11-02-2014, 01:33 PM
Quote:Mark, you state that DC is a straight line, DC is unidirectional but not always of a constant value in voltage/current terms.
Maybe it's better to think of it this way: DC is 0Hz, AC is anything else.
A pure DC source will never change, and will have no AC component. If a DC source does vary, however slowly, then it has an AC component. Let's pretend that it's normally a solid 5V, but that it rises up to 6V, then falls to 4V, then finally rises back to 5V. That variation might look like a single cycle of a sine wave. If all that took a leisurely 10 seconds to take place, then during that time, there was an AC component of 10Hz, 2V peak-to-peak, sitting on a 5V DC offset.
Quote:I still don't see how the direction of current flow is reversed in the context of my original post.
Let's pretend that our DC source above was connected to a 1000uF capacitor. When it sat there at 5V, the capacitor was fully charged, and no current was flowing.
When the output of the power supply rose to 6V, what about the capacitor? Well, it would have drawn current from the supply in order to charge up to 6V...
What next? When the power supply started to fall, there was a capacitor charged to 6V. Now, the capacitor is discharging back into the power supply. And that is where the negative current comes from.
Any practical circuit will have capacitance to earth, so this could be happening at any point you care to consider... The frequencies will probably be higher, and the capacitance and currents will be much less, no-doubt, but as a concept, there it is.
To be honest, that's a bit of a contrivance for illustration, but it might help... In pretty much all circuits I can think of, current does flow both ways with the signal, irrespective of the value of the DC offset.
