On 2/15/10 11:21 AM, Doug McNutt wrote:
At 21:00 -0800 2/14/10, Clark Martin wrote:
On 2/14/10 8:14 PM, Bill Christensen wrote:
At 10:07 AM -0700 2/9/10, Bruce Johnson wrote:
If the river is too big (high amperage), the water wheel will
work just fine, because it'll only use the amount of water that
fits in the paddles...the rest of the river flows on by.
Ok, if the "rest of the river flows on by", does that mean that
even if your end use device is drawing .9 amps, the wall wart
'river' is still going to draw 1.2 or 1.5 or whatever amp rating
it has, turning the 'excess' to more heat and a slightly higher
electric bill?
Or is the metaphor breaking down here?
Not exactly.
The components in a wall wart have a linear response (at least
idealized components would), if you reduce the current draw by
half, the current going in is reduced by half.
The parts are however not ideal but real. That means that the
transformer in particular isn't linear. In particular, smaller
transformers tend to more quiescent power as a percentage of the
total power drawn. Quiescent power is the power the transformer
draws with no load.
Methinks the metaphor is a bit mis-stated.
The hydraulic analogy, as taught in a first course in electrical
engineering, would replace the "river" with the "pipe" that is
connected to a large reservoir.
No course in electrical engineering uses the hydraulic analogy, at least
none I took. They figure (hope?) by that point you already know enough
about electricity.
The device using the current is analogous to a water valve with a
constant flow rate typical of newer shower heads. The water that's
not used never leaves the reservoir but it's still limited by the
resistance to flow caused by the pipe. It doesn't "flow on by".
And modern DC wall-warts, now required in California, typically do
not start with a transformer. They are high frequency converters that
use a much smaller and more efficient transformer after they convert
60 Hz power to something well above 100 kHz.. Those things are smart
enough not to draw current, "quiescent power", from the power mains
unless that valve between it and the user is open.
Switcher still have quiescent power, it's just typically smaller than a
linear supply.
And. . . Those new-fangled devices typically draw power from the
lines in short pulses 120 times per second. That's called harmonic
distortion which is illegal in Europe for larger units like ATX power
converters in a desktop computer. We'll hear more about that in a
while when California gets a bit brighter shade of green.
Actually they do it same as a linear power supply. It's not harmonic
distortion, it produces harmonic distortion.
--
Clark Martin
Redwood City, CA, USA
Macintosh / Internet Consulting
"I'm a designated driver on the Information Super Highway"
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