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Re: [Phys-l] Ugh - RLC circuit issues



On 05/19/2007 11:41 AM, Josh Gates-fac wrote:

#1- they have a crystal radio kit, and they're trying to identify the
frequency of the radio station that they're picking up.

Practical but non-physicsy answer: Listen for the station
identification at or near the top of the hour.

#2- another circuits project, a la Mythbusters this time. They're
looking for the "break even" time for a fluorescent light bulb
to make up its high energy start-up cost in low steady-state consumption. We trying to measure the current drawn with a PasPort current probe. The
max current for these is listed at 1.1 A, and the steady state current for
the 20W tube should be .167 A, but the start-up current is apparently high
enough to spark, fry, and let the smoke out of the probe...

That sounds expensive.

Are we talking about an honest-to-goodness current probe,
or just a differential voltage probe? The latter has a
10V common-mode voltage limit, so depending on where you
put it in the circuit, it will blow out instantly, no
matter whether any current is flowing or not.

The second-best solution is to use a differential voltage
probe and a low-impedance resistor. It is easier to
make them than to buy them, if you're not fussy about
the temperature dependence:

22AWG 16.1 milliohms per foot
20AWG 10.2 milliohms per foot
18AWG 6.39 milliohms per foot
16AWG 4.01 milliohms per foot
14AWG 2.53 milliohms per foot
12AWG 1.59 milliohms per foot

Be sure to use a four-point probe geometry
http://www.av8n.com/physics/img48/four-point-probe.png
The point being that if you inject a current via terminals
1 and 4, and measure V23 using a high-impedance differential
voltmeter, you are sensitive only to the resistance R0 and
/not/ sensitive to any of the parasitic resistances in the
leads (R1, R2, R3, and/or R4).

You still have to take care to put the sensor in the right
place in the circuit; otherwise you will be fooled by the
less-than-perfect CMRR of the differential voltmeter, and/or
you will blow out the voltmeter entirely by exceeding its
common-mode limits.

At some point in here we should mention the idea of current
divider. That's just like a voltage divider, but divides
current rather than voltage. Impedances in parallel rather
than in series. I don't think you need one in this case,
but they are quite handy when dealing with larger currents
(e.g. 400 amps).

=============

In any case, what you really want is a wrap-around current
probe. Low-tech ones use a little transformer and only
work for AC, while fancy ones use a Hall-effect sensor
and work right down to DC.
http://www.aemc.com/techinfo/techworkbooks/current_measurement_probes/tech_current.pdf
http://www.pqmeterstore.com/index.php?products_id=1365

Such a thing is better than a whole bottle of Excedrin:
it makes headaches go away.