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the12volt`s installbay - Mobile Electronics Forums the12volt's install bay / Lights, Neon, LEDs, HIDs

Subject Topic: LED in my ashtray, resistor size? (Topic Closed Topic Closed)

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stillrun
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Posted: March 18, 2004 at 8:00 PM - IP Logged  

I'm installing a Blue 5mm LED in my ashtray at the moment. I'm going to hook it up so it only goes on when the ashtray is pulled out and the headlights are on.

I have all the hardware part finished and i just have a question about some of the wiring. I was planning on using just a 560 ohm resistor, but when i mention what i was planning on doing to the salesman at the local electronics shop he mentioned i should also use a voltage regulator. 

Is he correct, or would it be fine with just the resistor i was going to use. What would be the best way (for brightness, Saftey)  to hook it up. Any help would be greatly appreciated.

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Teamrf
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Posted: March 18, 2004 at 8:47 PM - IP Logged  

I've done multiple LED's without a voltage reulator..I would use a resistor and see how that works first.
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xetmes
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Posted: March 18, 2004 at 8:50 PM - IP Logged  

Just use a resistor. Most people seem to think that LEDs are voltage controlled, they are NOT, they are current controlled. A resistor is a much better idea, in fact I would never recommend a voltage regulator for an LED

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stillrun
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Posted: March 18, 2004 at 8:56 PM - IP Logged  

cool!  thanks guys. How many ohms should the resistor be to safe that i don't blow the LED?
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xetmes
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Posted: March 18, 2004 at 8:58 PM - IP Logged  

many people just use a 470 ohm and that should be fine 560 should be fine as well

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stillrun
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Posted: March 18, 2004 at 11:35 PM - IP Logged  

Thanks Again. Is it alright to put shrink tubing over the resistor, or would that cause a problem?

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xetmes
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Posted: March 19, 2004 at 6:34 AM - IP Logged  

nah I do that all the time just get a piece to cover the resistor and the connections and you will be fine
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Dan_uk
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Posted: March 20, 2004 at 11:16 AM - IP Logged  

A voltage regulator for a single LED??? Christ! I bet he had one there that would do the job too... "only $30 to you, sir"

Ive done a couple of LEDs in mine, in the interior door handles (so that they can be located in the dark). I just used a resistor in series with the anode leg of the LED, shrinkwrapped them and installed them. Works fine.

Voltage regulators should only be used with actual devices, like cellphone chargers etc.

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Mad Scientists
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Posted: March 20, 2004 at 9:54 PM - IP Logged  

 To determine the correct size resistor for a given LED you need to know the foward voltage of the LED (known as Vf), and the typical foward current. A quick search for blue LEDs gives me these numbers for one particular example; Vf 3.2v and current of 0.03 amps.

 First, determine your supply voltage. Typically in a vehicle 12.8 volts is used, but your range can be from just under 12v to just over 14v. (as an aside, this is perhaps why a voltage regulator might be suggested..)

 12.8 volts minus 3.2 volts equals 9.6v drop across the resistor. The current is 30 mA, so use Ohm's Law to figure out that Resistance = Volts divided by Amps.

 resistance = 9.6v / 0.03 amps

 resistance = 320 ohms.

 I have seen Blue LEDs with Vf of 5v, and I of 0.01 amps.

 12.8v - 5.0 = 7.8 volts.

 Dropping 7.8 volts on a current of 0.01 amps would require a resistor of  780 ohm. (using 12.8 as a voltage figure.

 Different color LEDs have different Vf.. current requirements vary also.

 Regards,

 Jim

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stillrun
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Posted: March 21, 2004 at 5:16 PM - IP Logged  

Cool Guys, thanks for the help. LED is installed and working fine!!! 
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