Re: [Gumstix-users] A power regulation example...

[Chris P. <par...@ie...>]

True, but since the vast majority of my power consumption is due to 
motor current, which is unregulated, maximizing the efficiency of my 
electronics' power regulator is not worth the effort.

Lets assume for argument's sake that Vbatt = 8V.  That means that there 
is a 3V overhead, and if my electronics are drawing 750mA (which is an 
overestimate, I am sure), my regulator losses are 2.25W and my 
electronics consumption is 3.75W.  The motors (3 of them) can draw 1.5A 
each, which is 36W.  That means that I am throwing away 5.4% of my power 
into regultor losses.  Going to a DC-DC regulator (assume near 100% 
efficiency) can at best increase my system efficiency by ~5%.  For my 
app, it is not worth the extra cost to use a DC-DC instead of a very 
cheap linear solution.

In practice, we don't see a 6-battery NiMH pack above 7.2-7.5V for very 
long, so the actual gains are even less.  Of course, if you can't muster 
5.5V from your batteries (e.g. a 2-cell system), you have to go with 
some sort of DC boost converter.

-Chris
As one of my engineering profs used to say: good design is knowing what 
5-10% you can discard without it mattering.

Dave Hylands wrote:
> Hi Chris,
> 
> On 2/16/07, Chris Parker <par...@ie...> wrote:
> 
>>I am running off of 5 or six NiMH batteries, but I want to keep it
>>flexible, as the boards that I have been designing are modular for
>>teaching purposes.  I'm not too worried about the overhead heat
>>dissipation, though.  With just no heat sink, the gumstix just warms up
>>the regulator (comfortable to touch).  With both the PIC and Gumstix
>>boards, adding a small heatsink keeps the regulator touchable.  I'm just
>>  developing the GUI to tune the robot's sensors now.  I'll post a link
>>to the design page for the 'bot when it is a complete (3 weeks or so
>>from now).
> 
> 
> Ahh. Yeah with 5 batteries, you're talking about a voltage range of 5v
> to 7.5v and with 6 batteries, it would be 6v to 9v.
> 
> If added a couple more batteries, then you could use a switching
> regulator and get alot more efficiency (i.e. longer runtimes), and
> even less heat dissapation.
>