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Re: Electrical Theory

Subject: Re: Electrical Theory
Date: Thu, 6 May 1999 00:25:37 EDT
OK Now I'll throw my $.02 in.
Yes, voltage is like electrical pressure (PSI) in your water system.  Current 
flow (Amps) is like the amount of water that comes out of the tap (gallons 
per minute).
You are correct that 12 volts flows through the circuit looking for a ground. 
 In your example of a light bulb with a disconnected ground you are correct 
that anywhere before the open circuit there would be 12 volts available.  
This is a very handy piece of info to keep in mind the next time you are 
trying to find where the dark lord put the open circuit.
Your comment about the lower voltage after the bulb is technically correct 
but it kinda bothers me.  After the bulb in a single bulb circuit there 
should be little to no voltage (less than two-tenths of a volt).  
One thing to keep in mind is that the load(s) in the circuit use up all the 
voltage.  If there is a single load it uses up all the voltage, if there is 
second load in line with the first the two together use up all the voltage.  
Before everybody hits the reply button and goes flame on saying that series 
circuits aren't used on cars let me explain.
If you have a dim headlight caused by a loose ground the bad ground has 
resistance which causes a voltage drop, which makes it in effect a resistor 
in a series.  For example lets say the bad ground is causing a 4 volt drop.  
The headlight is not getting 12 V like it is should because the bad ground is 
using up 4 volts. 12 - 4 means that the bulb is running on 8 volts.
The same holds true for your brake switch.  Corrosion causes resistance which 
causes a voltage drop, etc.
I hope this helps clear some of the fog.
Richard Ewald

In a message dated 5/4/99 11:21:32 PM Pacific Daylight Time, writes:

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

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