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resistor help, ohms law is owning me

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airjordan223

Joined: Jul 02 2011
Posts: 19


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Post Sun Jul 03, 2011 9:20 am

So I have 16 of the super flux leds wired up.

I've changed the power source to them a few times and now I have them wired to turn on when the car is unlocked and turn off when the car is locked.

The issue is that this power source draws, from what I'm told, 0.01V when the car is locked. During the daytime this is fine, but at night, its juuuuust enough voltage for you to see some light coming from the LEDs.

So I'm wondering what size resistor I should use from that power source to get rid of that 0.01V when the car is locked. TIA.

And heres a quick vid for anyone interested...



bad venge

Joined: Jul 28 2007
Posts: 3550


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Post Sun Jul 03, 2011 11:38 am

A single Diode should be plenty ,
airjordan223

Joined: Jul 02 2011
Posts: 19


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Post Sun Jul 03, 2011 2:16 pm

Don't diodes just make sure the flow is going in one direction?
bad venge

Joined: Jul 28 2007
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Post Sun Jul 03, 2011 3:31 pm

There is a voltage drop that must be overcome before they conduct (.07 volts average)
airjordan223

Joined: Jul 02 2011
Posts: 19


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Post Sun Jul 03, 2011 5:53 pm

Alright, I'll get a DMM out there and see exactly what the voltage is when the car is locked.

If it's something bigger than .07 volts what should I do?
bad venge

Joined: Jul 28 2007
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Post Sun Jul 03, 2011 7:09 pm

Your POST states .01 volts ... so the .07 would be PLENTY Now you say you need to check it ...
You should check it first before posting
wolf36

Joined: Feb 26 2010
Posts: 96
Location: San Bernardino, CA


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Post Sun Jul 03, 2011 7:51 pm

Sweet, love to see new projects, its dead as hell around here lately.

Dosent that just happen when its negative switched? Cause most white LEDs wouldnt turn on until it had about 2.5 volts. I had some lights that were negative switched, and the - wire had a slight cut in it and was touching the chasis, causing the lights to slightly be on, you could only tell at night.
airjordan223

Joined: Jul 02 2011
Posts: 19


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Post Mon Jul 04, 2011 10:18 am

Well I got a DMM and went out there today and a big surprise... 4.8 volts. LAME.

So now if I throw a big resistor on there, then when the car is on I'll only get ~7 volts going to the LEDs.

So I'm guessing now I'll need a relay of some sort that only kicks on when there are a full 12 volts supplied to it?
PwrRngr

Joined: Jul 19 2007
Posts: 4407


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Post Mon Jul 04, 2011 10:26 am

bad venge wrote:
There is a voltage drop that must be overcome before they conduct (.07 volts average)


That should be 0.7V average.
airjordan223

Joined: Jul 02 2011
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Post Tue Jul 05, 2011 5:44 pm

anyone know what i can do?
bad venge

Joined: Jul 28 2007
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Post Tue Jul 05, 2011 6:26 pm

I LISTED what to do ... You stated you were checking the voltage I NEVER heard results so REALLY can't help without the info icon_rolleyes.gif
airjordan223

Joined: Jul 02 2011
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Post Wed Jul 06, 2011 7:53 am

Um, its two posts up.

4.8 volts, not 0.01 as was previously told
bad venge

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Post Wed Jul 06, 2011 8:34 am

What are you using as a SOURCE ?
why is it such a strange voltage ?
this is when it's OFF ?
You can try a 12 volt relay but the voltage may try to close at 4.8 volts
I'd wire a clean 12v source and ground to feed the LED's then use the relay to trigger from the source you are using now
airjordan223

Joined: Jul 02 2011
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Post Wed Jul 06, 2011 8:44 am

Originally I had just a regular 12v source wired to a switch.

I wanted to make a "coming home feature" ie the lights turn on when the car is unlocked, then turn off when the car is locked.

To do this I changed my power source to the one that controls the dome lights.

When the car is unlocked the source supplies 12v. When the car is locked, the source supplies 4.8 volts. Ideally I wouldn't want any voltage when the car is locked.
bad venge

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Post Wed Jul 06, 2011 9:34 am

I'd try the relay route first , they are cheap and reliable
airjordan223

Joined: Jul 02 2011
Posts: 19


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Post Wed Jul 06, 2011 9:45 am

I tried the relay method before, but I'll give it a try again I guess.

I got it at autozone, but now I cant find any info on it. It's a Tridon RL44
bad venge

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Post Wed Jul 06, 2011 10:31 am

I'd get a boshe 30 A bulletproof and will last forever.
As mentioned I would run a seperate 12v source thru a fuse and then run the current source you ae using to power the relay
airjordan223

Joined: Jul 02 2011
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Post Wed Jul 06, 2011 10:57 am

Whats the purpose of the second source?
bad venge

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Post Wed Jul 06, 2011 12:02 pm

I think your existing source is dirty thats why you have 4 + volts floating when off
airjordan223

Joined: Jul 02 2011
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Post Wed Jul 06, 2011 12:40 pm

I think its supplying the 5 volts so when the car is off you can still turn on the overhead lights
bad venge

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Post Wed Jul 06, 2011 1:49 pm

Almost all car electronics run at 12 volts , (more like 13.8 ), I doubt they'd try to run them at 5 volts
pcheng

Joined: Dec 12 2011
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Post Fri Dec 30, 2011 6:49 pm

was this every figured out?
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