Need resistor size to dim 12v light


Old 02-26-08, 12:25 PM
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Need resistor size to dim 12v light

I am building a custom headlight for my motorcycle and need to dim the light by about 25% so I can then switch between full and 75% pwr. Assuming the pwr. source is 12V DC, I am using three 35W halogen bulbs in parallel with each other, what size resistor including wattage will I need to put in line to reduce the light by 25%. Secondly, I am supplying pwr. through two switches. the first being for low beam or 75% pwr. and the second being for full pwr. which will attach after the resistor to accomplish this.When the high beam switch is enabled it cuts out pwr from the low beam switch. What I need to know is the type and size of diode to add so that I don't back feed through the high beam switch when I have the low beam enabled which will stop my high beam indicator from lighting all the time.
Old 02-26-08, 04:47 PM
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resistor size

3 bulbs @ 35 watts each = 105 watts SO; P = V X I or I = P/V; or I = 105/12 = 8.75 amps is needed. better make sure your bike alternator can output this much current.

you are drawing 8.75 amps on a 12 volt DC circuit at 100% on; to reduce the "CURRENT" and assuming the Lux output is proportional (usually not though); then 75% of the current is about 6.5 amps and you need 75% of the voltage, to drop across the resistor ... so 3 volts will be dropped, ... then

the resistor value is I = V x R or R = I/V or 6.5/3 = 2.1 ohms (2.2 is standard)

B U T the power rating for that resistor, should be :

P = E x I or 3 x 6.5 or about 20 watts MINIMUM

usually you double it, so a 40 watt Wire Wound is needed, a BIGGY !!!, and about 40 bucks! try ... in the USA

instead :a MUCH less bulky was is to put 6 amp silicone diodes in SERIES, it's allot cheaper. With 0.7 Volt Drop across EACH silicone diode, forward biased (drawing current), you will need 4 or 5 diodes to drop 3 volts (4 more likely, but more likely 3... Lux is non linear) and very little, if any, heating problems, ... ANY 6 AMP DIODE WILL WORK MR 750 , 6A05 ,
or any cheap, working 6 amp diode you can find, as long as its forward bias voltage is 0.7 volts
forward biased, one direction it will work, the other way won't ... don't worry, you can't burn them out if you have them in reverse.

its no bigger than 3/8" diameter and about 23 cents each in bulk ... e-mail me, i use thousands if you want 'samples'

you know, ... DC light dimmers, ... which are just current limiters using regulators such as LM7805 wired as a DC current limiter and an output transistor to handle the DC current you need, the 7805 is rated to 0.5 amps max, but the driving transistor can handle dozens of amps. This could give you any brilliance you want, most automotive bulbs have 2 or filaments inside, halogen doesn't, there is a voltage level that these bulbs will not work at, so be careful just how low you can go.

Last edited by pdp8e; 02-26-08 at 09:56 PM. Reason: added info / correction

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