voltage divider

A voltage divider is a simple circuit which takes a voltage source, Vin, and divides the voltage across two resistors in series, R1 and R2.

The divided voltage or the output voltage, Vout, is then measured across one of the resistors, either R1 or R2.

To calculate the output voltage across R1 use the following equation: Vout = (R1*Vin)/(R1+R2). Likewise to measure the output voltage across R2 use the equation: Vout = (R2*Vin)/(R1+R2).

If a circuit has more than two resistors, an equivalent circuit can be created to reduce the number of resistors down to two. Or a new voltage divider equation can be created. For more information on how to do this, read about Kirchhoff's Voltage Laws.

In its most simple form, an adjustable voltage divider circuit consists of a voltage source and a potentiometer. As the sweep arm moves across the potentiometer, it increases the resistance value of R1 and decreases the value of R2, or vice versa. The voltage drop from the sweep to either terminal is then measured or used as an input to another part of the circuit. This is cheap, easy to implement, provides its own control using the potentiometer's sweep arm, and provides a linear output voltage (if a linear potentiometer is used).

Voltage Divider in General

 +-----/\/\/-----+
 |       R1      |
 |               +-----o +
 |               |
 |               >     V                  R2
---           R2 >     o     Vout = V1 ---------
 -  V1           >     u                R1 + R2
 |               >     t
 |               |
 |               +-----o -
 |               |
 +---------------+

 +----------+
 |          |
 |          >
 |          >
 |       p  > sweep
---      o  ><------o +
 -  V1   t  >     V
 |          >     o
 |          >     u
 |          |     t
 +----------+-------o -

At the extreme ends of the sweep travel, V_out either encompasses the entire potentiometer (making V_out = V1) or none of the potentiometer at all (making V_out = 0V). To use the equation above, consider R1 to be the portion of the potentiometer above the sweep, and R2 to be the portion of the potentiometer below the sweep.

To size your potentiometer's resistance correctly, make it as small as possible without requiring the voltage source to provide more current than it can handle. Ideally, the input terminals which V_out is feeding will have infinite resistance. This of course is not possible in real life so the best thing to do is keep the resistance of the potentiometer as small as possible so the input resistance looks as large as possible in comparison. The more the input resistance looks like an open circuit, the less it will interfere with the operation of the voltage divider.

If your resistance is too small, however, the voltage source will need to produce too much current to feed the voltage divider. If it cannot provide the required current, it may not provide as much voltage as it is expected to, it may overheat, or it may blow a fuse.

Adjustable voltage dividers are often used in control and automation to provide manual control over a process. For example, the potentiometer could be adjusted by an operator's control knob, which in turn adjusts the voltage at V_out which is connected to the speed reference input of a variable frequency drive to provide motor speed control.