26 Jan
2004
26 Jan
'04
5:12 p.m.
At 05:21 PM 1/26/04 -0800, you wrote:
I take exception with your reply! While technically true, in this case
we were clearly talking about a battery powered telephone therefore we're
talking about pure DC with no AC. Therefore phase, frequency and impedence
are irrelevent. Throwing in the talk of impedence just to show off is only
confusing the issue. The orginal poster that thought voltage didn't matter
clearly doesn't clearly understand electricity and I didn't won't to
confuse him further by introducing AC and it's effects.
Joe
Oh man this thread is gettin' wacky!
Not only are there resistive loads, slightly-variable resistive loads,
phase-dominated AC loads, there are also ACTIVE loads, eg. 99% of
everything made today that uses a wallwart! The (example) telephone
draws little current quiescent, more when it rings, more when charging a
dead vs. full battery, a lot when you're talking, etc and nauseum.
They got them new-fangled micro-compu-compacitor thingies in 'em what
make's em all complicated. Chips, non-corn, silicon.
I think this dead horse of a thread is beaten into a thin film.
You CAN substitute wallwarts, but you gotta know what you're doing or be
lucky. If a device wants regulated voltage (eg. my Linksys WAP) it often
-- but not always! -- will say something like "5.0V x mA" on it. If a
device comes with a (say) 12V AC wart, you (the YOU that knows about
electronics, if you don't have a you that does, don't) you can
substitute a 13V, 14V, maybe even an 18V wallwart, even a DC one! Maybe.
Actually it's not all that complicated. You just have to know something
about what you're doing and use some common sense. The one serious gottcha
that will get you is when the load expects to have a significant resistance
(or impedence) in the power source. Most (99%) loads don't care and 12VDC
is 12VDc no matter what the source but if you connect a low impedence
source (like a lead acid battery) to something that expects a high
impedence source (like the photo flash that I mentioned) then the device
will draw too much current and fry itself. However I've found very few
things that operate that way. I keep a BIG box of wall warts and plenty of
batteries around and I've made up hundreds of power supplies using
non-original wall warts and/or batteries. As far as I can remember, the
photo flash was the only one that gave me trouble.
You definitely need to know if the device needs AC or DC and what
polarity. But that's frequently marked on the chassis or the old wall wart.
in fact, in the US it's required to be marked by law. If it's not marked
then you have to open up the device and look at the power input circuits.
Does it go to a rectifier circuit or transformer? If so then it uses an AC
input. If it connects **directly** to rechargeable batteries, logic
circuits or to three terminal regulator or across a capcitor then it uses
DC input. You can look at the input device and get the polarity. Count the
number of cells in a battery pack and multiply by their voltage (~1.3V/cell
for NiCads) and that will tell you the minimum input requirement for that
circuit. Raise that by about 20% to force the batteries to charge and it
should work fine. Capacitors are almost always marked for polarity and
voltage. Derate the voltage about 2/3 and it will almost certainly work.
Logic circuits have well known voltage input levels. Look up their specs if
you don't know what it is. I shouldn't have to explain that. You can get
the model number from the three terminal regulators and look at the specs
and get the absolute maximum input voltage and output voltage. Feed it a
voltage at least 1 1/2 volts higher than the output voltage but less than
the absolute max and it will work. But be aware that anything over Vout
plus ~1 1/2 will be converted to heat so you may need to stay under the
maximum just to prevent excess heat dissapation. The 1 1/2 volts is a
typical number, if you really want to be precise, look up the exact value
in the specs. They do make regulators that drop as little at .1 volts. But
in most cases the regulator will be a common 780x and 1 1/2 volt drop will
work fine. Or if you like, connect a variable DC supply to the input and
monitor the output while you slowly vary the input voltage. You'll quickly
find the point at which the regulator output starts to drop. That input
voltage will be the minimum input voltage. Again, you can get the Vin Max
from the regulator specs but it's usually MUCH
higher than the minimum and
it you stay within a reasonable amount (+50%) of the
minimum you won't have
any trouble.
This procedure will take care of 95% of the cases.
Joe
There's no substitute for knowledge except exhaustive N x M matrix
testing with a lot of wall warts and many, expendable, copies of your
appliance. (Eg. stick with the non-smoking, functional combination.)
Someone else can have the last word. I gotta go charge my orgone
battery.
tomj
On Mon, 2004-01-26 at 14:19, Joe R. wrote:
> At 01:22 PM 1/26/04 -0700, you wrote:
> >
> >> 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
> >
> >Close, but although both are measuered in Ohms, impedence includes
> >another factor: Phase. Resistance is non reactive. That is, it does not
> >change with the frequency of the source voltage. Impedence is a
> >combination of resistance and reactance (capacitive and/or inductive),
> >and along with phase, can change with the frequency of the applied
voltage.