26 Jan
2004
26 Jan
'04
7:22 a.m.
At 09:19 AM 1/26/04 -0800, you wrote:
>Just last week I fried a 2.4GHz wireless phone by
applying a power
>supply that had exactly the rated voltge but was 1A instead of the
>required 800mA. I smelled smoke not long after plugging it in. All
>this in a unit that worked properly with the right PSU just 2 weeks
>before... the original PSU being lost. Need I say any more? Current
>is everything... voltage is nothing.
Wrong! Voltage (V) determines current flow (I). Ohm's Law I = V/R. The
reason that your phone smoked was the it's design relied on the internal
impedence (a fancy word for resistance(R) ) to limit the current. When you
plugged in a different supply that had a lower impedence it allowed more
current to flow even though the voltage was the same. FWIW I did the same
thing once when I tried to used a small 6 volt lead acid battery to power a
camera flash that was designed to run off of four 1 1/2 volt AA batteries.
The voltage was the same but the impedence was lower, that made the current
much higher and the result was to let all the magic smoke out of the flash.
If you want to get a good idea of what I'm talking about, go short out 9V
transistor radio battery. The shorting wirre and battery probably won't
even get warm. Then go try it with a 6 or 12 volt lead acid battery. But be
warned, use gloves and wear googles!
Joe
Perhaps people need a DMM so they can check polarity, or maybe get
glasses to read the markings?
OMG, this is the funniest thread I've read in a while.
True story: My sister last year was trying to plug in a computer
modem after moving some things around. I think the input is 9V DC,
well she grabbed the cord from the HP inkjet that happened to be the
same size, but pumps out *24V*! All of a sudden we hear screaming and
she comes running down the stairs screaming "THE MODEM IS
SMOKING!!!!!!!!!!!!!!!!!!!" Now THAT is funny. Don't ask me how, but
she didn't fry the modem(not totally anyhow, it still powers up OK).
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John Rollins | KD7BCY | http://www.kd7bcy.com
DALnet #Apollo_Domain | Ham-Mac mailing list http://mailman.qth.net
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