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Engineering
Electrical Engineering
Voltage source and current source
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[QUOTE="DaveE, post: 6560405, member: 644223"] Voltage and current sources are ideal circuit elements used to simplify circuit models. They don't exist IRL as perfect sources. Of course, a perfect voltage source will always, no matter what, create a defined voltage across it. Similar for a perfect current source. You can model any voltage/current source as a perfect source with an associated internal resistance ##R_T##. Using the Thevenin/Norton models these could either be a perfect voltage source with a series resistance (impedance, really), or a perfect current source with a parallel resistance. Both models are equivalent according to those theorems. You would calculate or measure the quality of a voltage source by examining how much does the terminal voltage difference change when you (the circuit) changes the current drawn from it. This metric ## \frac{\Delta V}{\Delta I} = R_T ## is just the Thevenin equivalent resistance. This is ## \frac{\partial V}{\partial I} = R_T ## if you know calculus. There is a dual case for current sources, constant current regardless of the voltage imposed on it's terminals. So, a perfect voltage source has ## \frac{\Delta V}{\Delta I} = R_T = 0##. A perfect current source has ## \frac{\Delta V}{\Delta I} = R_T = \infty##. So, the distinction between voltage and current sources really comes down to whether the internal impedance is high or low (relative to the circuit it's connected to). Real world sources are somewhere between a voltage or a current source. For example the battery in your car is a good voltage source, but not perfect. That's why they have specifications for "cold cranking amps" for example. If it was perfect, CCR would be infinite. [/QUOTE]
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Voltage source and current source
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