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Increasing voltage with no error.

  1. Jan 30, 2010 #1
    I am trying to install larger better lights inside my car, it runs at 3.5volts sometimes, and at 10.25volts sometimes. I am trying to take the voltage up to 12+volts so I can use a relay. I used a 32v 4700uf capictor and it gave it enough power to activate the relay, but then the car doesn't get enough resistance so it turns off the power thinking the original bulb burned out. So at this point I installed a .005 Ohm 50 watt resistor, and it didn't work. When I remove the capictor and only have the resistor connected it takes the load and the car doesn't error.

    So my question here is, how can I bump the power to 12+volts to get a signal to turn the relay on, and take the load coming from the car so the car doesn't think the bulb burned out and turn the signal off. Is there a way to make the power only go one direction into the capictor, would a diode do that? I am thinking the capictor sends power back into the direction of the resistor/car. Thank you guys in advance.
  2. jcsd
  3. Jan 30, 2010 #2


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    Most of that doesn't make any sense at all.

    Is this a real car with a 12 volt ignition system, or a toy or model car?

    If it is real, why is the voltage that gets to the internal lights varying so much?

    What is the relay for?
  4. Jan 30, 2010 #3
    It's a 2003 E Class Benz, the interior lights stay dim, so I tested it with a volt meter and confirmed it with another owner it runs at 3.5volts during the day, and 10.25volts at night, I want a stable 12+volts coming out of it. The cars computer gives the lights a low voltage so it stays dim.

    If I use a diode between the resistor and the capacitor will it make sure the capacitor doesn't give power back the direction of the resistor? Thank you.
  5. Jan 30, 2010 #4


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    I'm not sure why they would do that.

    Anyway, if you want 12 volts for the interior lights of your car, you should get an auto electrician to wire it directly from the battery or from under the dashboard.

    The battery is a very stable 12 volts which you could switch with door switches and interior switches to give a nice bright light.

    This is probably not a job you should do yourself.
  6. Jan 30, 2010 #5
    I am running a wire from the battery, I need a signal from the light to turn the relay on.
  7. Jan 31, 2010 #6


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    Maybe you need something like this:
    relay driver2.JPG

    You would power the relay from the 12 volts and when it pulls in, connect the 12 volts to your lights, using the switch contacts shown at the top of the relay.

    Drive for the transistor would come from the 3 or 10 volts from the computer. This would be connected to the left side of the resistor marked "R".

    R would have to be adjusted to suit the transistor and relay, but it may be something like 1000 ohms.

    The transistor should be a small, power transistor like a BD139.
  8. Jan 31, 2010 #7
    Thank you very much vk6kr I will try it :)
  9. Jan 31, 2010 #8
    Hello kuiyt2,

    Please take seriously vk6kro's advice to involve an electrician. A short-circuit on a vehicle electrical system may cause large currents to pass, which can easily cause a fire. That wire "directly from the battery" would be much better going via a suitably rated fuse.

    Bear in mind that making any unauthorised mods to your wiring could invalidate your vehicle insurance, particularly in the case of your making a claim involving fire.
  10. Jan 31, 2010 #9
    Thank you very much for the heads up, I have a fuse right next to the battery with the wire I am running, and another fuse right next to the light area. Very appreciate the advice.
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