Voltage and Current Sources
Definition
A voltage source is an active circuit element that maintains a fixed voltage across its terminals regardless of the current drawn by the load, within its operating limits. A current source is an active circuit element that maintains a fixed current through the load regardless of the voltage across it, within its operating limits.
In simple terms:
- A voltage source “pushes” charges to maintain voltage.
- A current source “drives” a fixed amount of charge per unit time to maintain current.
Main Content
1. Ideal Voltage Source and Ideal Current Source
- An ideal voltage source provides constant terminal voltage no matter how much current is drawn from it. Its internal resistance is assumed to be zero, so there is no internal voltage drop. Example: an ideal 12 V source always gives 12 V across its terminals.
- An ideal current source provides constant current no matter what voltage appears across its terminals. Its internal resistance is assumed to be infinite, so it can adjust its terminal voltage as needed to keep the current constant. Example: an ideal 2 A source always delivers 2 A to the circuit.
Key characteristics:
- Ideal voltage source: voltage remains constant; current depends on load.
- Ideal current source: current remains constant; voltage depends on load.
Graphical behavior:
- The V-I characteristic of an ideal voltage source is a vertical line at the source voltage.
- The V-I characteristic of an ideal current source is a horizontal line at the source current.
Examples in practice:
- A laboratory DC power supply behaves approximately like a voltage source.
- A transistor-based current regulator behaves approximately like a current source.
2. Practical Voltage Source and Practical Current Source
- A practical voltage source is an actual source that has a small internal resistance in series with the ideal source. Because of this resistance, the terminal voltage decreases as load current increases. Example: a battery has internal resistance, so its terminal voltage drops when supplying heavy current.
- A practical current source is an actual source that has a large internal resistance in parallel with the ideal source. Because of leakage and finite internal resistance, the current is not perfectly constant and changes slightly with load voltage.
Important behavior of practical voltage sources:
- At no-load, terminal voltage is close to source emf.
- Under load, some voltage is lost inside the source due to internal resistance.
- The more current drawn, the larger the internal drop.
Important behavior of practical current sources:
- They maintain nearly constant current over a wide range of load voltages.
- The current changes when the load causes voltage to exceed the source’s compliance range.
- They are widely used in electronics and instrumentation.
Mathematical form:
- Practical voltage source:
- : terminal voltage
- : source emf
- : load current
- : internal series resistance
- Practical current source: in an approximate parallel model
- : large shunt resistance
3. Source Transformation and Real-World Significance
Source transformation
- is the process of converting a practical voltage source into an equivalent practical current source, or vice versa, without changing the behavior seen by the external circuit. This is based on the equivalence:
- Voltage source in series with resistance
- Current source in parallel with the same resistance
- This concept is very useful in simplifying circuit analysis using Thevenin and Norton equivalent circuits.
Why this matters:
- It helps solve complex circuits more easily.
- It allows one to choose the form that is more convenient for analysis.
- It is extensively used in network theorems, electronics, and power system studies.
Examples:
- A 10 V source in series with 5 Ω can be transformed into a 2 A current source in parallel with 5 Ω.
- A 4 A current source in parallel with 8 Ω can be transformed into a 32 V voltage source in series with 8 Ω.
Real-world applications:
- Batteries act like voltage sources with internal resistance.
- Solar panels and photodiodes can often be modeled as current sources.
- Biasing circuits in transistors often use current-source behavior for stability.
Working / Process
1. Identify the source type
- Determine whether the device is acting mainly as a voltage source or a current source.
- Check whether the circuit aims to keep voltage fixed or current fixed.
2. Apply the appropriate model
- For a voltage source, use an ideal source or a practical model with a series internal resistance.
- For a current source, use an ideal source or a practical model with a parallel internal resistance.
3. Analyze load interaction
- For a voltage source, calculate how load current affects terminal voltage using internal resistance.
- For a current source, calculate how load voltage changes while current remains nearly constant.
- If needed, convert between source forms using source transformation for easier circuit solving.
Advantages / Applications
- Voltage sources provide stable and convenient supply for most electronic and electrical devices, such as batteries, adapters, and DC power supplies.
- Current sources are useful where precise current control is required, such as LED drivers, transistor biasing, sensor excitation, and charging circuits.
- Source models simplify network analysis and are essential in Thevenin, Norton, and equivalent-circuit methods used in circuit design and troubleshooting.
Summary
- Voltage sources maintain voltage, while current sources maintain current.
- Ideal sources are theoretical; practical sources include internal resistance and real operating limits.
- Source transformation helps convert one source form into the other for easier circuit analysis.