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Re: data search.. RL circuits examples



It does not matter if the meter is analog or digital. The response should
be slow enough that almost any high impedance voltmeter will work.
Remember, the experiment design results in a long time constant (100
seconds). The A/D conversion time will not be significant. The resulting
curves look very good. If the capacitors leak (just a bit), the resultant
curves offer an interesting puzzle to the better students.

On the other hand, many oscilloscopes do not have a high enough input
impedance to obtain a 100 sec time constant without a LARGE capacitor.

Paul Hutta
PENN STATE at Abington



On Thu, 25 Apr 1996, Paul Hutta wrote:

I recommend using a DVM as a series resistor along with a 10 uF capacitor.
This provides a time constant of about 100 seconds( assuming a meter
resistance of 10 M Ohms). The students can take the data with a stop watch.
In fact if they plot a discharge curve, they can take the "ln" and estimate
the actual resistance of the meter.

This experiment works well with a wide range of students from engineering
majors to technicians.


Paul Hutta
PENN STATE at Abington


DVM's are "sampling" devices. The regularily "sample and hold" the
voltage at the probe tips and then digitize the value. This is why they
do not immediately respond to changing voltages. I recommend that people
use an ANALOG voltmeter for such observations or an OSCILLOSCOPE. These
will work for time constants a few times greater than the observer's
reaction times for starting and stopping a stop watch. The oscilloscope
will also show the whole voltage vs time curve!

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* bob muir voice: (910) 334-5669 *
* physics & astron. e-mail: bob_muir@uncg.edu *
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Paul Hutta; Engineering
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