[ale] [way OT]: Workbench power supply?

[Sean Kilpatrick]

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On Thursday 01 May 2003 10:38 pm, Christopher Bergeron wrote:
> Jeff H actually gave me some good advice tonight.  Am I right in
> thinking that if I try to draw more current from a supply than it can
> handle, the power supply will be the only casualty?  I'm asking this
> because I have MANY PC power supplies that can give out 10A at 12V.


Let me try. 
A run of the mill car battery should provide about 600 Cold
Cranking Amps.  This is a measure of how much juice the battery
can be expected to provide over (I think) 30 seconds at (I think)
30 degrees F.  Note that many modern cars now run halogen headlight
bulbs which draw 45-50 watts each on low beam. A healthy battery
will be able to run those two bulbs for more than an hour and 
still have enough juice to start the car (at STP). So I would
anticipate that a fully charged battery should be able to power
a 10 amp, 12 volt, transformer for at least 10 hours.
As long as you are sure to provide your power supply with the
juice it is expecting (12 volt, DC in this case) you should not
be in danger of damaging your MB.
If you are going to be hooking several such power supplies up
to one terminal, make sure that you use a big enough piece of
copper wire between the terminal/junction box and the battery --
and the same size piece of wire back to ground.  Always err
on the side of too big.  To much copper wire never hurt anything
except maybe your wallet. If you try to pull too much juice
through wire that is too small, Bad Things happen. I wouldn't
use anything smaller than 10 gauge wire for a 50 amp load at
12 volts -- and that for only a short run - - say less than 5 feet.
Stay away from aluminum wire.
I suspect that the "Rubber Book" will have the data on wire gauge
relative to current draw.

An excellent place to get answers to your questions is at a marine
supply store. There is one on the North-bound service road for
I-85 immediately outside the loop. These days boats have to deal 
with  lots of 12 volt DC devices and 120 volt AC at the same time.
When a boat is tied to the dock at a marina, it is plugged into
120 volt AC and uses inverters to get the 12 volts DC it needs to
run various sub-systems.  Figure out how many DC amps you might
be drawing at any one time, add 30 percent, and take that number
to your friendly Marine supply dealer.  He can provide you with
the proper piece of equipment.  Please keep in mind that a
350 watt power supply in a mid-tower computer will be drawing about 
3 amps at 120 volts AC if it is working at capacity. Laptops draw
a fraction that much power. Unless you are trying to power 12volt DC
CRTs you shouldn't have a problem.

Sean

PS  The "Rubber Book" is titled "Handbook of Chemistry and Physics"
and (was) published by the Chemical Rubber Publishing Co., Cleveland,
Ohio. My ancient copy is the 38th edition, copyright 1956. I 
suspect that much of its data is now out of date. <grin>

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