On Jan 20, 2008 12:13 PM, Eben King <eben01@verizon.net> wrote:
> On Sun, 20 Jan 2008, Chuck Hast wrote:
>
> > On Jan 20, 2008 2:24 PM, Richard Smoot <rsmoot@tampabay.rr.com> wrote:
> >> On Sunday 20 January 2008 10:05, Eben King wrote:
> >>> A friend gave me a router. When powered up, it comes on for a few seconds
> >>> (at first), then goes off. As you try repeatedly, the "on" time gets
> >>> shorter. If you wait some between tries, the "on" time gets longer. This
> >>> behavior makes me think something small is heating up. (The behavior is
> >>> the same with the router's case open AFAICT.)
> >>>
> >>> He said that when he first got it, the transformer was noisy (buzzing?),
> >>> but there was a mixup with the receipt and they (the store?) would not
> >>> honor the warranty. We think the transformer's finally died. Anyhow, this
> >>> is a practically unused Linksys WRTP54G. Anyone know where I can get a 9V
> >>> _AC_ 1A power supply? Most of them out there are 9V _DC_. Cheap is good,
> >>> in case it's _not_ the transformer that's bad.
> >> If my memory is correct an old US Robotics external power cube should work,
> >> because they were 9vac 1amp.
> >>
> > And a 9v DC psu will work just fine, the diode bridge in the WRTP54G will
> > make sure that the signs get applied to the right circuits... Just make
> > sure that it meets the current requirement of 1A or greater...
>
> Are you sure no components use AC? Since (depending on what the device
> draws) the actual resulting voltage could be anywhere up to 12.7 V, I may
> need an adjustable (or higher-voltage) xformer. Also, depending on
> transformer sag, it could be below 9 V. A 9 VDC transformer would probably
> have about the same voltage characteristics if everything works right.
>
I have had no problems with the AC devices when I could not find a AC
wall wort. I had a pretty good collection of them, sold most of them last
year at the Orlando Hamfest. I have run into a lot of 'lost wall wort'
situtations
where I had the correct voltage but only a DC wall wort. Most of them just
have a transformer and a bridge inside of them with little or no filtering. So
you are just putting a bridge behind a bridge, you will see a .7 v DC drop in
voltage across the second bridge due to the diode drop but that is all. By
the time the caps inside the unit in question filter and clean up the power it
is near where it should be.
Of course going the other way is usually verboten, i.e. AC into a DC input.
That will probably give you at best a non-responsive device and worst a dead
one...
Another interesting one are the compact florescent lamps, these devices
have a bridge rectifier at the input followed by a swiitcher running at a free
oscillation frequency of about 20-40Khz. They generate loads of RFI, but the
fix is to run them on DC. That way the 20-40Khz switcher is not loaded with
60Hz harmonics, and so produces a lot less RFI. Indeed you could have a
house full of CFL's running off of a battery bank which is charged by solar,
you now have a nice clean lighting system which is pretty efficient too, and
not dependent on the mains power at that... Most switched psu's have as
the input a diode bridge and some filtering, then the switching and and final
filter circuits to remove the switching components from the DC output. In the
case of the CFL's not even that, they run better when operated at the switch
frequency.
When doing these tricks just make sure your DC output sets at the RMS
voltage (in the case of mains voltage 110-125 VAC) since when you rectify
and filter say mains 110v ac, you will end up with 155.56 v DC (110/0.707)
so you will need to buck it down with a transformer prior to rectification or
if you have a wide voltage psu (one of those that will run on anything from
100 to 240 v ac) you will not need to worry about it. These switched psu's
are a plague or they are a blessing depending on what you are trying to do.
I guess I better drop this one, it is way off topic. Sorry folks...
These switchers
are fun to play with though and you have some flexibility that was not there
with the old transformer power supplies.
-- Chuck Hast -- KP4DJT -- To paraphrase my flight instructor; "the only dumb question is the one you DID NOT ask resulting in my going out and having to identify your bits and pieces in the midst of torn and twisted metal." ----------------------------------------------------------------------- This list is provided as an unmoderated internet service by Networked Knowledge Systems (NKS). Views and opinions expressed in messages posted are those of the author and do not necessarily reflect the official policy or position of NKS or any of its employees.
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