Michael Rigby-Jones wrote:

>>-----Original Message-----
>>From: Randy Ott [mailto:randy@THEOTTS.NET]
>>Sent: 28 May 2004 16:12
>>To: PICLIST@MITVMA.MIT.EDU
>>Subject: Re: [EE:] Obtaining 5 Volts from live wire
>>
>>
>>On Fri, 28 May 2004 08:52:32 +1000, David Duffy wrote
>>
>>
>>>Martin McCormick wrote:
>>>
>>>
>>>
>>>>       Some of those electronic light switches that are for
>>>>incandescent bulbs only use a SCR instead of a triac so the
>>>>
>>>>
>>bulb runs
>>
>>
>>>>on half-wave pulsating DC.  If they do that in the circuit,
>>>>
>>>>
>>there is
>>
>>
>>>>the other half-cycle of AC that is un-touched and will be at about
>>>>the same voltage no matter if the light is on or off.  The
>>>>
>>>>
>>bulb will
>>
>>
>>>>burn at a slightly lower brightness because the over all
>>>>
>>>>
>>energy level
>>
>>
>>>>is .707 times the RMS level, but it will basically work normally.
>>>>
>>>>
>>>>
>>>>
>>>"Slightly lower brightness" ?? Try it for yourself one day.
>>>
>>>
>>Running a
>>
>>
>>>lamp with half wave is more than slightly reduced! David...
>>>
>>>
>>>
>>That's because the lamp sees 1/2 the voltage not .707 as was suggested.
>>
>>Randy
>>
>>
>
>No, it's running at half *power* (ignoring the reduction in filament
>resistance from running cooler).  RMS voltage is indeed reduced to SQR(2)/2
>= 0.707
>
>

The filament resistance won't be any different when running with half wave.
It doesn't change much over most of the range. The largest change is in the
first few % of the voltage range. Half wave or 50% phase control setting
will have the lamp typically at much less then half brightness. The curve of
a typical incandescent lamp is very non linear. More like an S curve. Not
as much change in light output at either end than at the middle. (vs
Voltage)
David...

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