> The argument given for item 1 was a bit fallacious.  If it takes (for 
> instance) 2W to run a transmitter and you supply 5V to it, then you will 
> be obliged to reduce its resistance enough to draw 400 mA to run it.

Don't forget, we're using switching supplies here. So we're not obliged
to reduce the resistance. We might want to, but for other reasons, we
might want to just live with the voltage drop.

> you supply it with 12V, then you only need 167 mA to get the 2W output.  
> The 14 V wiring  could actually be smaller (and hence lighter) than for 
> the 5V.

I didn't mention that, but yes that's right, that's another reason to go
with the higher voltage. In fact when we did the system analysis, I
believe we chose the wire size to lose no more than 1% as I2R losses.
But I'd have to dig it up.

> I completely agree with your reasoning that two sets of wires is a good 
> solution.  I don't see building a voltage divider circuit (to give 5V 
> from the 14 wiring) as cost effective or weight effective.

As I mentioned, I think it's not worth it. We have have an amazing
little switching power supply design that the last capstone group
designed that integrates very, very nicely with the LPC2
microcontroller. I just don't think we need another parallel power
supply for it: it'd be designing something that the rocket doesn't need.
And it's easy to use a wall wort to give the boards the 12V supply they
need to run if you really want to plug it into a standard USB connector.

Let it be said that I love resuse, and I'm for the concept of open
hardware. But like open software, things (sometimes?) get forked for a
real reason. I'm loath to start massaging our own systems for other
projects. We'll wind up with lots of compromising designs that will end
up being a pain down the road.


Andrew Greenberg

Portland State Aerospace Society (http://psas.pdx.edu/)
[EMAIL PROTECTED]  P: 503.788.1343  C: 503.708.7711

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