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Electronics Grounding question

Discussion in 'Modding' started by shotgunefx, 15 Jun 2007.

  1. shotgunefx

    shotgunefx What's a Dremel?

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    I've got a question on grounds as a follow up on my relay questions. Please bear with me if this is meandering.

    Basically, I'm replacing my car HVAC controls with a PIC driven replacment. I need to work a few servos to control the cables, the rest is pretty much replacing the equivalent of mechanical switches.

    So I'm going through the schematics for my car and designing the replacment. Here's my question.

    Currently, the parts I'm replacing are grounded to one of two points, either in the engine bay (#13) or in the cabin (#5)

    Most of the switches are hooked up to the compressor controller and the blower motor and grounded through (#13), but there are a few switches that aren't (they go to #5) , those are the AC On/Off, Foot/Def and Inside/Outside switch.

    Seeming I'm using a ULN2803 for the fan switch positions, I'd like to use the one ULN2803 for all the switches for simplicity, but they share a common ground.

    I'm wondering if I'm opening up myself for problems here. Can anyone think of any reasons why the grounds might need to be seperate? I'm thinking it's probably just for the convienence of how the parts are assembled but I dunno.
     
  2. Macaba

    Macaba What's a Dremel?

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    Run a continuity test between the two ground terminals (or check the resistance, continuity = 0-0.2 ohms). If there is continuity, as should be the case with a car, then its fine.

    Its interesting how ground connections can pose huge problems sometimes, I received a diy CNC mill from someone a few months ago, and it had a big electrical problem (motors randomly jittering on the x/y axis). Basically, the guy who made it was a bad electrician, because he had grounded a motor (z axis!) to the Mill chassis, which is actually floating with respect to earth. Rerouted this to mains earth, and voila, everything was perfect.

    Hopefully, no such problems in a car. The other thing i'd say about having a pic in the car, is design the PCB as a single sided board, then use double sided copper clad board, and ground the other side. This reduces electrical noise around the pic... in theory.
     
  3. shotgunefx

    shotgunefx What's a Dremel?

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    Thanks for the suggestions. In addition to the ground potential, I still need to measure the current draw through the #5 switches, considering their tiny size, I'm going to guess it's not a lot, or not enough to cause a problem with channeling both through the same ground, but measure twice, cut once ;)

    Don't think I can get away with a single sided board with the size constraints and the number of components though.

    On the plus side, I've got a few PICs in the car and so far, haven't had any issues as of yet with resets, etc.
     
  4. AJB2K3

    AJB2K3 What's a Dremel?

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    Doesn't use use a positive ground were as uk and eu use negitive ground?
    Any way if you want to ground anything in a car drill and tap a hole in the metal work then just put in a screw but make sure you have the correct ground?
    Or am i just remembering some old cackhanded wiring practice?
     
  5. shotgunefx

    shotgunefx What's a Dremel?

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    I'll be honest, the whole discussion of positive ground has always confused me :)

    I'm not adding any grounds or even cutting any wires though there are a number of good factory grounding points all over the vehicle.

    The goal of this is to be a plug-in replacement for the various components of the HVAC controls (Assuming I can mate all the harness connectors), which are all basic switches and some wires that go through as well just to simplify the number of wires needed at the factory.

    The front harness for my car runs close to $1000 dollars so I'd rather not start hacking it up (though I bought a cheap spare at a salvage yard ;) ).

    So I'm basically just trying to reimplement the mechanical stuff with digital version and some smarts in a pin compatable form factor.

    FYI, From what I can tell in the schematics, the switches grounded through #5 don't route through anything else.
     
  6. cpemma

    cpemma Ecky thump

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    In a car, the chassis is just replacing return wires, so "ground" is a relative term; there was some idea which particular battery terminal was connected to chassis affected body corrosion, so some cars were one, some t'other.
    If you use the same spot for all the electronics grounds as far as poss you should be OK, digital circuitry shouldn't be affected by a few millivolts variation in the 'low' level and you shouldn't get mains hum in a car. ;)
     
  7. shotgunefx

    shotgunefx What's a Dremel?

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    So does positive ground mean it's not grounded through the body or that it is?

    Makes me think of dad's old Crown Victoria. Had a couple of anode-rod like bars in the engine bay. Once they went, it would get a ton of corrosion on the terminals, etc.
     
  8. shotgunefx

    shotgunefx What's a Dremel?

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    I'm retarded, you answered my question in the first post, probably shouldn't reply first thing waking up :lol:
     
  9. AJB2K3

    AJB2K3 What's a Dremel?

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    Positve ground = live side of battery connected to chassie (odd method)
    negitive ground = negitive side of battery connected to chassie (Normal method)
    Lood in the engine bay at the battery, which side goes to the chassie.

    As to your question if your making direct plugin replacement, as long as the connections are the same as the old units i don't see a problem
     

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