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Ignore 6v but let 12v pass

shaneuk

Apr 1, 2010
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Hi All, I have a little problem I am hoping someone can help me with, I have a bulb that has 6v constant going to it, then when I press the brake pedal 12v is applied to it. What I want to do is put something in line with the bulb so that 6v is ignored and the bulb only lights up when the 12v is applied.

How would I do this? The only way I can think of is to put something like a 18v relay on there which usually wont switch on with just 6v but will with 12v and then of course link a direct 12v supply- the problem with this is I may get relay chatter is the current increases for any reason.

Thanks in advance for your help! :)

Shane.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
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My first thought is something like this. Choose resistors and the transistor as appropriate.


edit: bugger, a senior moment. That transistor should be NPN.

And it won't work anyway. The lamp would have to be connected to the collector, and that means you'd have to isolate it from ground. Also the max voltage would be reduced by at least Vce(sat). I'm surprised 55p didn't chastise me for that! :)
 

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shaneuk

Apr 1, 2010
3
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Thanks Steve, the bulb is 25w 12v, Im not too up on my components what component vaues would you recommend?

Thanks

Shane.
 

55pilot

Feb 23, 2010
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I have a more fundamental question. Way do you want to do this? Sounds like you are trying to defeat the running lights on a vehicle.

---55p
 

shaneuk

Apr 1, 2010
3
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Apr 1, 2010
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I have updated some rear lights on a z4 from a newer model, the old lights use the tail light and brake light on one bulb, the new lights have separate bulbs for each job while the 6v going to the brake lamp gives extra tail light illumination it doesnt really look that great as I am already supplying 12v to the proper tail light section and in my opinion detracts from when you apply the brake. Im in UK, but know what you mean about daytime running lights, I think in the US guys have 6v going to the inidcator markers on the front then 12v when you inidcate?

thanks

Shane
 

55pilot

Feb 23, 2010
434
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Feb 23, 2010
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Running light was maybe not the best choice of words. Basically, the light stays on at a low level all the time and then comes on bright when it gets a higher voltage.

You can accomplish what you are doing by adding a switch that has an on-off voltage higher than 6V but less than 9V. The reason I am using 9V is because that is how low an almost dead battery can go. You do not want to be in a situation where your alternator dies and your brake lights die with it.

I am assuming that you have a negative ground system, so you do not have access to the ground side of the bulb and need to control everything from the + side.

I can think of a few ways to do this, but none of which are elegant and really easy to implement. Let me think about for a bit and I will be back.....

---55p
 
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