[Soekris] Re: Power Supply

[Joe Carey]

Joel Dare wrote:

> I should have prefaced my comments with the fact that I know nothing 
> other than the specs.  I know very little about electronics, but I'd 
> like to learn.
>
> My understanding (which is probably wrong) is that voltage is the 
> measure of force that pushes the electricity through the wires.  

This is correct.

> You measure the speed of that flow in Amperes.  

Yes.  Amperes can be expressed as electrons/second.

> So, typical power supplies are rated by volts and amps (at least 
> that's what all mine say).  Isn't wattage the measurement of heat 
> generated?  

Wattage is a measure of energy vs. time (this is the definition of 
power).  Sometimes that means heat.  Sometimes it's radio power.  
Sometimes it's physical movement.

> In which case, it seems silly for a power supply to list wattage, 
> unless it is the heat that "it" generates.  So, might the wattage of a 
> power supply have nothing to do with the Soekris at all?

Sort of.  You want a power supply that is capable of delivering enough 
energy to the board that will be drawing power.  The supply may be 
_capable_ of delivering more, but the board won't draw it. 

>
> The Soekris specs say "6-20V DC, max 10 Watt".  Does that mean that it 
> will take 6 to 20V DC power and that it generates 10W of heat at the 
> most?
>
>
Yes.  The Soekris uses a switching power supply, so it consumes nearly 
the same amount of power regardless of the voltage (within reason - 
please don't pick at the nits).  If you increase the supply voltage, the 
board will draw less current.

It also means that you need a supply capable of delivering at least 10W 
at whatever voltage you're running, and that that voltage had better be 
at least 6V and less than 20V.

I'm not sure whether or not Soren has included the power estimate for 
the wireless cards in that, but I believe he has.  Our measurements 
indicate that most of the time the Soekris boards consume considerably 
less than 10W.  Lots of things can affect the power consumption, and 
Soren's being cautious.


>
> Thanks,
> Joel
>
> At 07:24 PM 3/14/2004, you wrote:
>
>> On Sun, 14 Mar 2004, Joel Dare wrote:
>>
>> > the 4501 can take 6V to 20V DC with a 10 watt maximum.
>> > So, if your 12V 1Amp supply is 12 WATTS, then it's probably too high.
>>
>> A 12 volt 12 watt supply should be *fine* for something that wants 10
>> watts' worth of 6 to 20 volts. A power supply being over *voltage* would
>> be a problem; having spare current-supplying capacity is not. If you 
>> draw
>> less than the nominal supply current, the voltage supplied may increase
>> slightly; but in this case I'd imagine it's unlikely to go over 12.5V,
>> which is still well under the 20V maximum. It's pretty simple physics...
>>
>> cjb
>>
>> http://ocean.ucc.ie/~cjvdb1/
>
>
>
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