From: "Walter Harley"
Subject: Re: HELP WITH INDUCTION SETUP
Date: Fri, 15 Nov 2002 18:44:41 -0800
Organization: Posted via Supernews, http://www.supernews.com
Reply-To: "Walter Harley"
References: <3DD40746.4C3B0E33@xympatico.ca> <3DD5A5A7.717E1C22@sympatico.ca>
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"Joseph Legris" wrote in message
> You had better find someone who can supervise you directly if you really
> want to build a high-power pulse transformer - it is much too
> complicated for a beginner.
Joseph is quite right about finding someone to supervise you directly. But,
to answer your question about what he meant about the resistances:
The simple answer is "550 amps is one heck of a lot of current."
Ordinary-sized wires just aren't going to work. To carry that kind of
current you need big thick bars of metal, not little wires.
The longer answer is: Ohm's Law, one of the basic laws of electricity, says
that when a certain amount of current flows through a certain resistance, it
creates a certain amount of voltage drop. It's like saying that when you
push water through a pipe at a certain speed, the friction of the pipe makes
the pressure at the output be less than the pressure at the input.
(Resistance is like friction, water current is like electrical current,
voltage is like pressure.) Mathematically, it's written E = IR, where E is
voltage (in volts), I is current (in amps), and R is resistance (in ohms).
By the way, this "hydraulic" analogy is not quite perfect. But it's good
enough for this simple case.
Wire, unless it's superconducting, has finite resistance. Small, but
finite. Suppose you have a length of wire that has one ohm of resistance
(easy to do). Then, to get 550 amps to flow through it, you need to have
550 volts to push with. If you only have 1.2 volts, the most current you
can get is 1.2 amps. If you have 1.2 volts and you want to get 550 amps,
you need to have lower resistance: specifically, 1.2 / 550 = 0.0022 ohms.
That's where Joseph's number came from.
Your windings are probably a fraction of an ohm or even an ohm. So Ohm's
Law says there's just no way you can get the current you want without having
a lot higher voltage. (Which, by the way, is risky.)
There's also a law that says that power (in watts) equals current (amps)
times voltage (volts): P = EI. So, if you had 1.2 volts pushing 1.2 amps
through a 1-ohm length of wire, it would dissipate 1.2 * 1.2 = 1.44 watts of
power, as heat. That would not get it very hot. But if you had 550 volts
pushing 550 amps through that same wire, it would be 550 * 550 = 302.5
kilowatts, which is a decent fraction (say, 0.1%) of the output of a power
station, and the wire would instantly vaporize.