# Difference between Voltage source and Current source?

So what's the difference between a Voltage source and a Current source? Aren't they the same? Because after all they are related with the formula $V = IR$. So it means that If we produce a voltage, there should be a current and vice-versa. So really what's the difference?

• I don't want to add to the flurry of similar answers below, but I do want to point out that when an ideal voltage source or and ideal current source is used in circuit analysis it is not necessarily true that the source provides power to the circuit. For example, we might model a zener diode as a voltage source but it absorbs power. – Joe Hass Jan 30 '14 at 12:02

Voltage and current sources are not related by $V=IR$. This applies to resistances. However, you can connect a resistor to a voltage and current source, and then see what happens to voltage across the resistor and current through it.

A voltage source will maintain a constant V, regardless of the load connected to it. A current source will maintain a constant I, regardless of the load. Consider what happens as R changes in these two cases:

simulate this circuit – Schematic created using CircuitLab

For the case on the left, consider the resistance $R$, and the voltage across it $V_R$, and the current through it, $I_R$:

R   Vr  Ir
0Ω  1V  ∞A
1Ω  1V  1A
2Ω  1V  0.5A
3Ω  1V  0.33A
4Ω  1V  0.25A
∞Ω  1V  0A


Now consider the case on the right:

R   Vr  Ir
0Ω  0V  1A
1Ω  1V  1A
2Ω  2V  1A
3Ω  3V  1A
4Ω  4V  1A
∞Ω  ∞V  1A


In all cases, $V_R = I_R R$. On the left, $V_R = 1V$ (by Kirchhoff's voltage law), so $I_R$ is whatever it needs to be to satisfy Ohm's law.

On the right, $I_R = 1A$ (by Kirchhoff's current law), so $V_R$ is whatever it needs to be to satisfy Ohm's law.

Also notice that 0Ω is equivalent to a short circuit, and ∞Ω is equivalent to an open circuit. In some cases this results in infinite voltage or infinite current, which is an indication that these things can't physically happen. For example, if you actually short out a real voltage source, like a battery, the wire has actually some small resistance. A lot of current flows, but not an infinite current.

If you like, you can think of a voltage source as something that moves current, but adjusts the amount such that a constant voltage is maintained. You can think of a current source as something that adjusts the voltage across itself so that a constant current is moved. Realize of course that you are endowing the power of thought to inanimate objects, which isn't really true. Really all that's happening is that a current or voltage source introduces one constraint to a system of equations that must be solved.

You should also think about what happens when the thing connected is not a resistor. For example, what if it's a diode, like an LED? The voltage source still attempts to maintain a constant voltage, and the current source attempts to maintain a common current, but no longer is the equation being solved $V=IR$. That describes the behavior of resistances, but now the equation will be something else, describing whatever it is you have connected.

• @akki I don't see what was wrong with my grammar, and otherwise you just added a lot of words that I think made the answer harder to read. I've rolled back your edit. – Phil Frost Jan 31 '14 at 18:04

You fix the current source to a specific number of AMPS and then it will deliver it no matter what and it will adjust it's voltage to keep that current constant. So in that case only voltage varies. For the Voltage source you fix the voltage to a specfic Voltage and it will stay constant. It will pump out as much current as is necessary to keep the Voltage constant. In that case only current varies.

Because after all they are related with the formula V = IR. So it means that If we produce a voltage, there should be a current and vice-versa. So really what's the difference??

The difference is that a constant voltage source tries to maintain the V parameter in the above equation constant (irrelevant of the current level) while a constant current source tries to maintain the I parameter constant (irrelevant of the voltage level), so the resulting effect is quite different.

Ideally a constant voltage source will have an unlimited current capacity so will be able to provide as many amperes as needed to the load and keep the voltage steady.
In reality when the load impedance changes and it tries to get more current than what is available the voltage will drop lower than the set value.

With a constant current source things are slightly inverted, an ideal current source will have an unlimited voltage potential ability in order to increase the output voltage as high as needed in order to keep the load current steady (when the load impedance increases).
In reality when the load impedance increases and the current source has reached the max voltage level it can provide the current will drop lower than the set value.

A basic voltage source produces a constant terminal voltage. Let's say it produces 1 V. When 1 ohm is connected across it, 1 A will flow. When it is open circuit the terminal voltage is 1 V and no current flows. When it is shorted infinite current will flow.

A basic current source is different. Let's say it produces 1 A. When 1 ohm is placed across its terminals, 1 volt will appear due to ohms law. When it is open circuited infinite voltage will appear across its terminals in order to continue driving 1 A through fresh air! When shorted, 0 V appears across its terminals and 1 A is flowing.

• So suppose I connect two resistors in series with a current source but parallel to each other. Does that mean that the current source will drive 1 A through each resistor?? – radiantshaw Jan 30 '14 at 11:45
• @TheDreamCoder17 the source can only see a single load, that is the combined resistance of the two resistors. The 1A will be shared between the two resistors, if they are equal then each will get half the current if the curent will be shared according to the resistance relation between them. – alexan_e Jan 30 '14 at 11:55
• @Andyaka I agree with your rollback (that was a radical change), but I think it's wrong to say that thinking of voltage sources as things that adjust the current they pump in order to maintain a constant voltage is wrong. I think of voltage sources that way. Of course, it's personifying an inanimate thing, but it's a natural way to reason. I can also "confuse" a voltage source, for example by connecting a negative resistance load. An ideal voltage source may not be "confused", but real voltage sources (say, batteries) will be. – Phil Frost Jan 30 '14 at 13:14
• I appreciate that @Andyaka... – radiantshaw Jan 30 '14 at 15:30

A voltage source is assumed to deliver energy with a specific terminal voltage which does not depend upon the current from the source. A current source on the other hand is assumed to deliver energy with a specified current through the terminals.

Both current and voltage sources are ideal. In practice, we represent a real voltage source as an ideal voltage source in series with a resistance and a real current source as an ideal current source in parallel with a resistance.

Since student times, a curiosity about what is the basic difference between capacitor and inductor discharge, drive me to learn and simplify the issue of constant voltage and current sources. Later I was discover the usefulness of this achievement in circuit analysis and synthesis, replacing the active parts like zener and transistor with equivalent constant voltage and constant current sources respectively.

First of all there are DC as well as AC sources

but what is an Ideal Voltage Source? An ideal "voltage source" is simply an element providing voltage across its output that does not depend upon the current flowing, and a practical Voltage source is a kind of Voltage source whose internal resistance is very low! Such that the supplied voltage does not changes even if the external load resistance is changes.

And what is an Ideal Current Source? A current source which provides a constant current without any relation with the voltage supplied to the load is called Ideal Current Source, and in practical current source, internal resistance is very high compared to the external resistance of the circuit but not infinite.

An interesting point of this basic theory is that we can modify (or convert) current to voltage source and vice versa. How?

-Constant Current Source into a Constant Voltage: Find the resistance parallel of the Constant current source and place it in series with the current source to convert it into a Constant Voltage Source.

-Constant Voltage Source into a Constant Current: Find the Internal resistance of the Constant Voltage source and place it in parallel with the Voltage source to convert it into a Constant Current Source

So the answer to your question on how this two type of sources are related with ohms low is that the current source (If) is equal to voltage source (Vf) divided by the resistor If=Vf/R

Sourses in this very simple form are dependend some how by the load variations, so you have to balance the circuit variyng the series or the parallel resistance accordingly.Souch a dynamic resistor it is a transistor offcourse, making your source more independed.

There are several physical voltage source like batteries (except the underdevelopment nuclear batteries!) , as well as current sources like photoelectric cells.

There are lot of practical source circuits. For example your bench PSU that have adjustable output voltage and current limit, then based on the load connected, the supply will automatically switch to constant current from constant voltage when current limit condition is satisfied.

The symbol of a voltage source is

to indicate a device which can produce a continuous force to move the electrons (or, continuous voltage) through the wire connected into the two terminals of the device

The symbol of a current source is

to indicate a device which provides the regular flow or electrons or current on a circuit.