(6)The Potential Divider

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  • Created on: 25-03-14 13:24
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The Potential Divider.
What is a potential divider? Background
At its simplest, a potential divider is a circuit with a voltage source and resistors in series.
The potential of the voltage source (e.g. power supply) is divided in the ratio of the
resistances. This means that if you had a 2 ohm resistor and a 3 ohm resistor, there would
be 2/5 of the p.d. across the 2 ohm resistor and 3/5 of p.d. across the 3 ohms. Therefore
resistances can be chosen to get a required voltage across one of the resistors.
For example:
A series circuit is set up with a switch and two resistors. In the circuit R1 has R1 / R1+R2 of
total resistance. Therefore Vout = R1 / R1+R2 * Vs (voltage at switch)
If Vs = 9V and you wanted Vout to be 6V then R1 / R1+R2 would have to be 2 / 3 which gives
the ratio of R1 to R2 as 1:2. This would mean that you could have a resistance of 200 ohms
for R1 and 100 ohms for R2.
This type of circuit is mainly used for calibrating voltmeters, which have a very high
resistance. If something with a relatively low resistance is put through R1 there can be
problems. Effectively you would have two resistors in parallel, which will always have a total
resistance less than R1. That would mean that Vout would be less than the value calculated,
and will depend on what is connected to R1.
For two different Vout values i.e. V1 and V2
V1 / V2 = R1 / R2
This equation shows that the ratio of the p.d.s across each resistor is equal to the resistance
ratio of the two resistors.
Light or Temperature switch
An LDR (lightdependent resistor) has a very high resistance in the dark, but a lower
resistance in the light. An NTC thermistor has a high resistance at low temperatures, but a
much lower resistance at high temperatures (it varies oppositely to a normal resistor, only
more so). When resistance decreases Vout will increase (and vice versa).
Either of the components mentioned above can be used as one of the resistors in a potential
divider, giving an output voltage which varies with the light level or temperature. Adding a
transistor creates a switch (e.g. to turn on a light or heating system). A transistor is on when
voltage across it is high and off when voltage is low.
A potentiometer has a variable resistor replacing R1 and R2 of the potential divider, but it
uses the same idea (it can sometimes be called a potential divider). The relative sizes of R1
and R2 can be adjusted. This enables Vout to be varied from 0V up to the source voltage.
This can be useful when needing to change a voltage continuously, such as the volume
control of a stereo.

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Side note: Practice a good amount of questions for this section.
· A potential divider is basically two resistors.
· The current flows through the two resistors which will split the potential
· From the equations show right, you can see that the total potential difference across the
two resistors have been divided up by a ratio of their two resistances.…read more

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