Internal Impedance of a Source
Any source that produces voltage output continuously is a generator.All sources or generator have an internal resistance or impedance in case of an ac source.The internal impedance of a source may be due to following reasons-
1.The resistance of the wire or armature winding in an alternator or dc generator.
2.The output impedance of transistor or vacuum tube in rectifier, oscillator or signal generator.
3.The resistance of the electrolyte between electrode in a chemical cell.
The voltage available at the terminal of a voltage source is its terminal voltage. Due to the internal impedance,the terminal voltage drops when the voltage source supplies power to a load. But when load is not connected, there is no load current. With no current, the voltage drop across internal impedance is zero. Then the full generated voltage is available across the output terminal.This value is open circuit voltage or no-load voltage.
A open circuit voltage is 1.5 V. Load resistance of 0.85 Ohm is connected to the source. Internal
resistance of a source is 0.15 Ohm.
Total resistance = 0.85 + 0.15 = 1 Ohm = R
The current in the circuit
=1.5 A
Terminal voltage = V(AB)= V(RL)= I / R(L)
=1.5/0.85= 1.275V
The voltage drop because of internal resistance is
=1.5 - 1.275 =0.225V
The terminal voltage drop as the load current increases. The terminal voltage is less than its open-circuit voltage due to the internal resistance. If there is a short circuit across the source,
its internal resistance prevents the current from becoming infinitely high.
Any source that produces voltage output continuously is a generator.All sources or generator have an internal resistance or impedance in case of an ac source.The internal impedance of a source may be due to following reasons-
1.The resistance of the wire or armature winding in an alternator or dc generator.
2.The output impedance of transistor or vacuum tube in rectifier, oscillator or signal generator.
3.The resistance of the electrolyte between electrode in a chemical cell.
The voltage available at the terminal of a voltage source is its terminal voltage. Due to the internal impedance,the terminal voltage drops when the voltage source supplies power to a load. But when load is not connected, there is no load current. With no current, the voltage drop across internal impedance is zero. Then the full generated voltage is available across the output terminal.This value is open circuit voltage or no-load voltage.
A open circuit voltage is 1.5 V. Load resistance of 0.85 Ohm is connected to the source. Internal
resistance of a source is 0.15 Ohm.
Total resistance = 0.85 + 0.15 = 1 Ohm = R
The current in the circuit
=1.5 A
Terminal voltage = V(AB)= V(RL)= I / R(L)
=1.5/0.85= 1.275V
The voltage drop because of internal resistance is
=1.5 - 1.275 =0.225V
The terminal voltage drop as the load current increases. The terminal voltage is less than its open-circuit voltage due to the internal resistance. If there is a short circuit across the source,
its internal resistance prevents the current from becoming infinitely high.
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