R 2 ,(by Kirchhoff’s voltage law),




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V s

R 1

R 2

V out


and V s = V R 1 + V R 2 ,(by Kirchhoff’s voltage law),so V s = I 1R 1 + I 1R 2




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g ( )

V s

R 1

R 2

V out


and V s = V R 1 + V R 2 ,(by Kirchhoff’s voltage law),so V s = I 1R 1 + I 1R 2 = I (R 1 + R 2)




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( )

V s

R 1

R 2

V out


and V s = V R 1 + V R 2 ,(by Kirchhoff’s voltage law),so V s = I 1R 1 + I 1R 2 = I (R 1 + R 2)

I = V s R 1+R 2




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V s

R 1

R 2

V out

So V out = V 2 = IR 2




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V s

R 1

R 2

V out

So V out = V 2 = IR 2 = V s R 1+R 2

R 2




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V s

R 1

R 2

V out

So V out = V 2 = IR 2 = V s R 1+R 2

R 2

= V s

R 2R 1+R 2




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V s

R 1

R 2

V out

If R 1 gets smaller , then

V out = V s R 2R 1+R 2

gets bigger .




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V s

R 1

R 2

V out

V out = V s

R 2R 1+R 2




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V s

R 1

R 2

V out

If R 1 gets bigger , then

V out = V s R 2R 1+R 2

gets smaller .




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Voltage divider (no load)




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If R 1 = 5Ω , R 2 = 10Ω, V s = 5V




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If R 1 = 5Ω , R 2 = 10Ω, V s = 5V

V out




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If R 1 = 5Ω , R 2 = 10Ω, V s = 5V

V out

= V s

R 2R 1+R 2

= 5

105+10


Voltage dividersVariable resistors Voltage divider (no load)Voltage divider (with load RL )

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If R 1 = 5Ω , R 2 = 10Ω, V s = 5V

V out

= V s

R 2R 1+R 2

= 5

105+10

= 3.3V



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Voltage divider (with load RL )

V s R 1

R 2 R L

Load will reduce the output voltage



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V s R 1

R 2 R L



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V s R 1

R 2 R L

,

Some current goes through R 2, but some goes through R L so theeffective value of R 2 is reduced.



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If R 1 = 5Ω , R 2 = 10Ω, V s = 5V and R L = 10Ω


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Parallel resistance of R 2 and R L = R 2R LR 2+R L

= 10×1010+10 = 5Ω



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= 10×1010+10 = 5Ω

Thus V out



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= 10×1010+10 = 5Ω

Thus V out

= V s R p

R 1+R p = 5

55+5





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= 10×1010+10 = 5Ω

Thus V out

= V s R p

R 1+R p = 5

55+5

= 2.5V


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V i bl i t


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Variable resistors

Often it is useful to have variable resistors in a circuit.



Variable resistors

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Variable resistors

Often it is useful to have variable resistors in a circuit.These are sometimes called potentiometers or trimmers.



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Here is a trimmer.



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Here is a trimmer. The top line should look familiar.


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The potentiometer has three pins.

The resistance given is between the two end pins.





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The third pin is called the wiper.



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A small screwdriver can be used to move the wiper from one

end to the other, or anywhere in between.



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end to the other, or anywhere in between.The resistance between the two end pins will be constant.





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end to the other, or anywhere in between.The resistance between the two end pins will be constant.

If you want a resistance which varies, just use the wiper andone end pin.



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Here’s a different view. The wiper is in the middle.



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Voltage dividers

Variable resistors

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From the top, this one has 10 dashes to represent intervals of roughlyR /10.


Voltage dividers

Variable resistors

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Voltage dividers

Variable resistors

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This is a slightly different style.


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Voltage dividers

Variable resistors


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The potentiometer can be used for a variable voltage divider.


Voltage dividers

Variable resistors

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Connect the two ends of your supply to the two end pins.


Voltage dividers

Variable resistors

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Measure the output voltage on the wiper.


Voltage dividers

Variable resistors

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Measure the output voltage on the wiper.

Adjusting the wiper will change the output voltage from oneend of the supply to the other, or to anywhere in between.

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Current Voltage Dividers2

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