Alfa 156 third brake light LED conversion - Alfa Romeo Forum
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Alexandrus
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Alfa 156 third brake light LED conversion

Lately I became obsessed with LEDs, so I bought a set of 10 of these LED bulbs http://cgi.ebay.co.uk/ws/eBayISAPI.d...m=220708045286
I replaced the normal T5 wedge bulbs in the third brake light with the mentioned LEDs.
They work just fine, with a single exception, the third brake light stays very slighly lit even when brake is not applied.
I checked and there is no shortcircuit, so what is the problem and how do I solve it ?
Can anyone help ?
 
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fit a pulldown resistor... i'm guessing it's due to a circuit check?
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Alexandrus
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I guess it's because of a circuit check, but cannot be sure. At the same time I am trying to avoid the resistor solution because of the placement of the brake light, too much heat in that area might cause unpleasant things, but at the same time I cannot think of any other solution.
 
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no need for a big resistor, something that'll let 30mA pass is enough....

14.4v x 0.03A = 0.432W

14.4v /0.03A = 480 Ohm

fit this between the 12v lead for the brake light and a ground... anywhere on the circuit is good, doesn't need to be in the brake light itself
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Alexandrus
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The LEDs I used are 0.36W each at 12V, so yes, 30mA will be more than enough. The bulbs are 2.3W each.
I am guessing that the resistor will definitely work, but should it not be connected on the + wire, in series with the LEDs ? The connector for the 3rd beake light is composed just of a ground wire and the positive wire.
 
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no, you don't need to reduce the voltage over the leds, they seem to work fine....

what you need is to provide an alternative path for the circuit check so the current wont try to light the leds
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Alexandrus
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Understood
Thank you. Should be easy to solder the small resistor.

Did I get this right, I should solder it between the + and - wires ?
 
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yes
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Alexandrus
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Thank you.
 
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I kept searching for specifications and I just cannot find the continuous forward current for this type of LED.
Can anyone help ?
Some places say 20mA, some say 3x20mA, since it's a 3 chip basically.
 
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doesn't matter... what you need to know is the current that the circuit check tries to send through.... typically 30mA
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Alexandrus
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Oh, well, that I can measure
 
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Sorted.
Looks like the LEDs would only light up when brake NOT applied if the gauges were off the car, after I put the speedo and revo back on the car everything went OK.
I took those off to do a LED conversion to them as well.
 
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Any pictures ?
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Alexandrus
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Pictures of what ? The third brake light ? You must have one yourself.
 
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Figured it all in the end.
As long as the revo meter is not working or not on the car, the later was my case, the car will light up the third brake light all the time, but dim, to act as the runing tail light.
Once I put the revo meter on the car, it worked like magic. Very bright, these LEDs, tail lights as well custom made LEDs, but cannot get better pics right now as I do not have the steering wheel back as well as a lot of other interior bits.
But until I do, there.
Brake off.
http://img805.imageshack.us/img805/6520/img2545y.jpg
Brake ON.
http://img837.imageshack.us/img837/783/img2546q.jpg
 
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