how do you reduce a car brake light bulb from 21w to 5w with resistors?

Here's a little project i want to see if I can do , change a 21w BA15s (Red) brake light bulb to a 21/5w without having to change the bulb holder.  

So its a single contact 21w single filament bulb. once powered by 12v+ DC it burns at 21w . I would like to adapt it to burn at 5w for sidelights and 21w for brake light.

I have done this diagram as to what components I need to do this , a resistor to reduce the power to 5w and a Diode to make the positive go in one direction only from brake light to the bulb so the positive does not travel down the tail light circuit and potentially blow the fuse for either the brake light circuit or the tail light circuit of the car.

So, the things I have to know (I am am intermediate electronics knowledge lets say, I know what a resistor does and i know what a diode does and I am handy with a soldering iron) - here's what i need help on:

1. What value resistor will I need? (ohm / k/ohms and watt ie quarter or 0.5w or 1watt) to reduce the 21w brightness down to 5w 

2. How hot will the resistor get? - will it get really hot? - could it be covered with some heatshrink? 

3. are the Diodes in the right place and what value diodes would they need to be4. Can you see any problems or any irregularities looking at the diagram that could cause problems or the fuses to blow.  

I realise it will more than likely shorten the life of the brake light, it being one filament being used as a brake & tail bulb .. but that's no biggy. Thank you for any help 

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15 Answers

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  • 2 weeks ago

    Buy an LED light that fits in the same spot.  LED lights use about 1/5 the power as an incandescent.  It will also have the same brightness as your current bulb so should be street legal.

  • Jim
    Lv 7
    3 weeks ago

    Buy a couple 6 ohm load resistors and try in series with your lamp, about $3 each

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  • 3 weeks ago

    just use 12v/5w bulb

  • 3 weeks ago

    First of all, you don't need the two diodes in there. When you step on the brake then the resistor is simply bypassed. It's all the same 12 volt source.

    Probably the best thing to do if you insist on doing this, would be to replace the resistor with a power FET. Then use a 555 timer I. C. to generate pulses at a frequency of at least 60 hz and vary the duty cycle of the pulses with a pot. to control the brightness of the tail light. The advantages of this approach would be that you can vary the brightness to what it should be compared to another similar tail light. Also, the FET would be switching the power on and off and would be much more efficient than a resistor and much less heat would be produced. You might not even have to heat sink the FET. Just remember that the 12 volts of voltage is more like 13.5 volts when the engine is running.

    So just a 555 timer and a few parts and a FET and you should be good to go. 

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  • 3 weeks ago

    The resistor drops the wattage to the bulb by dropping the voltage to the bulb, where you would have 5 volts across the bulb @ 1 amp and 7 volts across the resistor at the same 1 amp, thus a 7watt minimum resistor would be needed and higher if you want it to feel cooler. Personally, I think it would be cheaper to install a twin pin socket.

  • garry
    Lv 4
    3 weeks ago

    please just look it up and calculate what size  resistor you need , its that simple .

  • Anonymous
    3 weeks ago

    1/4 20? means you need 4 times. is it so difficult? you should set up a proper regulator on the bulb using a fet obviously since you need to maintain brightness @5watts. its going to need to peak longer.

    or center peak 4x so people can see it.

    lastly.. did you get the diagram from a book or draw it?

    it looks like crap and probably wont work. 

    you need to change little but your firmware to change it.

  • 3 weeks ago

    It is illegal to mess with the lighting on a car.

    Lights must meet minimum brightness and color specs by federal law(NHTSB).

    You are messing with the tail lights of the vehicle. They have a minimum brightness spec.

  • 3 weeks ago

    I figure that a 22 ohm 5 watt resistor *might* do the job. On the other hand, the bulb might not get hot enough to work.

  • 3 weeks ago

    you will need a 10 or 20 watt power resistor, and yes it will get hot. 

    exact value cannot be calculated as resistance of the bulb is nonlinear, ie, it varies with voltage, and that curve we do not have.

    Why would you think the lifetime is reduced? reducing voltage increases the lifetime of a bulb.

    at 12 volts and 21 watts, current is 21/12 = 1.75 amps. Note that you will need 2 amp diodes (5 amps ones would be better).

    R = 12/1.75 = 6.9 Ω, but that will go down as voltage decreases. Filament bulbs start with a lower resistance, perhaps 5x lower, and that increases as the filament gets hotter. 

    I'd start out with a 10 Ω 10 watt resistor and see what it gets you. Or get a 10 Ω 10 watt rheostat (adjustable resistor).

    Or get a variable supply and some meters and determine what voltage and current get you the desired power. 

    BTW, do you know this may be illegal?

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