> I just re-read the entire Capstone report, and I can't find any
> justification for the use of a separate power supply.

Aaaargh, sorry. I clearly remember discussing the power supply voltage 
issue in detail during the capstone project. Switching the entire rocket 
avionics bus to 5V would of course make us pretty standard USB accept 
for the connector.

We decided against this, however. Unfortunately, the only record I could 
find of this decision is that the capstone *requirements* that say they 
must use the 14V avionics bus. See:

http://psas.pdx.edu/CapstoneLv2bRequirements/

Let me quickly summarize what I remember to be our arguments:

1. Many of our systems require higher voltages, in particular 12V. These 
12V loads tend to be our big power hungry loads, like RF power 
amplifiers. Efficiency is critical here, and boosting 5V to 12V means 
twice the current flow and thus four times the resistive power loss 
which is a minus. Switching supplies are more efficient when they're 
switching small voltages, and the 14V Li Ion back is ~ 12V for most of 
its life, so that's a plus. Of course, it's less efficient for the new 
3.3V nodes, but they draw very little current compared to the high power 
12 V loads, so it's still a win to have the power bus ~ 12V.

2. Making power distribution 5V means putting a switching supply between 
the battery pack and the rest of the rocket. This is a big time minus 
because:

a. A failure in this main supply kills *everything* on the rocket that 
doesn't have backup power.

b. There is now a tremendous amount of switching noise on the main power 
bus that was otherwise pretty quiet. In particular, RF loads, other 
switching supplies, etc., will all cause fluctuations on the power 
supply bus. This is particularly bad for our sensor systems, where we 
want very, very quiet power supplies.

... I think there's a few other things I missed, but I think that 
summarizes the main points. Anyone remember anything I forgot?

- Andrew

-- 
-------------------------------------------------------
Andrew Greenberg

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

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