Voltage Sources and Current Sources - NEW 29 APRIL 2020

Voltage sources and current sources are used as energy sources to drive circuits.

Voltage Sources

Voltage sources are two terminal devices that maintain a constant voltage between their terminals. Common examples of voltage sources include batteries, power supplies, and 230 V mains electricity.

An ideal voltage source will provide whatever current is required to maintain the voltage across its terminals.

The basic symbols for a voltage source are shown below.

Basic Symbol	Cell	Battery

On the round symbol, the '+' denotes the positive terminal, and the '-' denotes the  negative terminal.

Another common symbol is a cell or battery symbol. The dashed lines show that there are multiple cells in a battery. The wider line is the positive terminal.  The 'cell' symbol is often used to represent a battery.

A voltage source with a positive value will maintain a positive voltage between its positive and negative terminals. If it is connected to a circuit with only resistors, a conventional current will flow from the positive terminal to the negative terminal.

Behaviour in Circuit

The figure below shows a voltage source in circuit. The voltage source V is placed across a resistor, and as a result a current I flows through the resistor.

The behaviour of this circuit is given by Ohm's Law. The three laws are \( V = I R \), \( I = \frac{V}{R} \), and \( R = \frac{V}{I} \). If you know two quantities, you can use Ohm's Law to get the third.

Current Sources

Current sources are two terminal devices that maintain a constant current flow between their terminals. Current sources are less common than voltage sources, but examples include current regulated power supplies, and the Van de Graaff generator below. The Van de Graaff generator works by depositing a charge on an insulating belt (the wide brown rubber belt below) at the base and depositing it on the metallic electrode at the top. Since the belt is transferring charge at a constant rate, the Van de Graaff generator acts as a current source.

An ideal current source will provide whatever voltage is required to maintain the current flow across its terminals.

The basic symbols for a current source is shown below.

Basic Symbol

The arrow denotes the direction of conventional current flow.

A current source with a positive value will maintain a conventional current in the direction shown. If it is connected to a circuit with only resistors, a positive voltage will appear between the "arrow head" end and the "arrow tail" end.

Behaviour in Circuit

The figure below shows a current source in circuit. The current source I is placed across a resistor, and as a result a voltage V is developed across the resistor.

The behaviour of this circuit is given by Ohm's Law. The three laws are \( V = I R \), \( I = \frac{V}{R} \), and \( R = \frac{V}{I} \). If you know two quantities, you can use Ohm's Law to get the third.

Open Circuit and Short Circuit Behaviours

The following table shows what happens when an ideal source is "open circuited" (connected with no load across its terminals, or a load resistance of infinity) or "short circuited" (terminals connected directly together, or a load resistance of zero). Dangerous conditions are indicated in bold italic!

The main one to watch out for is short circuiting a voltage source, as voltage sources are far more common. In particular, the 230 V mains is designed to have a high short circuit current for reasons of efficiency.