potential-dividersx

Potential Dividers

In many projects, we use batteries to supply the power. The battery contains chemicals which are converted into an electrical current when you need it. So the battery contains POTENTIAL ENERGY. When the energy is released into the circuit, it flows round like water flowing through a pipe.
When you put the probes of a voltmeter across the terminals of a battery, you are measuring the POTENTIAL DIFFERENCE between the terminals.

The "Potential Divider" is a line of resistors in series that are used to give different voltages in parts of an electronic circuit. The voltages can either be set to fixed values or be adjustable. As the name says, it divides the "potential" (voltage) into different amounts.

2 resistor Potential Divider 3 resistor Potential Divider

Fixed Potential Divider.

The Supply voltage is 9V
The two resistors in series have equal values and they divide the supply voltage into two equal parts allowing a voltage of 4.5V to be supplied to another part of the circuit.
Three resistors of equal value would divide the supply voltage into three, allowing 3V, 6V and 9V to be used by the rest of the circuit.

Potential dividers are often placed directly after the supply source to allow different voltages to be feed directly to different parts of the circuit. In the Op-Amp comparator circuit below, the potential divider network of two 10K resistors give a fixed voltage on Pin 3 equal to half the supply voltage.

Op-Amp with2 fixed 10K resistors in a potential divider

To find the values of resistors in a Potential Divider network to obtain a particular output value, use the following method.

Calculations

Knowing the values of the resistors, it is simple to calculate the Output Voltage using the following formula.

Calculating Vo for known resistors

To find the values of resistors in a Potential Divider network to obtain a particular output value, use the following method.

Calculating restance values to obtain a specified output voltage

In practice, if you measure the output voltage of the Potential Divider (P.D.) network, you will find it will be lower than calculated. This is because the components feed by the P.D. output will act as a resistance in parallel to the P.D. The current through R2 will drop causing the voltage to drop.
One way to solve this problem is to use a variable or preset resistor instead of R2. Adjust it when the circuit is running under load. Remove the variable resistor and find its resistance using a multi-meter and replace it with a fixed resistor R2.

resistor equivalent of a load in a Potential Divider network

In most circuit drawings, you will see most P.D. networks have a total resitance of 20k. The higher the value, the lower the current passing through the P.D. so reducing battery drain.
If the current is too low, this can cause a problem in not allowing sufficient current to be feed to the rest of the circuit.
Electronics is more of an Art than a Science. Use trial and error and explore what will happen if .....

1. 3 Red Stars Potential Dividers 4/12/02
Theory of Potential Dividers showing temperature, light and sound sensor circuits. Generating voltages from a switch.
Issue 30 - 20/2/03