----- Original Message -----
From: "aussierob" <aussierob@odyssey.net>
To: "Military Vehicles Mailing List" <mil-veh@mil-veh.org>
Sent: Saturday, December 15, 2001 3:27 PM
Subject: [MV] Tail Lights - You Do the Math - Re Brightness
Oh dear, Rob and the rest, you have fallen into the classic trap as follows:
>
> Original Message ----- Tail light Brightness
>
>
> One of our listers, last night, (Friday) wrote correctly, about the tail
> light being less bright. However, they were way off in the math regarding
> the statement that I snipped below.
>
> It was mentioned, that a variable power supply made a 12 volt light work
> nice and normal.
> Then, when he halved the voltage to 6 volts ...the light was only half as
> bright.
>
> Not true.
> It's called the inverese square law... and you can't change physics.
>
Not wholly true either in the stated instance. You can't use your eyeball to
_measure_ light, its not linear, its almost logarithmic like your ears, plus the
iris compensates by opening and closing, visibly 4 times less light LOOKS like
about half, you need a professional light meter.
Ever used a semi-auto SLR camera with metering ? One f stop of the lens is
twice or half the light and one standard step in exposure is the same also, and
on a slightly grey day you find you need an f stop less and one speed down at
least !
> I have entered the formula showing what happens when you "halve" the
> voltage on "anything"
>
> Halve the voltage you get only one quarter the power (not half, as
> mentioned.)
>
> In other words, whether you speaking of anything electrical in a Direct
> Current - DC circuit, (which is our Mil-Vehicles - Willys vehicles etc.)
>
Can't fault your algebra, theory or maths, but, you have assumed an incandescent
filament bulb is a perfect resistance and that it certainly is not.
Why stop with DC ? AC circuits are the same without any impedance, (L or C),
Ohm's law is Ohm's law; except perhaps past the event horizon of a black hole
but you'd need to see Stephen Hawking about that. So we'll not go into AC
theory, reactance, imaginary resistance, vector diagrams and the j operator
here.
What it is, is a temperature dependant resistor and it makes a big difference.
I have just run out to the workshop and found a 12V 5/21W stop/tail bulb, using
only the 21W stop filament I connected this to my variable, stabilised bench
supply via a digital ammeter with another digital volt meter across the bulb
contacts so I could set voltages _exactly_ across the bulb; since the digital
ammeter necessarily has a tiny, but significant sense resistor in it. (these
things actually measure voltage across a very low value resistor for current and
do the I = V measured/known internal R for you).
At 12V applied it draws 1.78A giving 21.36 watts and therefore a hot filament
resistance of 6.742 ohms.
At 6V applied it draws 1.24A giving 7.44 watts and therefore a not so hot
filament resistance of 4.838 ohms.
As you can see the power variation for a nominal 21W _filament_ lamp from rated
to half voltage is not 4 times but only 2.871 times, and you can see the
filament resistance has not remained stable but changed by 28.24%.
Worse still the cold filament resistance actually measures 0.6 ohm so it changes
by a whole decimal order from cold to hot ! Try measuring a domestic light bulb
cold and doing the sum, you will not find it is anywhere near the printed
rating.
Similarly, for a diesel heater glow plug you can also expect the resistance to
change from one voltage to another, but probably to a lesser degree.
Richard
Southampton - England
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