[rescue] I'm not an EE Q - 48 vdc followup (LONG)

[rescue at sunhelp.org]

~ But really I have no clue. If a little knowledge is 
~ dangerous.. what about none?
~ 
~ got this nice looking (new) 48v DC rectifier on ebay too*..   
~ I'm hoping
~ once it gets here, it will be obvious.  and what I need
~ 
~ but feel free if anyone can give me a short primer..

DC Netra's are cheap for a reason ;) Nobody wants to deal with DC unless
someone else (data center) is providing the DC.

DC is similar to AC from a distribution perspective, except you must observe
polarity. There is a "correct wire to use" in AC distribution (hot, neutral
or ground), but if you mix up hot/neutral you usually won't hurt anything
(at least not at first, you may kill someone later if they plug in a device
with neutral bonded to ground with no ground link). Mix up polarity in DC
and you usually have smoke or flames in short order.

Your 1st problem is the DC source. You can use a regulated and filtered
power supply that takes AC from your house and converts it to DC. You can
also use batteries along with a charger. You *cannot* take a rectifier and
hope to charge batts or run your Netras. DC power supplies are expensive,
esp. for -48V. Some people will claim the battery will "smooth" the lumpy
and over-voltage output of a rectifier, but in reality your Netra will see
the lumps and not like it at all. Some people may also recomend a 1/2 wave
rectifier/diode (cause its "effective" output is 50V), but this is more than
insane.

So, for economy, lets assume batteries. Your biggest problem will be battery
charging. You will need to choose a battery chemistry (all have unique
problems and advantages). Once you have a chemistry you will need to charge
the battbank at the voltage *and* current that the particular chemistry
prefers. The other issue is that your charger will have to provide enough
current to both charge and meet the draw of the Netra(s). Also, if the
number of Netras is variable, you need to consider the charge capacity as
$Netras change. You may now think ?wtf, why not just get an AC power
supply?. You can build a charging system cheaper than a regulated/filtered
power supply AND the batts will provide a built-in UPS function.

So, if you aren't discouraged by now (you shouldn't be :), Use the wire
gauge tables you've seen in email to build a distribution system. Fuse
everything, esp. at places where wire gauge changes and at the closest point
to the batteries as possible. If using wet cells (car batteries), enclose
your batteries and vent them to the outside world (they generate hydrogen
when charging and the acid fumes will ruin your wallpaper over time). Those
big vinyl storage boxes at Home Despot make nice enclosures.

OR, alternatively, contact the people who sell phone systems in your area.
Tell them you want old power supplies from ancient phone systems (1A systems
especially). These boat anchors always have generous ammounts of
filtered/regulated -48DC.

Last note:
People think that for DC wiring, black=negative and red=positive. This is
not true.  black=source and red=return. In -48 your colors *may* be reversed
from what you are used to. You may also think what I said is backwards, but
the negative post on a battery is where electrons "come from" and the
positive post is "where they go back in".

~ 
~ hell. i dont even have the terminology straight
~ rectifier = AC in DC out ?
~ inverter = DC in AC out? ?

rectifier = AC in and lumpy DC out.
inverter = DC in and either AC out (sine wave) or square/step wave out
simulated AC.
DC-DC converter = inverter and rectifier/filter/regulator in one package.