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Old 10th Jul 2017, 7:28 pm   #227
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Default Re: 807 (maybe) amplifier build. Now EL34

Seems like a good tie to hit you with Kirchoff!

It's a lot simpler than Thevenin.

Think of a pipe junction, with any number of branches. If you add up all the water flowing into it from all the branches there is a flow that way, then the result must equal the sum of all the outward going flows, or else the pipe is going to start bulging (or else vacuum is going to break out)

Another way of saying it is to count flows into the junction as positive, and flows out as negative (you could do it the other way round, just so long as you are consistent) Then the total flow of all branches added up must be zero.

Kirchoff said the same thing about electricity. The total current into a circuit node has to be zero. You count currents in and out with opposite signs.

In other words the sum of all currents onto the node must equal the sum of all currents off of it.

If this wasn't so the node would reach silly voltages in an instant (if you consider an ideal node to have no capacitance, but if a real wrld node has some, then treat the capacitance as a connection off of the node.

Handy rules tend to go round in pairs like policemen at night. like Thevenin and Norton.

The one that is twinned with Kirchoff is that if you take a palk round a look on any circuit diagram and wind up back where you started, the sum of all the voltage differences you've passed over comes out as zero.

(Think of M C Esher's staircases as an illusion that looks like it breaks this rule.)

David
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