Mike wrote,
> What's the best way to step 12V down to 6V? I picked up an
> Ooogah horn that's 6V and want to install it in the Frontier.
Good question Mike. Fair answer follows:
To divide voltage requires resistors in series. In short, you need
to put a honkin' big resistor in series with the hot wire to the horn.
You need to know the current drawn by the horn. You can learn
that with a 6v battery and an ammeter. Perhaps the necessary current
is published in the specifications for the horn? The fuse size
recommended is LARGER than the current requirements of the horn.
It has to be or the fuse would blow every time the horn, uh, blows.
Here's the math:
There are two equations that see to most of our needs. They are:
P=IE and E=IR
where,
P is power in watts
I is current in amps
E is voltage in volts
R is resistance in ohms
So why not just measure the resistance of the horn? Because the
measurement, while correct for a static value, does not represent the
correct resistance when operating. An easier to understand example
is with a lamp. The resistance of a cold lamp filament is much lower
than that of a hot filament. Resistance changes with temperature.
(That's how temperature sensors work; many working in reverse.)
This horn is even more complex because the source of the 'noise' is a
small motor! Suffice it to say, we're better off measuring current than
resistance.
Let's pick a number - say, 10 amps. So in this example let's assume
we read 10 amps current with the horn operating on 6 volts. Using
E=IR and rearranging it to be R=E/I and substituting our knowns we
get,
R = 6/10
or,
R = 0.6
Therefore, the working resistance of the horn is 6/10ths of an ohm.
(Remember, this is our hypothetical case.) Now let's see how much
power is being used. Using P=IE and substituting our knowns we
get,
P = 10 x 6
or,
P = 60 watts
That's about the same as a single high-beam headlight bulb here in the
States (55w). So in our hypothetical case, to develop the same current
across the horn at 6 volts from a 12 volt source, we need a resistor 0.6
ohms and capable of intermittently handling 60 watts. Those values are
certainly within the resistance and power range of ignition ballast
resistors.
One thing to remember; These resistances are small so small variances
may cause large voltage differences. That is, one might be tempted to use
a 0.7 ohm resistor in place of a 0.6 (if that is the calculated value) but
the
voltage error would result in the horn seeing only 6/7ths of 6 volts (5.1
volts) while the resistor saw 7/6ths of 6 volts. In this example, the horn
will probably work okay on 5 volts but it should be obvious that the error,
if present, should be ONLY in the higher resistance direction. A resistor
too
low in value would cause the horn to see voltages too high, perhaps
damagingly high.
Okay, you asked a simple question and I wrote a book. Sorry. I hope
it helps anyway.
Rick - physicist, researcher, teacher, author
'76 Midget
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