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Re: LR demo circuit



There's an ubiquitous unpronounceably named coil with a removable iron
wires core and many accessories. The appropriate one is an Al ring. It
is ejected when the coil is energized with the line. I, independently,
but not originally, tried, say, 20 years ago, cooling the ring w/ lN2.
the effect is, of course, much increased.

bc

p.s. If a large choke or electro-magnet is unavailable, I suspect the
primary of a large power transformer will do.

sampere wrote:

I have a large electromagnet that I use as the inductor. The DC
resistance is about 2 ohms. I put this in series with an automobile
headlamp bulb and note the rise time for the bulb to come to full
brightness. This takes easily more than 1 second. Then swap the
inductor with a 2 ohm resistor and the rise time is much much faster,
almost instantaneous!

I like Denker's LN2 inductor. I've been meaning to make something like
that recalling conversations I had with Mike Roukes years ago. There's
a fun project. And something kinesthetic to boot!

Sam

Carl E. Mungan wrote:



I would like to build an LR circuit. For RC circuits, it is
relatively easy to get timescales on the order of seconds, which
makes demos and measurements easy with simple meters. However, it
doesn't seem to me to be quite so easy to do this for an LR circuit.
A reasonably well-wound coil has an inductance of a few tenths of
henries and a resistance of a few hundred ohms. Hence L/R is at least
3 orders of magnitude away from the seconds timescale and it seems it
would take a pretty unusual coil to bridge this gap.

So I'm willing to settle for having to using an oscilloscope to catch
the action. But now my question is what kind of switch to use?
Conservatively estimating an inductance of 0.1 H and a resistance of
one kilo-ohm (to allow use of an external variable resistor that
isn't dominated by the inductor's resistance), it seems to me that I
need to be able to switch cleanly on the timescale of about 10
microseconds. My experience suggests this is better than what
ordinary mechanical switches can do, which are subject to electrical
"bounces" upon contact. (I'm sure there's a technical name for this,
but you probably know what I mean.)

So what kind of switch should I use? Simple, readily available
solutions preferred. Thanks, Carl
--
Carl E. Mungan, Asst. Prof. of Physics 410-293-6680 (O) -3729 (F)
U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5040
mailto:mungan@usna.edu http://usna.edu/Users/physics/mungan/