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Date: Sat, 19 May 2007 12:49:03 -0400> From: jsd@av8n.com> To: phys-l@carnot.physics.buffalo.edu> Subject: 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.> > _______________________________________________> Forum for Physics Educators> Phys-l@carnot.physics.buffalo.edu> https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l_________________________________________________________________