Re: Insteon Observations

[Marc_F_Hult <MFHult@xxxxxxxxxxxxxxxxxxxxx>]

On Fri, 15 Dec 2006 22:27:03 GMT, nobody@xxxxxxxxxxxx (Dave Houston) wrote in
message  <45831e9f.29307109@xxxxxxxxxxxxx>:

>"Dennis Brothers" <brothers@xxxxxxxxxxxxxxxxxxxxxxx> wrote:
>>
>>Hard to imagine how a defective LampLinc could damage an ordinary 100W
>>incandescent bulb.  On the other hand, it's easy to see how a current surge
>>as the bulb dies (typically accompanied by a blue flash) could damage a
>>triac.
>>
>
>Why would there be a surge when the (resistive) circuit opens? The inrush
>current when a cold incandescent is turned on is significantly higher than
>after the filament heats up. If the filament is thin in a spot, the normal
>inrush current causes it to break.

Dave,

When a tungsten filament fails, a "tungsten arc" can occur through vaporized
tungsten connecting the ends of the foreshortened filament. There is a
positive feedback in this effect, so as more tungsten is consumed to support
the arc, the current path through the remaining filament becomes shorter, and
the current increases causes more vaporization. This can result in the bright
flash that is sometimes seen when a filaments fails.

This is a well-known phenomenon. Some lamps have fuses built into the base to
provide some measure of protection. One of the functions of the RC snubber
circuit across a TRIAC is to protect the TRIAC gates from inductive spike
during filament failure that can damage the TRIAC. This is also a problem with
IGBT and MOSFETs used in reverse phase dimmers.

... Marc
Marc_F_Hult
www.ECOntrol.org