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Two electical questions

  1. Jul 6, 2013 #1
    1. I have a car that has some electrical/electronic weirdness; the basic issue is apparently parasitic draw of about 2.5amps @ 12v. When I connect the (problematic) component the chassis ground (connected to battery ground) what I observe is that the battery chassis jumps from 0v to something like 1.1v and I see the 2.5 amp draw. I connected the related chassis ground connection directly to battery ground and saw the amp draw go to 0.5 amps. This tells me there is some sort of grounding weirdness. My basic question is what kinds of things could/would cause chassis ground and battery ground voltage disparity? I imagine that a bad diode would let current go back into a circuit creating a potential loop where the voltage never got back to battery ground. Ideas?

    2. Unrelated to the first question. V = I/R is Ohm's law but if you take a 12w fluorescent high efficiency bulb and run 120v into it the math says there's a 1200 ohm resistor sitting in the bulb given by 120v * (120v/1200ohms = 0.10 amps) = 120v*0.10 amps. Likewise you might have a 100w bulb that would have a smaller resistance. My question is does wattage take into account heat generated at the resistor(s) in the circuit or just I*V? If wattage doesn't define energy but power then what formula defines total energy consumed as opposed to power?

    Forum looks great. Sorry about undescriptive title.
    Last edited: Jul 6, 2013
  2. jcsd
  3. Jul 6, 2013 #2

    Simon Bridge

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    A lot of what you wrote does not make a lot of sense, i.e.
    Can you see how someone would have trouble seeing what you are describing? What did you do with the component and the chassis-ground? Presumably you connected it - but how? Where else is it connected? For that matter - what is the component?

    Does this mean that an ammeter read 0.5A or that an amplifier drew 0.5A? Is the "related chassis ground" not the same thing as the "chassis ground" above? Must be because you said, before, that it was connected to the battery ground and now you are connecting it!

    i.e. please post a clear description.
    It just sounds like what you said - there is an electrical connection someplace you have not accounted for. I'm guessing you have not been careful to isolate the variables - perhaps you are unclear about what the different "ground"s represent?

    Part 2.
    V=IR and P=VI ... P=ΔE/ΔT

    R is a constant for ohmic conductors only - the resistance of a bulb filament depends on it's temperature.

    Light bulbs are usually rated with a power level in Watts as well as a voltage or a current level.
    These are ratings not actual values. If you turned down the voltage, the bulb dissipates less power (it gets dimmer).

    Relationship between power and energy: 1W = 1J/s
    1kWhr = 1000J/s drawn for 3600s or 3600kJ

    This help?
  4. Jul 7, 2013 #3

    jim hardy

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    I'm assuming you have a steel car not a fiberglass corvette or other exotic.

    Old cars get corrosion underneath the ground lugs that connect chassis to a wire.
    Especially when the last guy who had it off leaves out the star washer whose purpose is to dig into the metal for good contact.

    Another problem is when somebody is tapping into wirres that he only THINKS he knows what they are...
    A "ground wire" that's intended to carry say 1/10 amp from a gas gage will develop significant drop when somebody taps into it for a 50 amp boomer-amplifier. It'll probably melt and destroy nearby wires too. Don't even think about using the ECU Sensor Common, even though by ohmmeter it reads close to chassis.

    Battery has only a positive and a negative terminal. Negative is usually connected to chassis and mis-named "Ground", 'cause the rubber tires separate chassis from "Ground". Pre 70's VW's are positive "ground".
    Using your voltmeter determine where along your wire that voltage is appearing - start from center of battery negative terminal to the battery clamp, then to frame then to engine.

    Many older Fords have a peculiarity in their negative battery wire -
    one end goes to battery, other to engine block, AND -
    there's a third terminal in the middle of the wire for connection to car frame. It looks like it's just a physical restraining clamp but it's an important electrical connection.
    When somebody replaces that OEM cable with an aftermarket cable lacking the middle terminal,
    the engine is now connected to car frame only through transmission bearings and the tiny voltage regulator remote sense wire which soon melts. That causes multiple troubles.

    So take your voltmeter and figure out where your voltage drop appears. You can go from a screwhead to the metal immediately adjacent to check for corrosion underneath it - a pocket knife or ice pick is handy for scraping through grease and rust, clip your meter lead to it and use it for a test prod..

    Good luck and have fun !

    old jim
    Last edited: Jul 7, 2013
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