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RE: K M Light Bulbs & Khz (fwd)



---------- Forwarded message ----------
Date: Mon, 15 Dec 1997 13:25:51 -0500
From: "Thornton, Russ #CSR2000" <ThorntoR-at-rc.pafb.af.mil>
To: 'Tesla discussion Group' <tesla-at-pupman-dot-com>
Subject: RE: K M Light Bulbs & Khz

Could there possibly be an added effect here. When I inspected a light
bulb filament under a magnifying lens or microscope I found that it was
a coil of a coil.  Could there be some inductance effects going on here?

Russ Thornton
CSR 2040, 
Building 989, Rm.  A1-N20
Phone: (407) 494-6430 
Email: thorntor-at-rc.pafb.af.mil


>----------
>From: 	Tesla List[SMTP:tesla-at-pupman-dot-com]
>Sent: 	Saturday, December 13, 1997 8:38 PM
>To: 	'Tesla List'
>Subject: 	Re: K M Light Bulbs & Khz
>
>
>From: 	Antonio Carlos M. de Queiroz[SMTP:acmq-at-compuland-dot-com.br]
>Sent: 	Saturday, December 13, 1997 7:18 PM
>To: 	Tesla List
>Subject: 	Re: K M Light Bulbs & Khz
>
>Tesla List wrote:
>
>> As regards the light bulb, I am afraid that it WILL cause an error in the
>> measurements. An applied 60 Hz sine wave AC voltage will NOT result in a
>> sine wave shaped CURRENT through the bulb. This is due to the fact that
>> the resistance of the bulb is a function of applied voltage. 
>
>Actually, for any metal resistor, the resistance is essentially linear, but
>varies significantly with the -temperature- of the metal. You can easily
>verify this trying to measure the resistance of a light bulb (better a small
>one) with a common ohmmeter. You can see the resistance increasing just as
>consequence of the heating caused by the test current.
>In a test at 60 Hz the bulb is maximally heated 120 times per second. Of 
>course its temperature varies a bit also at 120 Hz, causing resistance 
>changes and distortion. But with usual lamps the thermal time constant is 
>of at least a good fraction of a second, so there is no time for significant
>temperature change. I would not expect errors of more than 1-2% in a
>measurement of coupling coefficient using light bulbs as current limiters.
>
>> If you want
>> to see this on a scope, then put a 100 ohm fixed resistor in series with a
>>100
>> watt lightbulb. Observe the voltage waveform across the resistor and you
>>will
>> see that it is slightly distorted. You can see this best on a dual trace
>> scope by superimposing the original 120 VAC waveform and the waveform
>>across
>> the resistor. Scale and superimpose. In this case the resistor is being
>>used
>> as a current sensor for the non-linear lightbulb.
>
>I have made a similar test, using a signal generator and a 5 V sinusoid. I
>used
>a 10 Ohm resistor, since my 100 W bulb presented only 10 Ohms of DC cold
>resistance.
>I could not observe any distortion, even when subtracting from the voltage
>over
>the resistor an attenuated copy of the input sinusoid, cancelling the
>fundamental
>component. Bulbs of other powers resulted in the same. If there was some
>distortion,
>it was much below 0.1% in my tests.
>
>> And so I live with it and go on about life with great joy anyhow :)
>
>Ok. Other error sources are probably much higher.
> 
>> 60HZ is nice because it is FREE. I use the electric company's super 60HZ
>> low impedance signal generator that is available at my friendly
>> neighborhood wall outlet. How nice and convenient. Not to mention cheap.
>> And accurate enough for my needs. Most meters are optimzed for 60 HZ
>> anyway :)
>
>60 Hz is low enough to avoid other influences that could perturbate the
>measurement, as parasitic capacitances, and it is easy to get a signal at
>high power, avoiding difficulties with the measurement of low level signals.
>The coupling coefficient used in the idealized modelling of a transformer 
>-is- independent of frequency, exactly as the inductances. 
>How accurate is the model is another subject...
>
>-- 
>Antonio Carlos M. de Queiroz
>mailto:acmq-at-compuland-dot-com.br
>http://www.coe.ufrj.br/~acmq
>
>
>
>