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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