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Subject: Re: Dammit the guy sold me a Jukebox v2...
From: Fred Maxwell (rockbox_at_anti-spam.org)
Date: 2004-09-09


Stuart Tedford wrote:
>>This one is 1000ma and has a 6v option. Considering the
>>Archos universal charger sold by them is 12v rated at
>>1200ma output do you think this is erring on the safe side,
>>and will "Work" Or will it put too much stress
>>pushing 1000ma at the jukebox v2's internal regulator?
>>
>
> The adaptor dosen't "push", rather the JB "pulls" the current it requires,
> so the adaptor just needs to supply at least the current the JB requires, so
> over-rating the supply current is OK, far better than under-rating.

I'm sorry to have to tell you that you are incorrect. I've been over
this countless times, before, so I'll just cut and paste from a previous
post:

It is almost always a bad idea to substitute a higher current capacity
unregulated AC adapter (wall wart) for the one supplied with a piece of
equipment. That is because the voltage output is related to the load on
an unregulated adapter. If you have a low-load on a high-current
adapter, the voltage coming out of the adapter will be higher than the
rated voltage. So while the 12v/1800mah adapter will produce 12v with
an 1,800mah current draw, it's probably producing far more than that
with the smaller draw of a Rockbox.

At one time, I did a bit of study on this and posted a lengthy piece,
complete with links to the website of a firm which manufactures wall
warts. Here's that web site:

http://www.powerstream.com/Wall-mount-FAQ.htm

And here is the relevent section:

"Most wall-mount power supplies have no active regulation. They are
designed so that the voltage will be X when the current is Y, just like
the label says. Many engineers are confused by this, thinking that a 12
volt, 1 amp power supply can be substituted for a 12 volt 500 mA power
supply. This might be true, but the voltage at 500 mA will be higher
than the voltage at 1 amp. How much this varies depends on the load line
of the transformer. A load-line is a graph of voltage versus current.
PowerStream can supply load lines for our products, not every
manufacturer does. Cheaper transformers have fewer windings, and wilder
voltage swings with load."

Without matching the load lines of the stock supply and the aftermarket
supply, one cannot know if they have a satisfactory substitution. I
know that many people get lucky and nothing burns up, but often, it's
over-taxing the regulation in the unit into which it is plugged.

Regards,
   Fred Maxwell

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