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From: "Phil Allison"
Subject: Re: TRIAC "wears out" on dimmer circuit
X-Newsreader: Microsoft Outlook Express 5.50.4522.1200
Date: Tue, 29 Oct 2002 10:34:01 +1100
NNTP-Posting-Date: Tue, 29 Oct 2002 10:25:06 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"N. Thornton" wrote in message
> "Phil Allison" wrote in message
> > "N. Thornton" wrote in message
> > news:firstname.lastname@example.org...
> > > Secondly the bulb surge currents. 12x20w = 240 watts at 240v, 1A rms
> > > running. With bulbs at a tenth their R at switch on, that will be 10A.
> > > With mains at Vpeak in the cycle, that will be 14A. With noise on the
> > > mains as well it will be higher still at times.
> > >
> > > If you pick a 143 triac it should cope with that easily. But that
> > > isn't the end of the story.
> > ** The single cycle rating is 100 amps and 20 watt bulbs have
> > short time constants. The 12 amp triac should be good for 1000 watts
> > of them.
> OK, that sounds good, though that wasn't my experience.
> > > Triac voltage: 240v supplies see occasional spikes of way above 240 x
> > > 1.4. In fact if you dont have a filter on the input I would go for a
> > > 1kV rated part for reliability. Filtering is very desirable. A 400v
> > > triac with no filter is a no-no really.
> > ** Excess voltage merely triggers a triac on.
> When it is off, with cold bulbs. Excess voltage plus cold bulbs equals
> high surge current. Have to account for that.
** Done that already - no worse than a cold to full on start.
> > > Thirdly I assume you want this thing to survive. Dimmers are famous
> > > for dying when a bulb blows. When a filament bulb blows it sometimes
> > > produces an arc across the filament, and i can effortlessly exceed 60A
> > > until the bulb fuse blows. So I would want a triac that can handle
> > > more than just the running load.
> > ** An arc across the remaining ends will never be anything like
> > amps.
> Its higher in fact. There was a thread on here about it recently. You
> can check out the figures by checking it on google.
** Nonsense - please read what I wrote. You must be thinking of an
arc between the feed wires that connect to the filament. That is a big gap.
> > > Moral of the story: avoid capacitor surges, use a bigger triac, and
> > > fuse carefully to catch bulb failures.
> > ** The cap may be a problem - nothing else is.
> My own experience has been that triacs need a lot more headroom to
> survive long term. And triacs are known for dying when bulbs fail too,
> these are real issues I think.
** I service triac dimmers, light chasers and colour organs all the
time. A 12 amp triac will last indefinitely with a 4 amp (1kW) load made
from six or more lamps.
Only when the total load goes up to more like 2 kW or someone uses
a single 1 kW lamp do they protest seriously and the smoke escapes.
.. .................. Phil
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