# Help understanding current source

• mango21
In summary, the problem involves a closed circuit with a current source and a resistor. The goal is to express the voltage and current across the resistor in terms of the current from the source and the resistance of the resistor. The equations used are Ohm's law and Kirchoff's laws. The concept of a current source is explained as being the opposite of a voltage source and its output current being dependent on the rest of the circuit. In this problem, Ir is equal to I1.
mango21

## Homework Statement

This is a really basic problem, just a closed circuit with a current source and a resistor. We are meant to give the voltage and the current across the resistor in terms of the current from the source and the resistance of the resistor. (From the diagram, express Vr and Ir in terms of I1 and R). I have never encountered a current source before and am having trouble learning to deal with it.

## Homework Equations

Ohm's law: V = IR
Kirchoff's Laws: sum of voltages = 0, and current into a node = current out

## The Attempt at a Solution

My thought was to say that I1 = Vs/Rs, so Vs = I1/Rs = Vr = Ir/R.
But then I don't know how to solve these for Ir and Vr in terms of I1 and R.

Any help understanding the concept would be greatly appreciated! Thanks :)

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mango21 said:

## Homework Statement

This is a really basic problem, just a closed circuit with a current source and a resistor. We are meant to give the voltage and the current across the resistor in terms of the current from the source and the resistance of the resistor. (From the diagram, express Vr and Ir in terms of I1 and R). I have never encountered a current source before and am having trouble learning to deal with it.

## Homework Equations

Ohm's law: V = IR
Kirchoff's Laws: sum of voltages = 0, and current into a node = current out

## The Attempt at a Solution

My thought was to say that I1 = Vs/Rs, so Vs = I1/Rs = Vr = Ir/R.
But then I don't know how to solve these for Ir and Vr in terms of I1 and R.

Any help understanding the concept would be greatly appreciated! Thanks :)

Welcome to the PF.

There is nothing tricky here. The current source sets the series current that flows around the circuit. The resistor value is what determines what voltage is dropped across it, just based on Ohm's Law V=IR.

mango21 said:
I have never encountered a current source before and am having trouble learning to deal with it.

You are probably familiar with voltage sources, even if you didn't use the name. You can think of a voltage source as being like a battery. It always has the same voltage across its terminals, but the current depends on the circuit that is connected to it.

A current source is the opposite way round. It always produces the same current, and the voltage across it depends on the rest of the circuit.

If you increase the resistance of a circuit connected to a voltage source, voltage stays the same but the current decreases. If you increase the resistance connected to a current source, the current stays he same but the voltage increases.

Unlike a battery, it doesn't make any sense to have a current source "not connected to anything", because its output current has to go somewhere.

In real life you have to make a current source from a circuit containing transistors or op-amps, so the idea is a bit more abstract than a battery. But current and voltage sources are both very useful ideas in circuit analysis.

Note, all the above is a bit over-simplified. For example both current and voltage sources can produce a variable output that is related to something else in the circuit. But the important thing is to understand the basic idea before making it more complicated.

Last edited:
Thanks! So in this case Ir = I1?

mango21 said:
Thanks! So in this case Ir = I1?

Yep!

## What is a current source?

A current source is a device or circuit that is designed to provide a constant and steady flow of electric current, regardless of changes in the load or other external factors. It is the opposite of a voltage source, which maintains a constant voltage regardless of changes in the current.

## How does a current source work?

A current source works by using a feedback mechanism to regulate the output current based on the load and other external factors. This is usually achieved through components such as transistors, op-amps, or diodes. The goal is to maintain a constant current flow through the load, even if the load resistance changes.

## What is the difference between a current source and a voltage source?

The main difference between a current source and a voltage source is that a current source provides a constant current output, while a voltage source provides a constant voltage output. Another difference is that a voltage source is typically used to power electronic devices, while a current source is often used in electronic circuits for biasing or signal amplification purposes.

## What are some common applications of current sources?

Current sources are commonly used in electronic circuits for various purposes, including biasing transistors, generating reference currents, and providing a constant current source for sensors or feedback loops. They are also used in power supplies, battery charging circuits, and LED drivers. In scientific research, current sources are used in experiments and measurements involving electricity and electronics.

## Can a current source be converted into a voltage source?

Yes, a current source can be converted into a voltage source by using a resistor in series with the current source. The voltage across the resistor will be proportional to the current flowing through it, thus creating a voltage source. However, this conversion is not perfect and may introduce some fluctuations in the voltage output due to changes in the load resistance.

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