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blowing fuses

Discussion in 'General Electronics Discussion' started by car guy, Jul 7, 2016.

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  1. car guy

    car guy

    Jul 7, 2016
    Hey, all....broke the cover off the lighted mirror on my sun visor. Rather than spend $200 on a new visor, I got an aftermarket battery powered one. I thought rather than using batteries, I could use the existing hot wire from the original light. Well, either I'm wiring it incorrectly or the battery powered unit is not compatible with the cars 12 volt system. HELP!

    First pic is factory wiring
    Second pic is my hack job, original 2 wires coming out of the hole
    Third picture is the battery orientation
    20160704_141402.jpg 20160707_144704.jpg 2016-07-07 14.49.52.jpg
    KeithM likes this.
  2. mrmodify


    Feb 13, 2010
    What size or voltage batteries were in the aftermarket battery?
  3. Gryd3


    Jun 25, 2014
    It looks like the aftermarket one used AA batteries... 4 of them is only 6V
    Using a 12V source is asking for trouble...
    I would suggest more modifications... simply replace the lights with higher voltage lights or add more lights... or if they are LED we can calculate a resistor you can use.
  4. car guy

    car guy

    Jul 7, 2016
    Thanks for the responses, guys, It takes four AAA batteries.
  5. davenn

    davenn Moderator

    Sep 5, 2009
    check your wiring specially on the bottom 2 batteries .... it doesn't appear to be correct to have what would have been positive battery terminals going to negative supply
  6. BobK


    Jan 5, 2010
    You can check your wiring all you want, but if you end up getting it "right" and apply about 14V to something that was intended to run off 6V, it is not likely to end well.

    Mongrel Shark and hevans1944 like this.
  7. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    What a hatchet job! It looks like the aftermarket light is really two lights, each independently operated from a pair of AA cells. Two cells operate the light on the left and two more cells operate the light on the right. So, only 3 V is applied to each light via the central switch mechanism. Even if you somehow get all this wired correctly into the car's 12 V power, applying that power to lamps designed to operate from 3 V is not likely to end well. See post #6 above.

    Nice pictures though. We like that here.
  8. Gryd3


    Jun 25, 2014
    I didn't catch the 3V setup xD
    That could have been even worse!

    I do think you should disconnect it from the car though, gut the new light fixture and re-do it from scratch.
    You can use a pair of 6V or 12V incandescent lights and simply wire them to power. (In series or parallel respectively)
    Or you can take the LED route... either buy an automotive LED 'bulb' for a dome-light, or build your own from scratch.
    If you build your own, you will end up saving a bit of money... the LEDs and resistors will most likely cost you less than $3 .. The trick is putting the right value resistor in-series with your LED(s)

    If you do the DIY method... go buy two white LEDs, but be mindful of the current draw.

    == Math Ahead! ==
    First fine the Ω value of the resistor you need.
    [ SupplyVoltage - sum(LED1_Vf + LED2_Vf + ...) ] / min(LED1_Current, LED2_Current, ...) = ResisterΩValue

    Then find the power handling you may need.
    [ SupplyVoltage - sum(LED1_Vf + LED2_Vf + ...) ] * min(LED1_Current, LED2_Current, ...) = ResistorPowerValue

    If you use two white LEDs rated for 3.3V @ 20mA...
    Then you would need 370Ω resistor rated for more than 148mW ... of course, you can't buy that one exactly... so pick the nearest larger value for each requirement.
    Connect them all in series, and connect to the wires of the car and you are good to go ^^

    *Please note: Although your car is a '12V system', you actually end up dealing with a noisy 14.4V supply voltage when the motor is running. (Alternator provides 14.4V to charge the battery)
    This may also be quite noisy, so if you plan to use sensitive electronics, you should concern yourself with filtering.
  9. car guy

    car guy

    Jul 7, 2016
    So I guess you guys have figured out that I'm no electronics whiz! Just on here looking for advice. Anyways I think I'll just rip the lights out and use the lights from the original unit and all should be good.
  10. Gryd3


    Jun 25, 2014
    And everyone has to start somewhere, don't worry about it.

    Look at the voltage rating for the lights from the old unit. If they are 6V, then you need to connect two of them in a string to make the whole thing operate with 12V.
    If they are each 12V, connect them in parallel. (positive of all lights together, and negative of all lights together)

    Best of luck. Any more road-blocks, give us a shout.
    davenn likes this.
  11. car guy

    car guy

    Jul 7, 2016
    Success! 20160710_162622.jpg 20160710_163158.jpg
    hevans1944, davenn and Gryd3 like this.
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