J
John Fields
On Sat, 27 Nov 2004 12:48:37 -0600, John Fields
On Fri, 26 Nov 2004 18:02:46 -0800, John Larkin
On Fri, 26 Nov 2004 19:42:05 -0600, John Fields
On Fri, 26 Nov 2004 17:23:14 -0800, John Larkin
Which brings up the concept that an incandescent lamp appears to have
a capacitive component of impedance, which is itself a function of
frequency.
---
It may seem that it does if you're referring to the inrush current,
but put a resistor in series with the lamp and the voltage and current
will be in phase across and through them both, I believe, since all
that changes is the resistance of the lamp filament.
The filament has a substantial 120 Hz temperature cycle (you can hear
it with a photocell) and the tungsten has a positive TC. So the
resistance varies with time. The thermal lag results in the filament
resistance peaking later than the voltage peak. So the current leads
the voltage, which looks like a capacitive component.
---
Since there's no energy storage in the form of anything other than the
incidental capacitance and inductance of the filament, I don't see how
that can happen. That is, whether the resistance is parametric or
not, it's still just resistance and the current which will be forced
through the filament will remain in phase with the voltage forcing it
through.
Seems to me it would be akin to a simple resistive divider where one
of the resistors is variable, like this:
E1
|
[RV1]
|
+---E2
|
[R2]
|
0V
Since there's no reactive term in there, then the total impedance of
the string is simply the resistance, R1+R2, and E2 will always be
equal to
E1R2
E2 = --------
RV1+R2
for any instantaneous value of E1 and RV1 and any value of R2.
But the resistance in question is time-varying at 120 Hz. And phase
shift is not determined by an instantaneous measurement.
---
Yeah, poor choice of words. You can measure the phase shift by
measuring the time from the zero crossing of one signal to the zero
crossing of the other, measuring the direction of crossing, measuring
which one crossed "first", and all the rest of it...
---
To check, I did this:
240RMS>----+-----> TO SCOPE VERT A
|
[LAMP]
|
+-----> TO SCOPE VERT B
|
[576R]
|
240RMS>----+-----> TO SCOPE GND
The lamp was a 120V 25W incandescent, the resistor was 576 ohms worth
of wirewounds in a Clarostat power decade resistor box, and the scope
was an HP 54602B. I found a phase shift of about +/- 1.1° max which,
since it varied randomly about zero seemed to me like it might be
quantization noise.
But, there was the inductance of the decade box to consider, so in
order to rule it out I measured it and it came out to about 6mH, which
comes out to an Xl of 2.2 ohms at 60Hz, so the angle due to the
reactance of the box comes out to 0.109° which, being an order of
magnitude smaller than what the scope measured, puts it way down in
the noise.
How did you measure the phase shift? Looking at the zero crossings?
They will obviously *not* be shifted by a time-varying filament
resistance.
---
Excuse me???
You stated that there would be a phase shift between the current
through the lamp and the voltage across it, and my measurement, which
measured the difference in time between the voltage across the lamp
and the current through it showed that the voltage and current
waveforms were congruent, refuting your earlier statement which you
now seem to be abandoning.
Perhaps we're talking apples and oranges here, but I'm of the opinion
that if there's a difference in phase between current and voltage
their zero crossings will occur at different times.
The zero crossings obviously can't move, since there can be no current
anywhere in this setup when the line voltage is zero. But the time of
peak current is not simultaneous with the voltage peak, because the
filament resistance varies with time and doesn't peak at the voltage
peak. This is not a paradox, because harmonics are present to make
everything work out. If you were to measure the Fourier fundamental
component of current, *that* would lag the voltage.
You can google "incandescent filament harmonics" and such for some
references.
The old classic HP audio oscillators, the wein briges with
incandescent lamp amplitude levelers, had increased harmonic
distortion at low frequencies because of the wobble in the filament
resistance.
---
OK, but I think that was due to the fact that the filament was used as
a gain-changing element, so when the output frequency got down low
enough for the loop time constant to start looking like an appreciable
part of the output signal's period it wasn't capable of operating so
much like a gain control as a modulator.
---
The light intensity lags the voltage waveform because of the thermal
lag of the filament. And the filament resistance has a positive tc, so
it lags too.
---
OK, both true, _but_ the fact that the filament resistance lags
voltage doesn't mean that there's a phase difference between the
voltage across and the current through the lamp.
All you've got, in essence, is a pot and a fixed resistor in series
being driven by a sinusoidal source, and no matter where you choose to
look _in that circuit_ voltage and current will be precisely in phase.