From: tom@afthree.as.arizona.edu (Thomas J. Trebisky)
Newsgroups: rec.audio
Subject: Re: Subwoofer power supply
Date: 22 Dec 92 23:50:07 GMT
Organization: University of Arizona, Tucson, AZ

>Again a big hearty thanx to everyone who's helping me on this... ya
>just can't learn this stuff in school it seems... (it's much too
>practical)...

	:-)

>Anyway,  could someone tell me where in a power supply design the
>amout of output current get determined?  I understand the
>transformer determines the voltage, diodes do the rectification and
>capacitors/inductors will handle the filtering (leaving the
>regulation for some lucky IC!) - but I don't see where the output
>current is determined (V=IR somewhere I assume).  Is this equation
>true: Volts@primary supply x Amps@primary supply = Volts@other side
>of the transformer x Amps@other side of transformer x efficiency of
>transformer?

	See below.

	A power supply has a max current capability, but the actual
	amount delivered depends entirely on the load (unless the load
	tries to pull more than the supply can dish out).  This is
	exactly why amps can deliver more power to 4 ohm speakers than
	to 8 ohms.  Voltages being constant, twice as much current and
	thus power gets delivered to the 4 ohm speakers.

>Also (I swear this has something to do w/audio!) can I send my
>subwoofer amplifier more than 12V in a quest to give it 15amps or
>so?

	If it is a unit designed to go in a car, this would probably
	be a very bad idea, but notice that a car battery actually
	delivers more than 12V (check yours with a voltmeter, it is
	more like 13.2V, but don't quote me on that.)
	Just stick with 12V.

	Hey, do you have a subwoofer amp that is designed for use
	in a car and expects 12V supply, or are you building your
	own, or just what are you dealing with anyway.

	If, as I assume, you have a subw amp designed for car use
	and thus expecting 12V with plenty of amps, and you wanna set
	it up for in-home use.  You are in the market for a line-voltage
	to hefty 12V supply.  You could shop around (I wonder if Radio shack
	has something.....) or build your own.
	Do you wanna build your own? (Not very hard really).
	Start with a big transformer (see below), hefty diodes, a big filter
	cap.  And don't forget a line cord, fuse, and a box to put the
	whole mess in to avoid electrocuting visiting small children.
	It is fun, educational, and a potential source of lots of excitement!
	:-)

>Finally, what is the current that comes out of a wall socket? I
>see it's 110VAC @ 60Hz, fine, but what's the current?

  The load determines the current.  If you plug in a 15W nite-light, it will
  pull about 0.1 Amp.  If you plug in a 1000W toaster, it will pull 9 Amps
  or so.  At about 15 Amps or so, a circuit breaker calls the whole show off.

>But, does the amount of current not matter because the line current is
>isolated from the output current by the transformer?

	Um, er ....

>Is 110volts
>pulsating 60x/sec. all that's important to the transformer/rest of
>the power supply?

There are little dinky transformers that convert Line voltage to 12VAC,
and there are big mama transformers that do just the same thing.  Each
transformer has a VA rating that tells you how much power it can deliver.
As near as I can tell (some EE major will cringe) VA is short for volts*amps
and is some cryptic way to specify the Watts the transformer will deliver.
Just remember Watts = Volts*Amps and Ohms Law and you can figure all this
stuff out.

When you exceed (or try to do so) the VA rating of a transformer
several things happen, I am not sure in just what order, namely:
The transformer overheats, the voltage drops because the field in the
transformer begins to collapse, the fuse on the primary side of the
transformer blows.  Without an appropriate fuse, a variety of exciting,
smelly, perhaps pyrotechnic things can take place.  A good first cut for
a fuse value is Transformer VA rating / 100 rounded up to the nearest amp
(maybe add an amp or so for mama transformers, use fractional amp fuses for
baby transformers).  Fuses are your friends.

>THANK YOU EVERYONE WHO HAS A MUCH BETTER GRASP OF THIS STUFF THAN ME!

    Have fun, don't kill yourself!
--
	Tom Trebisky	ttrebisky@as.arizona.edu
"Did you know that 500,000 trees are consumed in America each week
to produce our Sunday newspapers.  Never mind recycling, why even
take the paper?"