From: firstname.lastname@example.org (N. Thornton)
Subject: Re: Smallest component to step down 240VAC to 18VDC?
Date: 10 Oct 2002 03:30:07 -0700
References: <email@example.com> <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org> <email@example.com>
NNTP-Posting-Date: 10 Oct 2002 10:30:08 GMT
From: The little lost angel (firstname.lastname@example.org)
>>Just because :
>>- your laptop will be *DIRECTLY* connected to mains without
>>a good way to kill yourself.
>>- 120V (mains) * sqrt(2) (rectification + cap) = 170V; then
>>4.5A = 690W to keep you hot in case your laptop couldn't succeed in
>>you cold for the rest of your brand new eternal life...
>I don't quite understand this isolation part.
>The 230VAC comes in, the diode rectifier would change it to 230VDC
No, 230v ac is the rms value. 230v ac has a peak value of about 330v.
So you'd get 330v on a rectifier/capacitor.
>Then spread across 10 (okay 6 might had been too little) regulators,
>each of them drops 7V, and puts out 16V to the laptop. If the laptop
>draws the full 4.5A, then the regulators would be burning something
>like 25W, which isn't too much right?
I explained that voltage is relative in my last post. That is the key
to understanding why your method won't get you a low voltage output.
>But I would like to understand why, since I'm still confused on this
>thousand watt figure being raised by everybody. I think I sorta get
>ie current through all parts should be the same, thus even after I
>split the 230V over x regulators, the 230V will still see the 4.5A
>I still get thousand over watt dissipation?
Well, if you have 4.5A going out, with no transformer, then you must
have 4.5A coming in. Dropping 330v down to 16v is a voltage drop of
315v, at 4.5A.
Power dissipated = V x i = 315 x 4.5 = 1400 watts. You could dry 4
people's hair with that!
And when you touch the laptop power connector the current will flow
from the mains through you to the nearest earth. Ouch. If you're
>Speaking of the no ground... I realize I don't really know which pin
>of the AC plug does power come in. Does the AC oscillates between the
>live and neutral, or does it do the +/-230V wrt to the ground pin?
Neutral sits at about 0v relative to the earth you stand on. Live is
230v ac, and supply current flows between live and neutral. The earth
connection is for safety, and does not normally carry any significant
current, just slight insulation leakage curent.
Neutral isn't quite at 0v because of small volt drops in the wiring,
its just close to 0v.