Voltage divider

In electronics, a voltage divider (also known as a potential divider) is a passive linear circuit that produces an output voltage (Vout) that is a fraction of its input voltage (Vin). Voltage division is the result of distributing the input voltage among the components of the divider. A simple example of a voltage divider is two resistors connected in series, with the input voltage applied across the resistor pair and the output voltage emerging from the connection between them.

Resistor voltage dividers are commonly used to create reference voltages, or to reduce the magnitude of a voltage so it can be measured, and may also be used as signal attenuators at low alternating current frequencies. For direct current and relatively low alternating current frequencies, a voltage divider may be sufficiently accurate if made only of resistors; where frequency response over a wide range is required (such as in an oscilloscope probe), a voltage divider may have capacitive elements added to compensate load capacitance. In electric power transmission, a capacitive voltage divider is used for measurement of high voltage.

General case

thumb|Figure 1: A simple voltage divider

A voltage divider referenced to ground is created by connecting two electrical impedances in series, as shown in Figure 1. The input voltage is applied across the series impedances Z1 and Z2 and the output is the voltage across Z2.

Z1 and Z2 may be composed of any combination of elements such as resistors, inductors and capacitors.

If the current in the output wire is zero then the relationship between the input voltage, Vin, and the output voltage, Vout, is:

:<math>V_\mathrm{out} = \frac{Z_2}{Z_1+Z_2} \cdot V_\mathrm{in}</math>

Proof (using Ohm's law):

:<math>V_\mathrm{in} = I\cdot(Z_1+Z_2)</math>

:<math>V_\mathrm{out} = I\cdot Z_2</math>

:<math>I = \frac {V_\mathrm{in{Z_1+Z_2}</math>

:<math>V_\mathrm{out} = V_\mathrm{in} \cdot\frac {Z_2}{Z_1+Z_2}</math>

The transfer function (also known as the divider's voltage ratio) of this circuit is:

:<math>H = \frac {V_\mathrm{out{V_\mathrm{in = \frac{Z_2}{Z_1+Z_2}</math>

In general this transfer function is a complex, rational function of frequency.

Examples

Resistive divider

thumb|Figure 2: Simple resistive voltage divider

A resistive divider is the case where both impedances, Z1 and Z2, are purely resistive (Figure 2).

Substituting Z1 = R1 and Z2 = R2 into the previous expression gives:

:<math>V_\mathrm{out} = \frac{R_2}{R_1+R_2} \cdot V_\mathrm{in}</math>

If R1 = R2 then

:<math>V_\mathrm{out} = \frac{1}{2} \cdot V_\mathrm{in}</math>

If Vout = 6 V and Vin = 9 V (both commonly used voltages), then:

:<math>\frac{V_\mathrm{out{V_\mathrm{in = \frac{R_2}{R_1+R_2} = \frac{6}{9} = \frac{2}{3}</math>

and by solving using algebra, R2 must be twice the value of R1.

To solve for R1:

:<math>R_1 = \frac{R_2 \cdot V_\mathrm{in{V_\mathrm{out - R_2 = R_2 \cdot \left({\frac{V_\mathrm{in{V_\mathrm{out-1}\right)</math>

To solve for R2:

:<math>R_2 = R_1 \cdot \frac{1} {\left({\frac{V_\mathrm{in{V_\mathrm{out-1}\right)}</math>

Any ratio Vout / Vin greater than 1 is not possible. That is, using resistors alone it is not possible to either invert the voltage or increase Vout above Vin.

Low-pass RC filter

thumb|200px|Figure 3: Resistor/capacitor voltage divider

Consider a divider consisting of a resistor and capacitor as shown in Figure 3.

Comparing with the general case, we see Z1 = R and Z2 is the impedance of the capacitor, given by

:<math>Z_2 = -\mathrm{j}X_{\mathrm{C =\frac{1}{\mathrm{j} \omega C} \ ,</math>

where XC is the reactance of the capacitor, C is the capacitance of the capacitor, j is the imaginary unit, and ω (omega) is the radian frequency of the input voltage.

This divider will then have the voltage ratio:

:<math>\frac{V_\mathrm{out{V_\mathrm{in

= \frac{Z_\mathrm{2{Z_\mathrm{1} + Z_\mathrm{2

= \frac{\frac{1}{\mathrm{j} \omega C{\frac{1}{\mathrm{j} \omega C} + R}

= \frac{1}{1 + \mathrm{j} \omega R C} \ .</math>

The product τ (tau) = RC is called the time constant of the circuit.

The ratio then depends on frequency, in this case decreasing as frequency increases. This circuit is, in fact, a basic (first-order) low-pass filter. The ratio contains an imaginary number, and actually contains both the amplitude and phase shift information of the filter. To extract just the amplitude ratio, calculate the magnitude of the ratio, that is:

:<math>\left| \frac{V_\mathrm{out{V_\mathrm{in \right| = \frac{1}{\sqrt{1 + (\omega R C)^2 \ .</math>

Inductive divider

Inductive dividers split AC input according to inductance:

<math>V_\mathrm{out} = \frac{L_2}{L_1 + L_2} \cdot V_\mathrm{in}</math>

(with components in the same positions as Figure 2.)

The above equation is for non-interacting inductors; mutual inductance (as in an autotransformer) will alter the results.

Inductive dividers split AC input according to the reactance of the elements as for the resistive divider above.

Capacitive divider

Capacitive dividers do not pass DC input.

For an AC input a simple capacitive equation is:

<math>V_\mathrm{out}

= \frac{Xc_2}{Xc_1 + Xc_2} \cdot V_\mathrm{in}

= \frac{1/C_2}{1/C_1 + 1/C_2} \cdot V_\mathrm{in}

= \frac{C_1}{C_1 + C_2} \cdot V_\mathrm{in}</math>

(with components in the same positions as Figure 2.)

Any leakage current in the capactive elements requires use of the generalized expression with two impedances. By selection of parallel R and C elements in the proper proportions, the same division ratio can be maintained over a useful range of frequencies. This is the principle applied in compensated oscilloscope probes to increase measurement bandwidth.

Loading effect

The output voltage of a voltage divider will vary according to the electric current it is supplying to its external electrical load. The effective source impedance coming from a divider of Z1 and Z2, as above, will be Z1 in parallel with Z2 (sometimes written Z1 parallel (operator)| Z2), that is: (Z1 Z2) / (Z1 + Z2) = HZ1.

To obtain a sufficiently stable output voltage, the output current must either be stable (and so be made part of the calculation of the potential divider values) or limited to an appropriately small percentage of the divider's input current. Load sensitivity can be decreased by reducing the impedance of both halves of the divider, though this increases the divider's quiescent input current and results in higher power consumption (and wasted heat) in the divider.

</references>

External links

Voltage Divider Calculator