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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 00:16:34 +1100
NNTP-Posting-Date: Tue, 29 Oct 2002 00:07:29 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"N. Thornton" wrote in message
> 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 extremely
short time constants. The 12 amp triac should be good for 1000 watts worth
> 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.
> 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 60
> 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.
. . . .. ............... Phil
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