Re: low voltage & bulb species

From: Michael M. Butler (
Date: Thu Jul 05 2001 - 21:13:57 MDT

There are two factors here. Simplifying slightly:

1) there is a thing called the "strike voltage" which must exist so that the mercury vapor inside the tube will turn
into plasma. If the strike voltage isn't present, at least briefly, the lamp will not light properly. The voltage
required to maintain the plasma is lower than the strike voltage, so once on, the lamp might stay on through some
voltage dips. I don't know the innards of your light bulb, but I am guessing they have an "electronic ballast"--that
circuit might be able to handle a slight undervoltage as long as it's a relatively clean near-sinusoid. This brings us

2) ordinary incandescent-compatible light dimmers are a no-no for fluorescents because they do not produce the
aforementioned RCNS. Instead, they typically use a thyristor (triac or quadrac) which is triggered periodically to
conduct current at some point in the applied external ("mains") sinusoid. So the waveform they produce looks like a sine
wave with part "chopped off"--in the extreme case of nearly-off, only a tiny part of the wave gets through, looking like
a series of tiny sawtooths separated by zero volts. As one increases the dimmer dial setting toward "bright", an
increasing part of the wave gets through. Problem: there is an abrupt rise in voltage every half-cycle. Fluorescents and
their driver circuitry tend to not like that.

Apples and mutant pomegranates, sort of.


Damien Broderick wrote:
> At 07:37 PM 7/5/01 -0700, Spike wrote:
> >> incandescent bulbs produce about half
> >> their normal
> >> illumination that low (and if they are 130 volt long life bulbs, they
> >> will be even less bright), while still using 91% of the energy, which
> >> will prompt people to have more lights running at the same time to keep
> >> up the same illumination, thus wasting 40% more energy.
> >it incentivizes her to replace her incandescent
> >bulbs with low power flourescent.
> I reveal my elementary ignorance: do fluoros work if you lower the voltage?
> I have a couple of dandy little long-life 15-watt lamps = 75 watt
> incandescent or maybe 100 watt, but they are explicitly said not to work
> with a standard light-dimmer rheostat.
> Damien Broderick

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