Switched 12V power supply

Discussion in 'Audio, Video & Gadget Tech' started by punkieys17, Jul 14, 2009.

  1. punkieys17

    punkieys17 New Member

    I am about to wire in 4 headrest DVD units. Is there a 12v switched power supply in the cabin area I can take the power from - I need to make sure the DVD's go off with the ignition or the entertainment system,
  2. bill190

    bill190 Rockstar 100 Posts

    To do this right and avoid fuse blowing problems, you need to do a bit of electrical design work/engineering...

    You would need to know the amperage draw total of what you are adding.

    Then you would need to know what circuits/fuses are for what (would be in a factory service manual set in the fuse section). Each fuses will typically go to several things.

    Then turn on all devices on a circuit, measure the amperage draw (with amp meter), then compare with the fuse rating. Then calculate 80% of the fuse value (maximum amperage draw per circuit), then subtract that from the actual amperage draw being used with all devices on. Then the remaining amperage would be what you could safely add to that circuit without having a fuse blowing problem.

    Then if a particular circuit had enough spare amperage available to power your devices, the factory service manual would show which color wires in which electrical harnesses that circuit was on (so you could tap into that circuit).

    If none of the circuits/fuses had enough spare amperage available, you could use a "continuous duty relay" tied into an ignition on circuit. And connect the contacts of the relay to its own new fused circuit connected directly to the + power distribution center. Relays get hot, so best to place this under the hood where it will get good air circulation.

    Here are wire sizes to use along with fuses. And remember the 80% rule, do not load up a circuit with more than 80% amperage draw of the circuit/fuse capacity...

    Wire gauge sizes for 12V wiring
    22 - 5Amps
    20 - 8Amps
    18 - 10Amps
    16 - 20Amps
    14 - 40Amps
    12 - 60Amps
    10 - 100Amps
    8 - 150Amps

    If you don't have an amp meter (ammeter), you might want to get one with a capacity of 600 amps DC. Then you could measure the amperage of anything on the vehicle including the starter.
    (Not 600 amps AC! Must also say 600 amps DC! Vehicles are DC. The specifications of the meter must say DC current 600A or Amperage 600A DC or Amperage 600 amps AC/DC. Or whatever amperage you would like to be able to test up to.)
    Here is one (click on Datasheet)...

    The 10 amp capacity multimeters they sell in auto parts stores are useless for todays vehicles! If you try to measure more than 10 amps, it will wreck the meter! And anyone can look in a vehicle fuse box and see that many fuses are more than 10 amps these days. Plus many alternators are 100 amps! Starters can draw 200 to 400 amps!

    You can order a GM Factory Service Manual Set for your year/model vehicle (has complete wiring diagrams for everything in vehicle) from a GM dealer or from helminc.com

    Also to avoid electrical and fuse blowing problems, make your wiring "look like factory". They wrap all those wires up for a reason! Wrap them in electrical tape and place that black plastic tubing over them. This protects the wires from rubbing against sharp metal objects and grounding out.
    Last edited: Jul 14, 2009
  3. Jimmiee

    Jimmiee Epic Member 5+ Years 1000 Posts


    Spoken like a true engineer! :)

    Would it not be easier to tap off a switched 25 or 30 amp circuit on the power side of the fuse and use a inline 10 amp fuse?
  4. bill190

    bill190 Rockstar 100 Posts

    Providing that the circuit is not using more than 80% of the available amperage on that circuit with everything on and with your new gizmos added and on.

    Everything has changed with these new vehicles. It used to be you could just add gizmos and no problem or if a fuse blew, just the A/C or radio would stop working.

    But now if certain fuses blow, computers go down and the vehicle will stop working or will not start.

    And there can be 2 or 3 computer local area networks and wiring for these all over the vehicle. If you tap into these wires, a network will go down. The 10 or more computers on that network will not be able to communicate, and your vehicle probably will not start.

    So *very* important to know exactly what wire you are connecting to. And to know in advance that you will not be overloading an existing circuit.

    BTW, so far as the 80% rule and not overloading, fuses and circuit breakers work by heat. And a vehicle can get to be quite hot inside in the summer. So if a circuit is near blowing a fuse, the additional heat from the sun might make the fuse blow or breaker trip.

    I've seen a LOT of fuse blowing problems caused by do-it-yourself add-on wiring. This is mostly with people who buy used vehicles and modify them. It will be quite interesting to see the used vehicle folks get ahold of and try to modify these newer vehicles when they get old.

    Also your owner's manual should have a warning about not adding anything electrical.

    Then there are safety devices like airbags and ABS which you would not want to tie into under any circumstances, nor any fuses ahead of these.

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