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Subject: RE: Question about rockbox charger
From: VanBaren, Gerald (AGRE) (Gerald.VanBaren_at_smiths-aerospace.com)
Date: 2004-09-21


> -----Original Message-----
> From: rockbox-bounces_at_cool.haxx.se
> [mailto:rockbox-bounces_at_cool.haxx.se]On Behalf Of Chuck Urwiler
> Sent: Tuesday, September 21, 2004 11:57 AM
> To: 'Rockbox development'
> Subject: RE: Question about rockbox charger
>
>
> Is there any simple way to tell if a power supply is
> regulated by looking at
> it or measuring it with a meter? Obviously if it's not marked
> as such, I
> mean...
>
> I've got a bunch laying round here (probably like most of you
> guys <g>) and
> wondering which are "better" than others, as I'm figuring a
> regulated supply
> is safer for most devices than unregulated (I think I
> remember what that
> means, but it's been a long time since I had an EE class...)
>
> -Chuck

Plug them in and measure the output with no load. If it is within 10% of the voltage stamped on it, it is probably regulated. If it is significantly higher than the rated voltage, it most likely is unregulated or broken. To confirm, connect a resistor that represents a decent load (say 50% to 80% of the rated output) and see if the voltage remains at Vrated +/- 10%. If it drops significantly, it is unregulated or broken.

Note that many switching power supplies won't run without a load, but they probably are not what you have kicking around. If you _do_ have a switcher, it is very likely to be regulated. Hooking up a load resistor will make it run so you can measure it.

To select a resistor, use Ohm's law:
  R = E / I
Be aware that P=I^2 * R -- you will need a pretty decent sized power resistor and it may burn your fingers if you are holding on to it :-/.

Example: 12v 1A => 12 watt power puck
  R = 12 / 0.5 = 24 ohm for 50% load
  P = I^2 * R = 0.25 * 24 = 6 watts

gvb

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