Voltage Source Ideal model VL vs IL graph Model of a Practical Source Why is there a Series Resistance? Backup Power Switching – An analogy

1. Basic Electric Circuits – © 2020 Mouli Sankaran Email: mouli.sankaran@yahoo.com

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Session 11A: Focus
 Voltage Source
◦ Ideal model
◦ VL vs IL graph
◦ Model of a Practical Source
◦ Why is there a Series Resistance?
 Backup Power Switching – An analogy

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Ideal Voltage Source

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Ideal (or Independent) Voltage Source
 An ideal voltage source is expected to maintain a constant
voltage independent of current drawn from it
 It means that an ideal 9 V source can supply a current of:
 As the resistance across an independent voltage source is
reduced, it is not possible for it to maintain the same voltage
across its terminals
◦ Ohm’s law is defied if voltage has to be maintained across a short circuit
I = 9 A
9 V 9 V9 V
I = 9 * 106 A
1 Ω 1 μΩ 0 Ω
I = 9/0 = A
Short circuit

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Practical Voltage Source

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Practical Voltage Source: In car
 A small resistor in series shows that the voltage across
the source terminals is going to come down
◦ As more current is drawn from it
 Note that this series resistance shown is the internal
resistance of the power source
◦ Not that a resistor is connected to the source externally
Ideal Independent Voltage Source Practical Voltage Source
12.6 V
12.6 V

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Relationship between VL and IL
Practical Voltage Source
 Draw the graph of ideal and practical source.
Note: In practice never try to short circuit a voltage source, which will damage
the source permanently

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Why is there a Series Resistance?
 An ideal source needs to be modeled to have a resistor
in series to depict the real-life operation of a voltage
source
 i.e., when more current is drawn from a voltage
source, the voltage across the source drops down
◦ Example: Dimming of car light when the engine is started
Practical Voltage Source

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Backup Power Switching
- An Analogy

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Backup Power Switching
 During power failure from the Grid
◦ Power is delivered to the load by switching to backup power
source (diesel generator)
◦ As long as the input power parameters (Voltage, frequency
and current) the household appliances function normal
What is important
is the incoming
power parameters
- Voltage (~230V)
- Frequency (50 Hz) – in AC
- Current (20 A or more)
The loads (lighting and fans)
function normal
This is irrespective of
who is sourcing!!!
Diesel
Generator

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Session 11A: Summary
 Voltage Source
◦ Ideal model
◦ VL vs IL graph
◦ Model of a Practical Source
◦ Why is there a Series Resistance?
 Backup Power Switching – An analogy

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References
Ref 1 Ref 2