[Techtalk] OT but I'm desperate.... Windows....!

Maria Blackmore mariab at cats.meow.at
Sat Oct 19 22:33:48 EST 2002


On Sat, 19 Oct 2002, Meredydd Luff wrote:

> Uhhmmmm - how does that work?

How does what work?

> A power source pumps electrons from the positive to the negative pole (of the 
> supply, that is). The current is how many it pushes across (one amp is 
> defined as a certain number of electrons per second).

The electricty grid has a *lot* of push :)

> A brownout occurs when the power source can't push electrons across fast 
> enough to keep up with the current that flows if you put 240V across your 
> load circuit. As a result, the pd decreases (V=IR again). Thus, a brown-out 
> is indeed a drop in pd, as well as a drop in current.

hmm, yes, but this is an AC circuit, plus we're dealing with inductances
and stuff too, so you have to worry impedances as well as resistance, and
all sorts of stuff.

*waves arms vaguely in the absence of explaining things fully*

So it's possible for a power supply to continue drawing the same amount
of power from a browned out supply by reducing its effective resistance.  
This is how my UPS won't actually switch to internal batteries until the
mains voltage drops as low as about 75V iirc.  I guess it's a little
selfish really, because if more and more things behave like this it pulls
the whole grid down faster, and here's me wanting to put my whole house
apart from the cooker and shower circuits on UPS power :)

Sorry about the abundance of arm waving, I'm feeling a little tired, and
feeling like being an A level physics teacher (who always grinned when I
called him on arm waving explainations :)

Maria

yes, "stuff" is a technical term, as is "gubbins"




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