A voltage divider is a simple circuit consisting of two resistors that has the useful property of
changing a higher voltage (Vin) into a lower one (Vout). It does this by dividing the input
voltage by a ratio determined by the values of two resistors (R1 and R2):
This circuit is best for low-current applications like sensor and data lines. If you draw too
much current through Vout it will affect the output voltage. Therefore this shouldn’t be used
for high-current applications like power supplies (voltage regulators are a much better
option).
To pick resistors, use the following equation:
You can also find a number of voltage divider calculators using Google.
Because the output voltage depends solely on the ratio of R1 to R2, you could use a number
of different R values to get the same output (for example, if R1 = R2, the output will always
be half of the input, whether R is 1 Ohm or 1M Ohms). For most of our purposes, the total
resistance (R1 + R2) should be between 1k Ohms and 10k Ohms. Less than that and the
circuit will waste a lot of power flowing through R1 and R2 to ground. More than that and
Vout may not be able to source enough current to drive an analog input.
This circuit is very useful for turning the output from a resistive sensor (such as a thermistor
or force-sensitive resistor) into a voltage you can measure using an analog to digital
converter. R2 will be your sensor and a good rule of thumb is to choose R1 to be halfway
between the lowest and highest resistance values of the sensor.
For example, our Mini Photocell has a light resistance of 1k Ohm, and a dark resistance of
10k Ohm. A 5.6k resistor is just about halfway between the high and low values. If we
connect the sensor as R2 and the 5.6k as R1, we get the following Vout:
light
level
light
dark
R2
(sensor)
1k
10k
R1
(fixed)
5.6k
5.6k
ratio (R2 / (R1 +
R2))
16%
67%
Vout (if Vin is
5V)
0.83 V
3.33 V
...so the voltage output will vary from 0.83V in bright light, to 3.33V in the dark. You can't
get a complete voltage swing from 0 to 5V without more complex circuitry, but hey, this isn't
bad for only one resistor.
A special case of this circuit is a potentiometer, which is a rotary control that allows you to
smoothly vary the ratio between R1 and R2, and thus the output voltage, allowing you to
create an easy to use analog control.
Inside a potentiometer is one big resistor with a moving connection (called a "wiper") that
can move from one end of the resistor to the other. Electrically, it looks like two resistors, just
like in our first schematic above. When you turn the knob, one resistor will get larger while
the other will get smaller. To use a potentiometer as a voltage divider, connect your power
and ground lines to the outside pins (these are the ends of the big resistor), and use the center
pin (the wiper) as your Vout. When you turn the knob, the ratio between the two resistors will
change, smoothly varying the output voltage between Vin and GND.
Much more information on Voltage Dividers is available at Wikipedia.
Sample Question:
If you make a voltage divider circuit with R1 = 10K and R2 = 10K, and your Vin is 12V,
what will be your Vout?
A. 12V
B. 10V
C. 6V
D. 5V
The correct answer is C: 6V. Whenever you have the same R1 and R2, no matter what those
values are, the equation (R2 / (R1 + R2)) will always come out to 1/2. Therefore, Vout will
always be one-half of Vin.