Greasings and lubrications to all...
I find this whole thread about halogen light bulbs shocking! :)
Ohms, amps, volts and watts are electrical terms for physical
realities which can be expressed by mathmatical terms...ie numbers. Still with
me? Let us start with watts. The formula for watts (the unit of power, which
expresses the amount of work being done) is AMPS X VOLTS = WATTS. Lets put
this to practical use...I mounted 2 55 watt road lamps on the MGB. I wanted
fuse protection. So, to figure for amps, devide total watts,110, by the volts,
12...110/12 = 9.17...so my road lights pull 9.17 amps in operation. Adding in
about 30% margin (You would always be changing a 10 amp fuse) I selected a 15
amp fuse.
Heat is directly proportional to watts. Look at a hair dryer. 800 watt
setting gives x amount of heat. The 1200 watt setting gives more. So, a 55
watt road lamp makes x heat whether its halogen or not. (Well, almost. There
will be some difference due to physical construction) Also, 55 watt lamp will
pull the same current, AMPS, regardless of its type. Watts is watts. The
advantage to halogen lamps is you get more CANDLEPOWER, LUMENS, for the same
amount of WATTS. Same volts. Same amps. Same heat. More light.
The debate here is how to get really bright tail lights without melting
the lens. If you replace a 25 watt bulb, any type, with a 50 watt bulb, any
type, you will get more heat as well as light. But if you replace an old Lucas
tugsten 20 watt bulb with a modern halogen 20 watt bulb, you will get a
noticeable increase in light, without melting your lenses or burning up your
wiring. The improvement will NOT be as great as with a 50 watt bulb, but then,
the trade off is you eliminate the heat problem.
Back to theory. Volts is defined as Electromotive Force. Analogy is
air pressure. Amps represent electrical flow. How many electrons pass a point
in a circuit in a given amount if time. Analogy...cubic foot per minute, or
gallons per minute. Ohms is the unit of electrical resistance.
Application...Lets use dim tail lights. Measuring from the ground side
of the lamp socket to chassis we should see very few Ohms. Very little resist-
ance. If we see 10 ohms its time to get out the emory cloth.
Another application...road lamp idiot light. After installing a set of
road lights and the dashboard indicator, we find the indicator much too bright
for comfortable night driving. We know this is a 5 watt bulb in a 12 volt
circuit. We will select a resistor to dim the bulb by half, but not put it out
totally. So 5 watts over 12 volts equals .42 amps. Knowing this, we compute the
resistance of the bulb, using the formula for Ohms law. Volts = Amps X Ohms.
So...12 / .42 = 28.57. The resistance of the bulb is 28.57 ohms. Now, if we
double the resistance we can half the current, therefore halfing the watts
dissapated by the bulb ...remember the watts formula. So its off to Radio Shack
to get a 30 ohm resistor. But wait...our current will be .21 amps (close)
passing through a 30 ohm resistor. Hmmm...that means a "voltage drop" of 6.3
volts will apear across the resistor.... .21 amps x 30 ohms = 6.3 volts. So?
How many watts will the resistor body dissapate? How HOT will it get?
Remember the hairdryer? Amps x volts = watts, so .21 amps x 6.3 volts = 1.32
watts. We will need a 30 ohm resistor rated at 2 or 3 watts. The resistor
would then be installed between the lamp and ground. Now, in reality, we'd
get a 30, 50 and mabye a 75 ohm resistor and try each one, making a totally
subjective judgement, using all our math and theory as a starting point.
Now, in confusion...
more watts, more light, more heat...
halogen bulbs make more light per watts applied than tugsten.
if watts stay the same, so does the heat, amps, and volts.
Volts = Amps X Ohms
Watts = Amps X Volts
Jennifer Joy`s writings on dim bulbs contains a lot of non-electrical
solutions that would certainly help. While you`ve got the lenses off, run em
through the dishwasher (when wife NOT home) then polish them with ordinary
buffing compound like you`d use on dull paint. Makes them shiny and new.
I wonder how often poor ol` Lucas gets blamed for ordinary road filth
stuck on headlights? Dirt you can see can reduce light transmission by 20%.
All of this from a guy who's never heard his B run, much less driven
the thing. But, I did pull all the wiring and electrical out and o/h`ed the
whole mess. And got it all working again!
Wade (what a bright idea!) Massengill
If everybody has one, it's not a sports car.
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