Interesting non-linear effects

[Karl W4KRL]

George WB5OYP made a fascinating presentation last night at the AMRAD
meeting. He showed two series RLC circuits energized at 60Hz. The resistance
elements were ordinary 100W incandescent bulbs and the inductances were the
primaries of common 120V transformers. By careful selection of the
capacitance and energizing voltage the circuit shows bistable effects with
the lights flashing regularly every few seconds.

 

There was discussion about the nature of the light bulb resistance as
possibly having negative resistance. Some notable components do exhibit
negative resistance over some range of voltages and currents. An arc in air
is a good example; exploited to great financial benefit by Marconi. The
filament of an incandescent bulb DOES NOT have negative resistance effects.
I made measurements of current versus voltage for a 100W and 60W bulb and at
no point does the current decrease with rising voltage. (See attached Excel
file and pdf printout)

 

The incandescent bulb (and vacuum tube filaments) have the well known
characteristic of low cold resistance that rises dramatically at operating
voltage. As George pointed out, this change of state combined with the
saturation of the transformer winding and consequent change in resonant
frequency results in bistable oscillations.

 

The Standard Handbook for Electrical Engineers 10th edition by Fink and
Carroll contains good information about the transient characteristics of
incandescent bulbs:

 

"Section 19-18 Transient Characteristics and Flicker

 

The low cold resistance of a tungsten filament results in a marked inrush of
current when the lamp is lighted. Though the current and the power
consumption rise momentarily to values far above those of steady operation,
the candlepower does not "overshoot" but rises gradually to its steady
value. The relative magnitude of the initial surge of current and power
depends to some extent upon the resistance and in greater degree upon the
inductance of the circuit in which the lamp is connected. If the lamp is
operated in an a-c circuit, the amount of the surge depends also upon the
phase of the voltage wave at which the switch is closed. Oscillographic
records of the starting of a 300-W, 120-V lamp, having hot-to-cold
resistance ratio 15.5, showed a maximum surge crest current of 47 A in a
circuit having 0.1 Ohm resistance and 0.04 Ohm reactance at 60 c. ."

 

I also found the interesting article "TPTDec99Filament" that shows
measurement of the light output and filament temperature.

 

I've got the transformers now I need the capacitors to replicate George's
apparatus!

 

73 Karl W4KRL

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