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14volt reducer resistor 

 

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 Post subject: 14volt reducer resistor
Posted: Mon Feb 25, 2008 8:34 pm 
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I've got some Autometer C2 gauges mounted on my A pillar. They have a blue backlight, but it's so birght and the gauges are mounted at eye level that I find it blinds my right eye somewhat.
I was thinkign if I put a resistor in the illumination power wire the light would not be so bright. But I've got no idea what size resistor - any recomendations - I was thinking or trying one that would reduce it to about 10volts initially?
The lights are only on when the cars running so it would be getting about 14volts then.

 

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Posted: Mon Feb 25, 2008 9:07 pm 
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Have a read of this champ. I was going to explain Ohm's law, but this is easier. :D
http://www.play-hookey.com/dc_theory/vo ... vider.html

 

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Posted: Mon Feb 25, 2008 9:45 pm 
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you could used a variable resistor hidden somewhere to get it to the perfect brightness you want.

 

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Posted: Mon Feb 25, 2008 9:47 pm 
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smiley235 wrote:
you could used a variable resistor hidden somewhere to get it to the perfect brightness you want.


I could, but it's too much effort. I'd rather just get a couple of different resistors and find one that's close and leave it as just that.

 

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Posted: Tue Feb 26, 2008 2:17 am 
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umm are they no hooked into the dash lights?
and so when you turn the dash lights down they dim?

or are the globes leds?

either way u need to work out the amp's being drawn by the globes before u can do anything

once u have the amps use V=IR
V= volts
I= amps
R= resistance

to get 10v from a load of say 2amps
10 divided by 2 = 5 ohms



i also like smileys idea as then u oly have to wire it in 1 time :)

 

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Posted: Tue Feb 26, 2008 9:38 am 
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Yeah go with smiley's idea you can get really small ones with a screw top and just hide it away when you're done.
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Posted: Tue Feb 26, 2008 10:38 am 
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Disco Frank wrote:
umm are they no hooked into the dash lights?
and so when you turn the dash lights down they dim?

or are the globes leds?

either way u need to work out the amp's being drawn by the globes before u can do anything

once u have the amps use V=IR
V= volts
I= amps
R= resistance

to get 10v from a load of say 2amps
10 divided by 2 = 5 ohms



i also like smileys idea as then u oly have to wire it in 1 time :)


What you want to do is to have 4v across the resistor and 10v across the illumination (globe / led whatever)

First step is to work out the resistance of the illumination circuit. You'll need a multimeter, and you'll need to measure it against ground.

Let's say for arguments sake it is 10 ohms

Next you need to work out how much current the setup would pull.
You know you want the globe to be at 10V and you know how much resistance is in the circuit (10 ohms)
I = V / R
I = 10 / 10
I = 1 Amp

So now you know you want the whole setup to pull 1 amp. To work out the value of the resistor you use the forumla
R = V / I
Since we know we want the voltage across the resitor to be 4V, and we know the current of the circuit (which is the same across both components) is 1 amp:
R = 4 / 1
R = 4 ohms

So in this example you would need a 4 ohm resistor wires in series with the illumination wire.

In short:
Let R1 = Resistance of globe
Let R2 = Value of resistor
R2 = 4 / (10 / R1)

Hope this makes sense.
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Posted: Tue Feb 26, 2008 3:52 pm 
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Use a 1K resistor, should bring it to a good illumination level.

Also remember the car will never be running at 14v more like 14.8v as the alternator runs a bit higher than the required charge voltage (13.8v)
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