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Subject: RE: Plugged in Car-charger - smoke out of my box - no charging
From: Fred Maxwell (rockbox_at_anti-spam.org)
Date: 2004-03-17


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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