The Voltage divider reference article from the English Wikipedia on 24-Apr-2004
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Voltage divider

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In electronics, a voltage divider or resistor divider is a design technique used to create a voltage (Vout) which is proportional to another voltage (Vin).

Resistor divider

Two resistors are connected as shown in Figure 1. The output voltage, Vout is related to Vin as follows: Vout = Vin*(R2/(R1+R2)). As a simple example, if R1 = R2 then Vout = Vin/2. Any other ratio between 0 and 1 is also possible.

Note that this rule only works if the load resistance is infinite, that is, if all of the current flowing through R1 goes into R2. If current flows into a load resistance (through Vout), that resistance must be considered in parallel with R2 (see: resistor).

Schematic of a resistor divider. R1 is connected to Vin and Vout, R2 is connected to Vout and GND
Figure 1: Resistor Divider

General impedance divider

A voltage divider is usually thought of as two resistors, but capacitors, inductors, or any combined impedance can be used. For instance, a divider with a resistor and capacitor:

Schematic of a voltage divider with a capacitor. Resistor is connected to Vin and Vout, Cap is connected to Vout and GND

will have voltage ratio:

since the capacitor's impedance is 1 / jωC (where j is the imaginary number, and ω is frequency in radians per second). The ratio then depends on frequency, in this case decreasing as frequency increases. This circuit is, in fact, a basic lowpass filter.