Overview
This lecture covers voltage and current sources, their symbols, types (independent and dependent), and how to analyze circuits with these elements, including practical examples and conventions.
Voltage and Current Sources
- Voltage sources provide a fixed voltage regardless of the current drawn, typically shown as a circle with +/– or as a battery symbol.
- Current sources provide a fixed current, represented by a circle with an arrow showing the direction of current flow.
- The value of a voltage or current source is usually written next to the symbol (e.g., 10V, 1A).
Independent vs. Dependent Sources
- Independent sources do not rely on any other element in the circuit; their values are fixed by definition.
- Dependent (or controlled) sources have a value determined by another variable (voltage or current) elsewhere in the circuit.
- Dependent sources use a diamond-shaped symbol.
Dependent Source Types and Symbols
- Voltage-controlled voltage source (VCVS): Diamond with +/–, labeled e.g. k·Vx.
- Current-controlled voltage source (CCVS): Diamond with +/–, labeled e.g. k·Ix.
- Voltage-controlled current source (VCCS): Diamond with arrow, labeled e.g. k·Vx.
- Current-controlled current source (CCCS): Diamond with arrow, labeled e.g. k·Ix.
Example: Analyzing Circuits with Sources
- Voltage across a load in parallel with a voltage source equals the source’s voltage.
- The polarity of connection affects the sign (positive or negative) of measured voltage.
- Practical voltage sources supply as much current as the load requires, within their limits.
- If too much current is drawn, the actual voltage may drop below the stated value.
Example: Dependent Source Circuit
- Given: 9V source in parallel with a load, a dependent voltage source (3·Vx), and current I = 2mA through the load.
- Solution: Vx = 9V (being in parallel with the 9V source), thus VL = 3×9V = 27V.
- Power across the load: P = IV = 2mA × 27V = 54mW.
- Positive power indicates absorption; negative would indicate supply.
Power Supply Practical Notes
- Rechargeable batteries absorb power when charging (positive power), and supply it when in use (negative power).
- Always interpret positive computed power as absorption/consumption and negative as delivery/supply.
Key Terms & Definitions
- Voltage Source — Element that maintains a fixed voltage across its terminals.
- Current Source — Element that maintains a fixed current through its terminals.
- Independent Source — Source whose value is set independently of the circuit.
- Dependent Source — Source whose value depends on another circuit variable.
- Load — Any component or group of components drawing electrical power.
- Power — Rate of energy transfer, P = IV (current × voltage).
Action Items / Next Steps
- Review the four types of dependent sources and their symbols.
- Practice drawing circuits with both independent and dependent sources.
- Solve circuit problems to reinforce voltage and power calculation methods.