# What is a Voltage Source? | DC and AC Voltage Sources Explained

- Link: https://studyelectrical.com/2021/12/voltage-source-dc-ac.html
- Published: 2021-12-20T10:01:05+00:00
- Author: Tesla

**Definition**: Any device that produces voltage output continuously is known as
a voltage source.

There are two types of voltage sources,

 1. Direct voltage source
 2. Alternating voltage source

## Direct voltage source

As the name implies, a direct voltage source continuously produces direct voltage
output. Typical examples are batteries and [DC generators](https://studyelectrical.com/electrical-machines/dc-generator).

An important characteristic of a direct voltage source is that it maintains the 
same polarity of the output voltage i.e. positive and negative terminals remain 
the same. 

When load resistance **_R_(L)_**, is connected across such a source, current flows
from positive terminal to negative terminal via the load. This current is the conventional
current however the flow of electrons are in the opposite direction.

![dc voltage source](https://studyelectrical.com/wp-content/uploads/2021/12/dc-voltage-
source.png)

DC voltage source

This is called direct current because it has just one direction. The current has
one direction as the source maintains the same polarity of output voltage. The opposition
to load current inside the DC source is known as internal resistance _**R_(i)**_.

![Equivalent circuit of DC source](https://studyelectrical.com/wp-content/uploads/
2021/12/Equivalent-circuit-of-DC-source.png)

Equivalent circuit of DC source

The equivalent circuit of a DC source is the generated emf **_E_(g)_**, in series
with internal resistance _**R_(i)**_ , of the source as shown in the figure.

Load Current **_I_ = _E_(g)_ / ( _R_(L)_ + _R_(i)_ ) **

Terminal Voltage **_V_** = **_E_(g)_** – **_I_** **_R_(i)_ ** or **_I_** **_R_(L)_**

## Alternating Voltage Source

A device which produces alternating voltage output continuously is known as alternating
voltage source. Example of ac voltage source is an [Alternator](https://studyelectrical.com/2015/12/working-principle-of-alternator.html).

An important characteristic of alternating voltage source is that it periodically
reverses the polarity of the output voltage. When load [impedance](https://www.sciencedirect.com/topics/materials-science/impedance)**
_Z_(L)_** is connected across such a source, current flows through the circuit that
periodically reverses in direction. This is called [alternating current](https://studyelectrical.com/2019/11/types-of-electric-current-steady-dc-varying-alternating-ac-current.html).

![AC voltage source](https://studyelectrical.com/wp-content/uploads/2021/12/AC-voltage-
source.png)

AC voltage source

The opposition to load current inside the AC source called its internal impedance**
_Z_(i)_** . The equivalent circuit of an AC source is the generated e.m.f. **_E_(
g)_** (rms) in series with internal impedance **_Z_(i)_** of the source as shown
in the figure above.

The equivalent circuit of an AC source can be drawn as below.

![Equivalent circuit of AC source](https://studyelectrical.com/wp-content/uploads/
2021/12/Equivalent-circuit-of-AC-source.png)

Equivalent circuit of AC source

From the images, it is clear that,

Load Current **_I_ (rms) = _E_(g)_ / ( _Z_(L)_ + _Z_(i)_ ) **

Terminal Voltage **_V_** = (**_E_(g)_** – **_I_** **_Z_(i)_ **) or **_I_** **_Z_(
L)_**

(Vector difference since AC quantities are vector quantities)

## Constant Voltage Source

The term constant voltage source refers to a voltage source whose internal impedance
is very low in comparison to the load impedance.

In such a case, the output voltage nearly remains the same when load current changes.

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