On Mon, 8 Nov 2004, 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 a normal switching supply. The input current is the maximum inrush 
current probably (rms equivalent of inrush current). You need to know this 
when you put several devices in a multiple distribution circuit (aka 
powerstrip, but think large). It is mandatory for the manufacturer to 
state either that or input power, whicherver is *larger*.

Simple calculations show that at 100V 0.6A will cause the wall wart to 
take 60W and output 10W (5V*2A) which would cause it to turn 50W into 
heat. That would make it about as hot as your soldering iron. Obviously it 
doesn't do that. The 600mA is rms inrush current.

As to how a switcher works, it works like a transformer, but it uses an 
electronic circuit to drive the transformer at high frequency (as opposed 
to 60Hz mains). That allows a tiny and lightweight transformer to be used 
instead of a bulky and heavy iron core needed for the same power at 60Hz.

Peter
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