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Re: Lamps

To: sweisman@gis.net, triumphs@Autox.Team.Net
Subject: Re: Lamps
From: JAMES_S_WALLACE@HP-Canada-om1.om.hp.com
Date: Wed, 22 Jul 1998 17:21:34 -0400
     Sumner,
     We do have them here alright. I'll check that out - thanks.
     
     Here's my Dan Masters impersonation:
     
     "14 volts at 200 mA is indeed 2.8 watts, since P=IE, where P is power, 
     I is current, and E is voltage. At 12 volts, the same bulb would then 
     consume 2.4 watts. Bear in mind however, that the resistance of the 
     filament varies with the temperature, such that start-up current is 
     higher than once the filament has been warmed to its nominal rated 
     operating temperature. Further, the voltage output from a properly 
     adjusted voltage regulator in a car is more like 13.8 volts than the 
     oft-quoted 12. Not only that, but there's some question as to the 
     voltage rating system for car lamps anyway - it appears that some 
     wattages are derived assuming a 12 volt input, while others more 
     realistically use around 14.
     
     Now 2.8 watts is of course 2.8/2.2 = 1.273 times as bright as the bulb 
     originally supplied with the vehicle. While this would seem like a 
     bonus, there is also the function of the rheostat to take into 
     consideration. The rheostat, being in series with the bulbs, inserts a 
     voltage drop when the control is rotated that is equal to the 
     resistance thus being inserted in the circuit multiplied by the 
     current through the bulbs. Now a 2.8 watt bulb would have 27.3% more 
     current flowing through it than a 2.2 watt bulb, and that extra 27.3% 
     extra would also go through the rheostat, yielding a greater voltage 
     drop across the rheostat for a given number of degrees of rotation. In 
     conclusion, if you use brighter bulbs, you may have to turn the knob a 
     different amount than you used to for a certain amount of dashboard 
     illumination."
     
     Dan, if you're there, this is only meant as a friendly poke at your 
     _much appreciated_ and very thorough explanations.
     
     Regards,
     Jim


Subject: Lamps
Author:  Non-HP-sweisman (sweisman@gis.net) at HP-USA,mimegw5

Jim,
     
Do you have a Radio Shack in Ottawa?  According to my catalog, they have 
Part No. RSU 11910783, rated at 14 volts at 200 Ma. That's 2.8 watts at 14 
volts, probably about 2.2 watts at 12 - 13 volts.  It's good to derate 
incandescent lamps, they last much longer.  I haven't tried it so can't say 
for sure that it will do the job, but the picture looks right.  If not, 
you're only out $1.19 (US) for two lamps.
     
Sumner Weisman
62 TR-3B
     
     
     


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