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Old 28th August 2005, 02:31 AM   #1
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Default Staying grounded

This may be a dumb question, probably is, but how can I make sure I'm grounded while working with the + and - wires coming from the car battery. I've heard they can send a lot of current through you if you're not careful (which also may be wrong). Just want to make sure when I'm testing my amp I'm not injured. Thanks in advance.
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Old 28th August 2005, 03:28 AM   #2
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The *last* thing you want to be is grounded if you're working with mega juice. The electrons in the wire you're touching want to go to ground -- don't give them a path!

A rule of thumb for working with worrisome electronics is to only work with one hand. That way, you won't send any electricity through your heart (in one hand and out the other to ground).

Wes
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Old 28th August 2005, 08:39 AM   #3
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Haha, and here I am, a potential electrical engineer. That's scary. Obviously I meant to say how can I be sure I'm NOT grounded, but I was at work, and not thinking properly. Anyway, thanks for that tip, I'll be sure to keep it in mind.
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Old 28th August 2005, 08:56 AM   #4
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a car battery can produce tremendous currents - but not into you, you're skin simply have a very high resistance, you can easily touch 12V connectors, and you wont feel a thing ( but NOT any higher voltage than that! )
What you should consider when working with car batterys, is the high current output - if you short anything to ground, then cables will burn , batterys can explode, you might not even have a car left afterwards ...
IF you dont use correct fusing, which you MUST. ALWAYS.
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Old 29th August 2005, 02:29 AM   #5
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You could get a shock from the output of a amp though
800Wrms at 4ohms is 56.6Vrms.
I would stay away from ignition related parts of course but am unsure of there current abilities. If you short something you could risk getting burnt also.
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Old 29th August 2005, 05:32 AM   #6
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almost no shock hazard from a car battery, but it cna melt/set fire to wires if shorted, and weld things. do NOT drop tools onto it, be careful when working with the wiring to it, and do not wear metal bracelets/watches around it.

Be sure to fuse all wires comming off the battery positive. also fuse at any point you switch to a lighter gauge wire (like at a D-block).

When working with the car's electrical system, it's a good idea to disconnect the negative from the battery.

The ignition system should be prettymuch out of your way when working on the car (and is only live when the engine's on), so it shouldn't be an issue. Do be careful around the plug wires and other stuff (there's also high voltage at the coil primary, too), since even though the shcok will only be painful and not dangerous in itself, it might make you recoil and fling your hands into something spinning on the engine. But that's only if the car is on at the time.
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Old 29th August 2005, 07:25 AM   #7
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Thanks for all the tips. I'm not doing anything complicated, just hooking up a simple amp to my speaker wires so I can plug my mp3 player in and hear it. It's only 18W per speaker, it's no sub amp or anything. I guess I'll need to look up fuses now, hadn't considered those in my original designs.
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Old 29th August 2005, 11:26 AM   #8
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A few things I'd like to point out.

The negative wire does NOT come from your battery, if it does you are creating a faulty instalation, the Battery's negative pole should be connected to the engine block or a solid part of the chassis and the negative wire for your amp connected to the chassis using as short a wire as possible.

Whenever you are working on your instalation and PARTICULARLY when working on the power feed REMOVE the fuse that you have on your positive wire from the battery as close to the battery as possible or disconnect the wire from the battery.

If you do not have a fuse right next to your battery install one NOW and feel free to give yourself a bit of a slap too :)

These are the first and most important rules to working with car audio. Do pay attention to them, please.

Keep signal leads away from power leads and speaker leads and yada yada do not use chassis ground for speaker negative in order to save wire, wear sunblock, look before you cross the street and so forth.
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Old 29th August 2005, 06:16 PM   #9
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here's the typical installation of a car amplifier:

you run a heavey guage wire from the positive of your car battery to the amplifier. There is a fuse inline with this wire by the battery, 18 inches or closer. remember to protect this wire and keep it from shorting out (watch runs with the chassis of the car, other wiring/hoses, getting crushed under the floor and in doors, etc.) . shorting out after the fuse will simply (hopefully) just pop the fuse. However, if the wire between the fuse and battery shorts out, it will metl/catch fire and there will be fireworks, and possibly flames and tears. if you have some extra electrical tape or some of that mesh or spiral plastic wire stuff, you might consider wrapping your wire in the from the batteyr to the fuse if there is any chance of it rubbing up against something. The rating of this fuse must be equal or great than the fuse and/or current rating of the amplifier. the fuse must be equal to or smaller than the maximum current that your wiring is capable of handling (see one of those charts that lists amps per variuos gauge sizes). The wiring must be rated to handle the current you will be pulling. If you plan to add more amps later, use bigger wire so you don't have to pull annother wire later!

The other end of this wire will go to the positive terminal of the amplifier. the ground of the amplifier will go to chassis with a short, several foot, length of wire that's of the same gauge as the power wire. Just connect the other end of this to some random screw that goes to bare metal in the car somewhere. don't fuse the ground. ground it to the nearest available spot, don't run it back to the battery. do not tie the speaker, power, and/or signal ground together, either.
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Old 29th August 2005, 06:48 PM   #10
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> When working with the car's electrical system, it's a good idea to disconnect the negative from the battery.

Assuming you know the combination to re-activate the stereo and/or any other anti-theft systems in the car.

When I don't know these details for a given car, I will hook up a 2A current-limited 12VDC power supply in parallel with the battery, THEN disconnect the negative lead. This will provide enough power to run the stereo, computer, etc in "car off" mode, but will limit the damage caused by short circuits and such.

> do not tie the speaker, power, and/or signal ground together, either.

Considering your amp is probably internally bridged, yeah, that's extremely good advice. Doing so could kill it.

And that plastic mesh stuff, used for making wiring looms etc., is cheap and plentiful at autoparts stores. There is no excuse for not using it in a potential rubbing situation. I even use it to tidy up my entertainment centre.

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