Re: LR demo circuit

[Gary Karshner <gkarshner@STMARYTX.EDU>]

Carl,
        Try a square wave generator for the switch. It is repeating action
makes it easy to generate an oscilloscope display.
        Your beginning to be aware of the problem of inductors. Not only
do they have a sizable internal resistance, but they also contain
capacitance. Remember to conductors separated by insulator make a
capacitor, as in the turns of the coil. So a coil has a natural resonance
frequency and tends to ring. Also inductors with a core don't have a
constant inductance over a very large frequency range.
        A few years ago we developed an RLC lab exercise and had quite a
time finding an inductor that would give us an easily measurable Q. Most
were either far to sharp and the students would miss it, or they were far
to broad and they couldn't find the resonance peak.
        Best of luck with these fickle parts.
Gary

At 03:49 PM 7/6/2004 -0400, you 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/