RE: LD11 failure

["Keith Doxey" <ukha@xxxxxxxxxxx>]

Hi Martin,

> -----Original Message-----
> From: Martin Greenwood [mailto:greengreengreen@xxxxxxx]
>
> Has anyone had this failure before...
>
> Lamp is on at half brightness. LD11 has 240V in.
>
> Cannot adjust or turn on or off using either momentary switch or X10
> commands.
> Powered it off and on a few times to reset it but no joy.
>
> Only clue is that other working LD11's buzz when at half brightness,
> whereas this one doesn't?
>

<SHORT_ANSWER>
Its Knackered :-(
</SHORT_ANSWER>

<LONG_ANSWER>
Almost cetainly easily repairable :-)

Havent had an X10 failure like that but have had plenty of lighting
controllers that have died with the same symptoms.

Sounds like the triac has failed.

Did this occur after a bulb failure by any chance ?

When triacs blow they normally go short circuit, sometimes fully short
circuit in which case the lamp is full on, sometimes only in one direction
in which case the lamp is on for either positive or negative half cycles of
the mains.

Sounds like yours has failed in one direction therefore the lamp is on for
50% of the time hence the half brightness.

The reason for the lack of buzzing is that the buzz comes from the filter
choke in the module and is caused by the sudden surce of current when the
triac turns on. This makes the windings of the choke rattle as they move
due
to the magnetic field created.

When the dimmer is working, this is worst at 50% as the triac is being
turned on at the peak of the mains cycle. The noise lessens either side of
50% as you are before or after the peak mains voltage therefore the current
surge is less. Approaching minimum or maximum the mains voltage is very low
at triac turn on and gives virtually no surge.

In the case of your module, because the triac has failed, it is permanently
conducting in one direction so the lamp current flows the moment the mains
voltage moves away from zero and continues to flow until the next zero
crossing. This means the current gradually builds up and decays therefore
there is no surge to cause the choke to buzz.

Replacing the triac should solve the 50% brightness problem. This could
also
be preventing the proper operation of the dimmer, or something else could
be
damaged. I suspect the triac will be the only fault though.

</LONG_ANSWER>

HTH

Keith