# Slightly OT - Electrical Puzzle

• posted on September 13, 2009, 7:05 pm

Using a handheld voltmeter, at the battery on my car, I get a reading of around 12.5 volts. When I use the same voltmeter at the cigarette lighter socket and at the accessory outlet, I get a reading of only around 4.5 volts. (The latter reading jumps about a volt if the engine is running.) Why aren't the readings on the cigarette lighter socket and the accessory outlet pretty much the same as at the battery? (Both the cigarette lighter socket and the accessory outlet work just fine to recharge my laptop computer.)
Enlightenment appreciated.
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CWLee
Former slayer of dragons; practice now limited to sacred
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• posted on September 13, 2009, 7:18 pm

Assuming the sockets work as you say, you may have a bad connection of your voltmeter test probes at the point you are trying to measure. The voltage should jump a volt or two with the motor running as it takes more voltage to charge the battery than what it is normally.
I have also seen bad connections on the wiring that measured the normal voltage, but under load the voltage would drop. That is not your case.
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• posted on September 13, 2009, 9:02 pm
Ralph Mowery wrote:

If it's the same with two jacks, it must be the ground. I'll bet if the OP measured between the point he thought was ground and the negative terminal of the battery, he would find approximately twice the resistance of his meter.
When humidity is low, clothing rubbing against car seats can cause static shocks when a person touches grounded metal. If the car manufacturer puts some megohms between that metal and ground, touching it will drain the charge painlessly.
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• posted on September 13, 2009, 10:29 pm

Actually the car metal does not have to be grounded. As the car is a large mass of metal it will have a differance of potential from the person that slides across the seat. PUtting a resistor between the car and ground will do nothing for this effect.
Years ago some gas trucks would drag a chain to ground the truck. This was thought to drain off any static charge the truck would have so the gas would not blow up when the hose was used to transfer the gas. A spark could jump from the end of the hose to the storage tank.
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• posted on September 14, 2009, 1:51 am
Ralph Mowery wrote:

Fuel trucks still use static dissipation devices. Instead of chains there are conductive straps that are less likely to cause a spark when dragged across metal. The straps are used on cars, trucks and motor cycles. Sometimes the chains you may see under a truck could be automatic snow chains. It's some interesting stuff.
http://www.mizter.com/index.htm
http://www.onspot.com /
Check out #9
http://www.wilsonantenna.com/tsswrt.htm
TDD
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• posted on September 14, 2009, 12:17 pm
wrote:

Looking at that automatic tire chain sitting so close to the ground got me to wondering.
If he ran over a piece of tire debris and knocked it loose, that could cause some serious damage to his truck.
Andy
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• posted on September 14, 2009, 1:57 am
On Sun, 13 Sep 2009 18:29:02 -0400, "Ralph Mowery"

Not what the guy said. Put a resistor between the metal part you might touch and the VEHICLE ground, and the likelihood of getting a static shock can be reduced dramatically.

When fueling aircraft the fuel tug (truck) MUST be grounded to the plane before the fuel hose gets CLOSE to the plane.

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• posted on September 14, 2009, 6:37 am
Ralph Mowery wrote:

Not exactly my field, but I have read that modern car tires have enough carbon in them to be a ground path. Might be high resistance?
I agree with other that the OP probably does not have a good connection.
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• posted on September 14, 2009, 12:28 am

~10 years ago, when 3M still made the "touch me first" anti-static strips for computer keyboards, I took one and stuck it to the armrest/door handle of my car then ran the wire in behind the trim and attached the wire to some point on the body. It worked great. Wish I could still get them!
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• posted on September 15, 2009, 8:49 pm
Bob M. wrote:

Window glass typically has the right amount of resistance to drain static safely. If you could pick up static at the arm rest and carry it to a conductor that brushed against the glass, that should work.
Another trick is to wipe an insulating surface, such upholstery or an arm rest, with a soapy wash cloth. The soap film will allow static to bleed across the surface to a ground.
Polyurethane soles, which are very common these days, insulate so well that static can build. If I start getting shocks, I use a soapy cloth to wipe the edges of my soles. I wish I knew something that wouldn't wash off the first time I walked on wet grass!
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• posted on September 16, 2009, 12:58 am
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Just hang on to something conductive and grounded to the chassis as you get out of the car, until your feet are firmly on the ground and you will never get a static shock getting out.
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• posted on September 17, 2009, 10:42 pm
E Z Peaces wrote:

Static electricity.... The local walmart got all new shopping carts (buggies down south). They are painted with a fairly thick coat of paint. If your hands are on the painted surface, or even the plastic handle, as you stroll along you keep getting zapped with static shocks. They also got some electric cart pushers with a remote so that one person can move a loooong line of carts. They stand up front and steer the first cart and the electric one is in the rear pushing, operated by remote control. The first thing they tried to fix the static was to hang chains from the remote cart pusher but that didn't do a thing. I think they have been using anti-static spray on the hard rubber wheels and that lasts for a little while. Even all summer although humid outside it's air conditioned enough inside so you get the shocks all year around.
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• posted on September 14, 2009, 1:21 am

You might try scratching the leads against the metal when you take a reading.
You should be getting at 12 volts.
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• posted on September 14, 2009, 2:18 am
CWLee wrote:

Hi, Picked a good grounding spot and pushed center connector inside the jack? If it is truly 4.5V Nothing plugged in there would work.
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• posted on September 14, 2009, 3:03 pm
The cigarette lighter should be about the same as the battery. This would be measuring DC volts with the multimeter.
1. Does the ignition switch need to be on for this to work? Also be sure you are measuring directly to the socket itself and not through any plug-in device (which could change the voltage).
2. The problem could be with the +12 volts which would be the center of the socket, or the ground which would be the round metal. Get a long wire or jumper cable and connect it to the + of the battery, then measure from that to the ground of the lighter socket (this tests the ground). Then switch the cable to the ground on the battery and measure from that to the +12 center connection on the socket (this tests the +12).
If the ground is bad, check the ground connection to the socket.
If the +12 is bad, check the fuse and measure for +12 at the fuse. If +12 good at fuse, but not at socket, then problem with the wire or connectors between the two.
If the +12 is not good at the fuse (you would probably be having problems with starting your car in this case), check all main electrical connections and wires.
How to test automotive fuses... http://www.dinosaurelectronics.com/Test_Fuse.htm
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