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Subject: RE: Plugged in Car-charger - smoke out of my box - no charging
From: Chris Weaver (cweaver_at_bellsouth.net)
Date: 2004-03-17


I agree with everything here except that it doesn't matter that the voltage is slightly higher from a 1amp supply when supplying
500ma than a 500ma supply providing the same 500ma.

For that matter a handful of cheaply made 500ma supplies providing 500ma will also vary in voltage.

My only point is that all these voltages will be in an acceptable range for the device as the load lines will all be in the same
basic range and a device designed to run on an unregulated supply will be able to accommodate the range of voltages. If it can not
then an unregulated supply was not an appropriate choice to begin with.

Thanks for the info

-----Original Message-----
From: rockbox-bounces_at_cool.haxx.se [mailto:rockbox-bounces_at_cool.haxx.se] On Behalf Of Fred Maxwell
Sent: Tuesday, March 16, 2004 11:52 PM
To: 'Rockbox development'
Subject: RE: Plugged in Car-charger - smoke out of my box - no charging

Chris Weaver wrote:

> A non regulated supply will certainly have a high
> output voltage with very little current draw,
> perhaps 10% of rated output and lower.
> Specifically, the open circuit DC voltage output
> is 1.414 times the loaded voltage when using a
> full wave bridge rectifier. However, this voltage
> will drop as soon as you start to draw any
> significant current.

I think that you are not taking RMS (Root Mean Square) AC voltage
measurement into account. The peaks on an AC line are 1.414 * the RMS
voltage. Thus, a 120 line has peaks that swing about 170 volts to each side
of ground. If you rectify that, you will get DC with peak ripples at 170
volts and 120hz (minus the voltage drop across the diodes). See this page:
http://ourworld.compuserve.com/homepages/g_knott/elect39.htm

As to the acceptability of swapping a higher current wall wart for a lower
one, I suggest that you read the following from PowerStream, a manufacturer
of wall-warts:

: Regulation in DC Adapters
:
: 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.

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

Without having the "load-lines" for the various wall warts, you really can't
say whether you have a suitable replacement.

Regards,
  Fred Maxwell

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