Re: Fwd: Wall switch dimmers work with 12v halogen?

["Keith Doxey" <keith.doxey@xxxxxxx>]

Hi Nigel

> Thanks for the further detail Keith- I didn't realise that asymmetry
was
> the reason- I assumed it was just the phase angle between voltage and
> current, so the switching point might be 10% in the voltage waveform,
but
> 50% in the current one.
>

The phase angle causes a different problem in that when the voltage drops
to
zero there is still current flowing. When the current drops to zero there
is
voltage across the triac. The net result of that is that once turned on,
the
triac stays on until power is removed. Careful design of the snubber
network
can overcome most of that. Also some dimmer manufacturers (Zero88 and Mode
Electronics are a couple that I know of) have an adjustment for the firing
angle when used on purely inductive loads like neon. Other manufacturers
advice fitting a parallel resistive load to damp down the inductive load.

I can sympathise with the guy who destroyed the transformers.....
I once connected some festoon lighting to one of my lighting controllers.
The total load was less than the 1000W rating of the unit and the triacs
were 16 amp devices, but the load was made up of 50 x 15W (total 750W)
bulbs
and the cold resistance of the bulbs was so low that the surge vaporised
the
triac and blew a hole in the PCB :-(((

Keith

Keith Doxey
http://www.btinternet.com/~krazy.keith
Krazy Keith's World of DIY HomeAutomation




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