the potential divider electricity lesson 10. learning objectives to know what a potential divider...
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The Potential Divider
Electricity Lesson 10
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Learning Objectives
To know what a potential divider is.
To derive and know how to use the potential divider equation.
To know that the load affects the output of a potential divider
To know the applications of potential dividers; to detect temperature or light levels.
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Definition
What is potential divider?
A potential divider consists of two or more resistors connected in series with each other and a source of fixed potential difference.
Draw a circuit diagram for this arrangement.
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Circuit Diagram
What is the pd across each resistor? We can use potential difference
between points A and B as a supply for another component.6 V
50 100
A B
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Uses of a Potential Divider
To supply a fixed pd between any value between zero and the source pd.
To supply a variable pd from a fixed one.
To supply a pd that varies with physical changes e.g. position, temperature or light level
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VS
V1
V2R2
R1
Another Example
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Supplying a fixed pd
The total resistance of the circuit = R1 + R2
The current through the resistors = V0/(R1 + R2)11 IRV
22 IRV
21
101
RR
RVV
21
202 RR
RVV
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Deriving Potential divider equation
For an unloaded potential divider the current is the same through both resistors
So the voltage is proportional to the resistance
2
1
2
1
IRIR
VV
2
1
2
1
RR
VV
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Potential divider equation
If R1 >> R2 then V1 is more or less the supply voltage
If R1 << R2 then V1 is close to 0 V.
V0 as an input to the potential divider and V1 as an output. The circuit itself provides a way to tap off a voltage between 0 V and V0.
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Confused?
So what’s the difference between a variable resistor and a potential divider?
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Confused?
When a variable resistor is used the total resistance of the circuit is being altered.
Also, the pd is not being taken across the variable resistor.
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VINV
V out
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Using potential dividers
Use as a volume, brightness or contrast control.
Making and designing a circuit to use as a temperature sensor
Making and designing a circuit to use as a light sensor
Controlling logic devices
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Effect of different resistive loads
Output voltage is affected by load resistance
Connecting a resistor across the output reduces the output voltage
Shorting out across a bulb reduces the total resistance of the bulb – the wire is in parallel with the bulb
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Questions A series circuit is connected as shown in the
diagram. 1. What is the potential difference between A
and B?
2. An additional resistor of 100 W is connected between the 50 W resistor and the cells. What is the potential difference between A and B now?
3. The additional 100 W resistor is now connected in parallel with the first 100 W resistor. What is the potential difference between A and B now?
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Question
How do we get a variable supply voltage from a fixed one?
How can we measure physical changes in position, temperature or light level etc.
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Answer
A potential divider is one way of producing a variable p.d.
A combination of a suitable sensor (angle / position sensor, thermistor or LDR) and a potential divider enables measurement / monitoring / control of physical changes.
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Alternative formula
The potential divider equation can be derived by rearranging the ratios above to give:
V output = R1 / (R1+R2) V input.