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Re: Electrical Theory

To: Fred Pixley <>
Subject: Re: Electrical Theory
From: Rick Huber <>
Date: Tue, 04 May 1999 17:45:35 -0500
Fred and others,

Since I started this brake switch is hot thread, I thought I'd throw my
2 cents in.

In my tiny mind I understand it this way.  Voltage is like the pressure
of water in a pipe and current is like the flow of water through the
pipe.  Resistance to flow causes pressure drop, roughly equivalent to
power.  With the system energized with the 12 volt battery to an open
switch, there is pressure, or 12 volts available to the switch, but no
current flow, like pressure up to a spray nozzle on the hose.  Once the
switch is closed, when you press the brake pedal, 12 volts is available
to the light bulb.  If the ground past the bulb is open, i.e. not
grounded, then there is 12 volts available through the whole circuit,
but still no current flow.  Once the ground past the bulb is closed, as
much current flows through the system as the element in the bulb will
allow (it's the orifice or the spray nozzle on the hose), so the element
is like an orifice in a pipe.  12 volts upstream, current flow of a
couple of amps through the element,  producing power and therefore light
and heat, and then much lower voltage on the downstream side of the bulb
back to ground.

As has been said already, normally, there is essentially no resistance
anywhere in the circuit except at the light bulb element.  However, when
the contacts in the switch become corroded over 24 years, they become
the resistance in the circuit more than the elements in the bulb, so the
power is taken across the contacts in the switch and it gets hot. 
Downstream of this orifice, there's not enough voltage left to light the
bulbs.  The analogy here is a kink in the hose upstream of the nozzle,
therefore no flow through the nozzle when it's open.

So what I did was take the switch apart, clean the contacts very
thoroughly (remove the kink in the hose), and now I have brake lights again.

It seems so simple now trying to explain it, why couldn't I figure it
our this easily when I was troubleshooting?

Safety Fast,

Rick Huber
75 V8 B Daily Driver
65 B undergoing lengthy restoration

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