just a quick electrical circuit question

[Gary]

hhhhhhhhhhhhi

ive recently fried a headunit at least im pretty sure i have

i wired in a kill switch for the car (off the battery negative), but ive ran a wire from the battery side of the switch to the headunit so i can still keep my settings etc. not sure how but the headunit has managed to earth itself out to other earths in the car, and when i tried to start the car (to see if my switch worked) i think ive pulled starting current through the headunit and its broke

ive replaced the headunit and im insulating it from the rest of the car, so that should do the trick, but i am quite useless so i want to make sure. will a 10A fuse on that negative wire prevent another $200 or so mistake?

basic diagram:

h/u ground
^
|
|
|
battery neg ----------------> kill switch ------> cars negative/ground circuit

[o_man_ra23]

Make the negative fuse the same size as the fuse on the back of the HU. Make sure that is the only earth going to the HU also, grounding the outer shell on the car is a bad thing. And that can include incidental contact with the metal frame of your dash (if your dash has a metal frame). You will want the earth lead going from the battery to your HU to be fairly thick too, or you will induce engine noise into the music. Any time you forget to turn the kill switch back on, it will be likely that you will lose your settings.

[Gary]

10A is the h/u fuse, weirdly enough it wasnt blown on my old one :\

the h/u circuitry might be internally grounded to its body which has come into contact with the metal box ive made to mount it (welded to the rest of the steel dash, with lots of other grounds id imagine)

as mentioned ill do my best to insulate the headunit this time, i didnt realise it'd be a problem but it obviously is! heh

just wondered if fusing a negative wire would actually do anything. i would have guessed a fuse will be a fuse anywhere you care to put it but i dunno

[litldv8]

why didn't you put the switch on the positive side?

[Gary]

i hear its safer, given its on the floor if it ever got wet for any reason the worst that will happen is that it wont isolate - if it was positive itd short. also the switch ive got isnt insulated so well, leading me to believe its for use on a negative lead not a positive. i didnt think there'd be a problem either way

the isolation is for a theft deterrent, not for racing or anything else

ive just insulated the headunit completely, it still manages to supply the car with positive and negative when plugged in (hazards/horn etc still work when the negative lead is isolated from the rest of the car)

when the h/u isnt plugged in, or i pull the fuse for the negative supply then nothing in the car works. obviously im missing something, but i dunno what

[o_man_ra23]

You have an earth path somewhere from your HU to the body. Perhaps through the speakers. If you try and draw too much current, the fuse will blow and you will be all good.

If you have a multimeter, measure the ohms with the fuse disconnected from the HU body to the car body. Until this is open circuit, you have not completely insulated, however as mentioned, there may be another earth coming through speakers etc. If you are using mounting bolts/screws into the HU, these may cause your fortuitous earth problem. Another source of grounding could be your antenna. This needs to have a ground plane to work properly, so I suspect the outer braiding of the antenna cable is earthed at the antenna.

I will bet that if you try to start the car, the current draw will blow the 10A fuse, despite the hazards etc working. This may work in your favour though, as you may require the hazards to be left on while you hail help along the side of the road, however you will still be able to have the isolator open to prevent anyone starting it and taking off with it.

[wilbo666]

Earthing via the radio antenna and hence radio perhaps?

Anyway something to think about... But if it was me I'd put the isolator on the more conventional positive side of the battery.

Cheers
Wilbo

[Gary]

ah hah! was through the antenna as you both mentioned excellent work

i tested the speakers but theyre isolated good already, ive isolated the antenna too and so far as i can tell reception hasnt changed. win!

the way it was does sound not so bad owen im weird though and feel a little uneasy about having such a hairy connection

thanks chaps! ive got a couple things to do to the car before the weekend, funny how the small shit takes decades and the bigger more mechanical stuff takes no time at all

edit i just noticed i can add rep in this section +madrepz to j00's

[o_man_ra23]

You think this little job takes up all your time?? Wait till you try and paint your car a pretty colour!! That's nuts how long that takes!!