Re: battery box supplying 4 different voltages?

Joe Strout wrote:
In article <132imvp1n4ae8a0@xxxxxxxxxxxxxxxxxx>,
BobH <WanderingMetalHead@xxxxxxxxxxxxxxxxxxx> wrote:

The 3 DC-DC converters approach is the best way to get the batteries to drain evenly.

OK, this is good feedback. On other bots I've tried it both ways, and did indeed find (especially on my servo-driven bots) that it was easier to just have separate motor and logic batteries. But in this case, I have plenty of room and weight allowance, and I'm willing to invest more time in getting this battery module now so I spend less time switching batteries later.

Separate motor and logic batteries makes things simpler. Especially if you use the optoisolators Wayne mentioned.

Proper grounding design will also be needed. Remember that as much current and voltage drop occurs on the ground path as the +V paths. Star or single-point grounding with the common point as close to the battery negative or bulk filtering capacitors as possible will help.
>
OK. I'm planning to have a power bus running the length of the bot, with the idea that anything that needs power can tap into the nearest 3V, 6V, 9V, or 12V line as needed. If I understand you correctly, I should have eight conductors on this bus, rather than five, so that each voltage can have its own ground line. Then tie the four ground lines together in the battery box, right next to the batteries or caps.

Yes. The low current voltages can have smaller buses, but definitely separate the high current return from the low current and or logic returns.

So the whole thing would look something like this?
http://www.strout.net/temp/power-schematic.gif

...where the boxes are switching regulators of four different ratings? And the capacitors would be sized bigger for the 12V line and smaller for the lower voltages?

The capacitors get sized more for transient load current than voltage (other than needing caps rated for the voltage you are using). If you think about power distribution buses without capacitors, short term heavy loads will require the heavy load current be supplied from the battery. Every wire has resistance and inductance and pulling those short term heavy loads through your whole distribution system causes voltage drops across the resistances and inductances. Large capacitors act as local reservoirs to supply the large currents locally, so you don't get all the voltage drops throughout your power distribution system. You need heavy enough buses to handle the average current, but the caps supply the peaks.

I have sketched out and scanned a couple of drawings, but I don't have a good place to post them. If you want, I will email them to you and we can discuss from there. If you post them like your schematic, other people can comment as well.

Regards,
Bob
.

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