Wiring from battery and switch questions

bombboyer · 11-17-2002, 11:06 PM

I'm going to run power directly form my battery to my inverter, and I know I need a 30A inline fuse. Where can I get one? Does the Shack carry em?

Also, I read somewhere about using the sequential door locks on factory keyless entry systems to power on a computer (http://members.aol.com/kustomz28/pro...ivebypass.html), and I was wondering if this would work. I have an AC pushbutton switch I'd like to use to turn my computer on and off from the front of the car.

I also want to be able to power on the car by doing a double press on the unlock button in the keyfob. How could I wire it so either method would power on the computer? I'm asking because I'm wondering if the DC current will flow through the AC switch and ruin it. Also, can I just hook the 12V DC pulse to the two pins on my mobo, or am I going to need this relay: http://www.radioshack.com/product.as...5Fid=275%2D248

Also, will the above progressive door lock method work on any factory system? (specifically my 1999.5 Pathfinder)?

Thanks.

Ohgary · 11-18-2002, 08:28 AM

To properly fuse a line you need four fues. A fuse on the + side near the battery, A fuse on the - side near the battery. A fuse on the + side after you pass through the firewall, a Fuse on the - side after you pass through the firewall.

This way, any short is fused.

Telek · 11-18-2002, 11:29 AM

Uhh, you only need a fuse on the + line, and you certainly don't need 4. Most people just run a cable from the battery to wherever you want the power, put on an inline fuse there (you can get inline fuse holders for like $8 from most automotive parts stores, Canadian Tire I know carries them), or you can get funky looking fuse holders from most car-audio stores for a little more expensive. If you want to be paranoid, then put an in-line fuse holder right after you take the power off the battery, and then you don't need a fuse holder anywhere else. This is usually very unnecessary because as long as you attach the cable securely where it can't get frayed or cut then you don't need to worry about the cable shorting out before you get to the fuse.

Putting fuses on the negative line is pointless, since the car uses negative earth ground. Since the ground is actually the negative connection, you cannot short anything from the negative terminal to anything except the positive terminal, and since you'll have a fuse on the + line already you don't need any more fuses.

Putting more than one fuse on the positive line is pointless as well.

Additionally you will need to use the relay. The computer electrical system ground is isolated from the car's ground, and the switches for power don't run off of +12V anyway (and so attaching the +12V to the computer might just fry it). You will also notice that you can't re-use pins to connect to other switches (i.e. you can't use the '-' pin on the reset switch and the '+' pin on the power switch to turn the computer on since it's not really + or -, it's just completing a connection).

Also switches don't care about what type of current you're using. DC or AC will both work through any switch.

Ohgary · 11-18-2002, 01:18 PM

Most people who do this are fools. What happens if you short the positive cable anyplace before the fuse? You get a melt down

In your config, what happens if you short a positive line with the negitive return line, You get a melt down. There should be 4 fueses
one on each leg of the power on each side of the fire wall. Otherwise your asking for a short and/or fire.

A single fuse inside the vehicle directly connected to the battery is asking for trouble.

Quote:

Originally posted by Telek
Uhh, you only need a fuse on the + line, and you certainly don't need 4. Most people just run a cable from the battery to wherever you want the power, put on an inline fuse there

Electronicsuk · 11-18-2002, 02:38 PM

It's best to have a couple of fuses, but if you put your fuse right next to the battery there is little chance of a short before the fuse. Since current flows in a full circle around the circuit, one fuse should protect the inverter, right? I have never quite understood the need for so many fuses. I could be opening up a can of worms here, but can someone explain the need for 4 fuses?!?!
Best regards,
Matthew

Ohgary · 11-18-2002, 02:55 PM

If you only fuse the positive line on the inside of the vehicle then you have a direct short to ground anyplace between the battery and the inside fuse.

If you only have fuse on at the battery on the positive side you can short the negative side to some other incorrectly fused positive line and get a dead short to the battery.

The reason for the fuse near the device is to prevent heavy currents during a short from going through the device. If your amp is direct attach to a battery with a fuse at the amp then a short of the ground to a + line will cause a high current flow through the + of the amp through the amp and down the ground line.

In a perfect world, you can get by with 2 fuses at the batter on both + and - lines. With the realworld and comming through a firewall, 4 fuses is the safest best.

TruckinMP3 · 11-18-2002, 02:56 PM

I read a different post awhile back indiacting a specific distance from the Battery (12 to 18 inches I think) for the positive line fuse.

If 4 fuses were needed, all autos would have 4 fuse blocks from the factory. Many have 2 some have 1. I think the big auto makers are right on this one.

TruckinMP3

TruckinMP3 · 11-18-2002, 02:57 PM

I read a different post awhile back indiacting a specific distance from the Battery 12 to 18 inches I think for the positive line fuse.

If 4 fuses were needed, all autos would have 4 fuse blocks from the factory. Many have 2 some have 1. I think the big auto makers are right on this one.

TruckinMP3
Sorry about the dup post, must have double clicked :-(

Ohgary · 11-18-2002, 03:26 PM

I agree, for factory cables that have a defined purpose, well know cable routes and million of dollars in planning to eliminate the risk then 1 fuse block inside is fine. However for everyone else 4 fuses are neeed. also keep in mind, many of the "wires" in cars are fuse wires that will break when they are shorted.

mp3z24 · 11-18-2002, 04:12 PM

well... ask any car stereo installer about fusing and the answer you will get is...

2 fuses

one RIGHT after the positive battery post. that one protects agains shorting due to a wire becoming exposed due to cutting, cracking, rubbing... etc

the second fuse goes near the device recieving the power. that is a general safety fuse for the equipment and is generally lower rating than the fuse at the battery.

the fuse at the battery is only for preventing a major electrical disaster and a subsequent fire.

NO ONE has ever told me or anyone to fuse the negative/ground wire. the ground wire is generally run from the equipment to the nearest solid mounting point on the frame or interior body. somewhere that you can get a nice metal to metal contact for the ground wire.... it is also usually as short as possible.

if you run a wire back to the battery for your ground, why would you want to put something in place that will prevent your equipment from being properly grounded...

~mike

my $0.02....

Ohgary · 11-18-2002, 04:40 PM

My background is RF and not AF installations. Power is always 2 wire. 1 hot, 1 ground from the battery. Going to the frame for power ground is asking for trouble. Might get away with it in audio installations, but in high power moble transmitters its going to get you.

With that I guess Ill go. 2 fuses on all wires comming directly from the battery. If you dont run both wires back then your covered. If you do your covered.

Telek · 11-18-2002, 04:43 PM

Ohgary, calm down. You don't need to be so obnoxious.

Quote:

What happens if you short the positive cable anyplace before the fuse? You get a melt down

Which is why I gave 2 propositions: Either put it close to the battery (thus making it difficult to short) or make sure that your cable is securely attached and won't be shorted out and then put it by your device that you're powering.

Quote:

what happens if you short a positive line with the negitive return line

Not if it's after the fuse. See response #1

Quote:

A single fuse inside the vehicle directly connected to the battery is asking for trouble

Funny that. I've used that setup for 4 years now and never had an issue. Also strange how my buddy who works as an engineer at GM agreed with my installation and the way that I did it. I just put a 30A fuse right next to the battery and then ran the cable to the trunk to my amp. If there is any shorts at all, the 30A fuse will blow. I modified this setup to put a second fuse in the trunk so that I could power my inverter for my carputer as well, but really that wasn't necessary.

Quote:

The reason for the fuse near the device is to prevent heavy currents during a short from going through the device. If your amp is direct attach to a battery with a fuse at the amp then a short of the ground to a + line will cause a high current flow through the + of the amp through the amp and down the ground line

No, if the line is shorted BEFORE the fuse near the amp, then you have a problem with the battery and wire, but any device attached to that line will be unaffected (current takes path of least resistance). If the fuse is near the battery and the wire is shorted anywhere between the fuse and the amp, then the fuse will blow and there is no problem.

Quote:

There should be 4 fueses [sic] one on each leg of the power on each side of the fire wall

I think your response is the best response to what you said there:

"most people who do this are fools".

Fusing the negative line is *pointless*. Unless you have done a lot of crappy wiring in your car then you're not going to find any incorrectly fused +ve lines. Additionally most "ground" connections are done nearby the device to the vehicle chassis, so they're not run by any rogue +ve lines anyway. For example I have a screw in my trunk that is into the chassis and that's my ground connection.

Even if, for some unknown reason, you want to run a second wire directly to the -ve terminal of the battery, since the entire chassis of the car is ALSO THE -VE TERMINAL, the only way that you can cause a short is to have that cable itself short out another +ve cable before a fuse. If you're going to run a second -ve cable to the battery, you're most likely going to do that away from any +ve leads anyway. But this is a dead point because nobody runs a 2nd -ve line straight to the battery.

So my opinion remains, you only need one fuse. If you want to be super careful, put 2 fuses, one near the battery and another near the device that you're powering. Note that almost all electrical wiring in the car is single fused, and

Quote:

many of the "wires" in cars are fuse wires that will break when they are shorted

no they're not. They're 14-16 guage wires which will melt when there is a short simply due to the current that passes through them. Unfortunately these melts can occur anywhere in the line which makes them a real PITA to replace, thus why fuses are used.

Telek · 11-18-2002, 04:46 PM

Quote:

Originally posted by Ohgary
My background is RF and not AF installations. Power is always 2 wire. 1 hot, 1 ground from the battery. Going to the frame for power ground is asking for trouble. Might get away with it in audio installations, but in high power moble transmitters its going to get you.

Why is using frame for -ve a problem?

I look under my hood, and the -ve terminal of the battery has 1 connection: to the engine block (some I've seen run to the frame near the battery instead). There is a thick wire from the +ve terminal to the starter motor, which is essentially using the chassis for ground. Similarly everything else in the car is grounded to the chassis, nothing else runs to the -ve terminal of the battery.

Ohgary · 11-18-2002, 06:39 PM

I am not being obnoxious, I am answering the question.

Now back to the issue, If you always use the chassie for ground this the entire issue of fusing the ground is eliminiated. If however you do run a wire to the battery +ve or -ve then it should be fused.

There are a number of unfused +ve connections under the hood, Heck even the new GMC trucks are using -ve switched head lights, and the headlamp circuit have +ve active even when the headlight is off. Now ould you prefer to replace a fuse or a headlight fused link? The fuse is cheap and easy to install.

As for making sure cables are tight to solve the problem. Fuses would be need anywhere if there were never problems. There are however going to be problems and therefore fuses.

As for your GMC engineer freiend. He might want to check GMC's service bulletin http://service.gm.com/techlineinfo/radio.html

Note the fueses on both +ve and -ve, also note they done use chassie for -ve. Also check out Icom install document http://www.icomamerica.com/support/d...c-706mkiig.pdf Not both + and - lines are fused.

As for short before the fuse taking the path of least resistance, 1 your thinking only of dead shorts, not equipment malfunctions and routed shorts. With 2way gear you also have a second ground source via the antenna connections. So a +ve Sort on the -ve power lead of a radio will run through the radio to -ve (ground) of the antenna.

bombboyer · 11-18-2002, 07:08 PM

OK guys, enough of the flames. Can anyone answer the question?

BTW: I got a 30A inline fuse to put near the + terminal, and my inverter has a 30A fuse built in.