Hi Roman.

What does the bulb see curent-wise with a resistor
at the peak of the sinusoid during turn-on on 240 VAC
RMS mains?

I once did a crude continuous-on (no switching)
life-test with a bulb (120 V RMS mains) I had on
an outside front porch.

I soldered one lead an appropriately-sized
rectifier-duty diode to the base of a bulb and made
the other the contact when the bulb was screwed into
the socket - normal bulb lasted a month (continuous)
and I recall that the dioded-bulb lasted 7 mos ...

During tests on the bench I noticed a considerable
change in the current waveform on a bulb which saw
both positive and negative swings of the AC mains
as opposed to a bulb which *only* saw one-half cycle
of the applied mains - the 1/2 cycle excited bulb
filament apparently cooled sufficiently during the
cycle it was off and the bulb then drew a noticable
amount of current more during the cycle it was on.

Jim

----- Original Message -----
From: "Roman Black" <fastvid@EZY.NET.AU>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Tuesday, June 11, 2002 7:02 AM
Subject: Re: [EE] Phase control dimming on a cheap pic


> Jim wrote:
>
> > Now the contest comes down to zero-crosing start for
> > a bulb as opposed to simple 'ballasting' with a
> > fixed resistor - although the resistor, by lowering
> > the operating temp of the filament, will also work
> > to lengthen the life of the bulb ...
>
>
> Hi Jim, the resistor will WIN this contest. :o)
> 60w 240vac bulb filament is about 60 ohms cold,
> which will dissipate close to 2kW at cold turn on.
> The filament does not heat fully in one half-cycle
> and even with a zero-crossing turn on device the
> filament will get almost 2kW the first half-cycle,
> reducing for each half cycle over the first 10 to
> 20. But yes, that will be SLIGHTLY gentler than
> turn on at wave peak.
>
> The resistor will give longer life than just using
> zero-cross turn on. But won't compare to proper
> soft-starting like many dimmers that slowly ramp the
> average voltage to the bulb over 1 second etc.
>
> I also don't like the soft-start thermistors, power
> thermistors undergo significant mechanical and thermal
> stresses everytime they start up (heat up) and are
> one of the least reliable devices i've come across.
> Yes the bulbs may last longer but the thermistors
> will self destruct.
>
> I have a incubator using a 100w light globe and a
> resistor dropping 7% average volts, even with the
> nasty 240vac supply we have here (often sees 255v)
> the thing has gone for many months switching on/off
> every few seconds. :o)
> -Roman
>

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