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Lecture 2

Lecture 2. Review Passive Sign Convention Power Generation, Absorption Power Sources Resistance Related educational modules: Sections 1.1, 1.2, 1.3. Passive sign convention – review. Passive sign convention:

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Lecture 2

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  1. Lecture 2 Review Passive Sign Convention Power Generation, Absorption Power Sources Resistance Related educational modules: Sections 1.1, 1.2, 1.3

  2. Passive sign convention – review • Passive sign convention: • For passive circuit elements, we assume that the current enters the node with the higher voltage potential • Your analyses will not be reliable unless you do this correctly • Examples:

  3. Subscript notation can denote voltage polarity • Voltage polarity is sometimes indicated by subscript notation • The order of the subscripts indicates the polarity • The first subscript indicates assumed higher-voltage node • The second subscript is the assumed lower-voltage node

  4. “Ground” • Voltages are often represented as relative to “ground”: • Ground (symbol: ) is a reference voltage; often 0V • Voltages relative to ground generally not called a voltage difference; they are a difference relative to zero volts • Voltages relative to ground often represented with a single subscript

  5. Power Generation and Dissipation • Circuit elements can either dissipate or generate power • Power is dissipated (or absorbed) if current enters the positive voltage node • Power is generated (or supplied) if current enters the negative voltage node

  6. Power Generation and Dissipation • Power = voltagecurrent (p= vi) • Power is absorbed if the power is positive (voltage and current are consistent with the passive sign convention) • Power is generated if the power is negative (voltage and current not consistent with the passive sign convention)

  7. Examples • Determine the power absorbed by the circuit element below. • The circuit element absorbs 10W. Determine the current in the element.

  8. Power Supplies • Power supplies provide a source of electrical power • Conceptual types of power supplies (models of physical supplies): • Voltage, current sources • Independent, dependent sources • Ideal and non-ideal sources

  9. Independent voltage sources • Common symbols: • Independent voltage sources maintain specified voltage, regardless of the current

  10. Independent voltage sources – continued • Voltage-current characteristic for constant voltage source:

  11. Independent current sources • Common symbol: • Independent current sources maintain specified current, regardless of the voltage

  12. Independent current sources – continued • Voltage-current characteristic for constant current source:

  13. Ideal power sources – limitations • Ideal sources can provide infinite power • Voltage sources provide specified voltage, regardless of the current  current can be infinite  power can be infinite • Current sources provide specified current, regardless of the voltage  voltage can be infinite  power can be infinite • These models can be unrealistic • We will examine more realistic power source models later

  14. Dependent Power Supplies • Some active circuit elements can be modeled as dependent power sources • The current or voltage delivered by the source is controlled by a current or voltage somewhere else in the circuit • Four possible combinations • Voltage controlled voltage source (VCVS) • Current controlled voltage source (CCVS) • Voltage controlled current source (VCCS) • Current controlled current source (CCCS)

  15. Dependent Power Supplies – continued • Examples:

  16. Resistors • Circuit symbol: • R is the resistance • Units are ohms () • Voltage-current relation (Ohm’s Law):

  17. Resistors – continued • Notes: • Resistors can only dissipate energy • The voltage-current relation is algebraic

  18. Resistor Power Dissipation • Ohm’s Law: • Power: • Combining:

  19. Example • Determine the power (generated or absorbed) by the resistor below:

  20. Conservation of energy • In an electrical circuit, the power generated is the same as the power absorbed • Slightly more mathematically, • Recall that power absorbed is positive and power generated is negative

  21. Conservation of power – example • Determine the power (absorbed or generated) by the voltage source VS

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