wb8foz@mthvax.cs.miami.edu (David Lesher) (12/09/90)
> How could one possibly have a fire, you may ask? Simple. Consider >a light bulb whose filament opens, with a short filament section drooping >against a support wire, thereby creating a *much* lower resistance. While >such lamp faults usually burn open within the lamp envelope, a virtual >short circuit can also result. Most readers have probably seen the effects >of a burned out lamp which is then jostled while under power. I have to second Larry Lippman's caution about shorted filaments. Once upon a time, in an agency far, far away.... I designed a LSI-11 subsystem that, as a minor part of its job, drove a bunch of those little lamps inside illuminated switches. These are those tiny bayonet base creatures. The 28 volt version is the 387, but these were 14 v. For several reasons, I drove them with 555's as the lamp drivers. They drew, ISTM, well under the rated current of the chip, even with both lamps installed. Several technicians built the boards, the wirewrap backplane, and the cables. After bench testing, we hauled it over to the main computer room where the testbed was, and installed it, connecting it to the installed switch panel. Now, I should point out that this was NOT one or two lamps. Each card had 16 channels, each rack had 32 cards, and no matter HOW we tried, it still added up to lots of amps! So we had gotten a 50 amp/14 vdc regulated supply just for the lamps. Well, we powered it up, and everything was fine, until I sent the command for a lamp test. At least 8 lamps were SHORTED. They did go blink, but their 555's also went BANG! and a cloud of smoke drifted up toward the ceiling, where a smoke detector just waited for an excuse to: sound LOUD horns, emptying 100+ people from the building notify the base fire dept. wet-charge the sprinkler system We were glad to notice the smoke drifting AWAY from the detector.;-} After that we tested all of the lamps, at the bench. A good 10% were FUBAR. We changed suppliers..... I'd hate to think what happens in 120vac lamps...... -- A host is a host from coast to coast.....wb8foz@mthvax.cs.miami.edu & no one will talk to a host that's close............(305) 255-RTFM Unless the host (that isn't close)......................pob 570-335 is busy, hung or dead....................................33257-0335