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