roines reenig wrote:

> Hi all,
> 
> I bought this fancy thermal massager thing that has a
> switching power supply. The interesting thing about
> the power supply (wall wart) is that it says input
> 100-240V AC at 50/60Hz 600mA and output 5.1V DC
> 2000mA. I've never noticed before this that power
> supplies state an input current requirement, assuming
> that the 600mA is the minimum input current
> requirement. Just curious, what are they doing in
> there to take a 600mA current and drive it up to a
> 2000mA? Is it just a capacitor? Is it normal?

  It's no magic.  It's power conversion.  It converts Power at one voltage 
(100-240VAC) to power at another voltage (5.1VDC) minus a little bit of wastage 
due to normal losses and inefficiency.

  Assuming a 100% efficient system the output is 10.2W (5.1V x 2A), that makes 
the input at 100vac RMS 102mA (10.2 / 100), and 43mA at 240vac RMS (10.2 / 240)

  Ok, as it turns out, that "little bit of wastage"  appears to be a lot of 
wastage (600mA verses the calculated 102mA with an ideal power conversion), but 
that specified 600mA might not actually be 600mA.  I imagine they take peak 
power requirements under worst case conditions.

-- 
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