thevenin
Example circuit has a voltage source, and 3 resistors.
When constructing Thevenin equivalent circuit, the voltage source is replaced with a short, while R1 & R2 resistance is calculated as though they were in parallel, even though in original circuit they were in series.
Why?
When using Thevenin's theorem to calculate current and voltage of R3,
simulate this circuit – Schematic created using CircuitLab
Why are dependent sources never deactivated in application of this theorem?
When solving a network with multiple independent sources, we can use the superposition principle. This means deactivating all except one independent source one by one and solving the network for each case, then combining the results to get the solution for the network with all sources active. This is probably what your text is talking about when it talks about "deactivating" some sources.
You can't deactivate controlled sources when doing this process because a controlled source in a network with no independent sources wouldn't produce any output, so it can't be analyzed independently. The behavior of the controlled source has to be analyzed as it responds to each of the independent sources in turn.
Why is in the mentioned case "resistance combination is not applicable"?
In a network with controlled sources, you can't simply combine resistors like you do in a purely resistive network. Because there's more than just resistors in the network.
There are in fact special cases where a controlled source is hooked up to behave like a resistor, but in the general case, you have to treat them differently.
Perhaps all you need is some visual aid. Here's your circuit and its equivalent version once I cut the top 22V rail in the middle. (You have to ask yourself why I can cut the circuit between those upper nodes when they both are at the same voltage).
And now I can redraw it so that I have a black box with one port. You can apply Thevenin to that one port and get your final circuit
It's as easy as that.
Best Answer
When calculating the Thevenin equivalent resistance, you're asking, if the current out the output port changes, how much will the voltage across it change.
So you can think of it like a superposition problem with a hypothetical current source connected to the output port. When calculating the effect of this source, you need to zero out the other sources in the circuit.
And when V1 is zero'd out (replaced with a short circuit) you'll see that R1 and R2 are connected in parallel.