# LED's, resistors, and Serial conection to a 21V car wire, Questions

Discussion in 'The Projects Forum' started by loganfinlay, Apr 2, 2010.

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1. ### loganfinlay Thread Starter New Member

Mar 31, 2010
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i have x2 3.3V (typical)-3.6V (max) 25mA, and i need to make a serial conection with these 2 LEDs to the 12V fog light electrical wire in my car. i just dont know what RESISTOR i should use,(how many ohms it should have). i dont know how to do the math to find out maximum W output for the Superbright LEDs without burning the LEDs out. can anyone tell me the resistor gauge/size i should be using?

2. ### SgtWookie Expert

Jul 17, 2007
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Better to use an LM317 regulator with a resistor.

The trouble with automotive electrical systems is that there can be a wide variation in voltages, even when the system is operating normally. You can get voltage spikes to 40v or even more. Battery voltage can drop below 10 when starting the engine, and jump up to 15v when the engine starts and the alternator comes on line.

An LM317 regulator with a 62 Ohm resistor wired from the OUT to ADJ terminal will source or sink about 20mA.

The basic formula for using the LM317 as a current regulator is:
R1 = 1.25v/DesiredCurrent, where 10mA <= DesiredCurrent <= 1.5A
By the same token:
Current Out= 1.25v/R1, where 0.833 Ohms <= R1 <= 125 Ohms.
So, 62 Ohms gives you 1.25/62 = 20.16mA

You can connect the IN terminal of the regulator to the +12v source, and your load between the ADJ terminal and GND.
Alternatively, you can connect your load between the +12v source and the IN terminal, and connect the ADJ terminal to GND.

Either way, you should use a 1A fuse between the +12v supply and the load. This protects your vehicle in case there is a wiring fault. If you don't use a fuse and you have a wiring fault, your car may burn up.

Don't use "electrical tape" on automotive wiring. Use appropriately-sized heat shrink tubing.

3. ### Bychon Member

Mar 12, 2010
469
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Dear Wookie,

When I was reading up on car electrical, the voltage mentioned was 60. Sixty volts happens when a sudden "unload" happens (like turning off the headlights) and the alternator overshoots for a few milliseconds.

4. ### SgtWookie Expert

Jul 17, 2007
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Yep, it can get up there.

But, there's a bit of a trade-off here. Sure, the LM317 is rated for a max of 40v differential between input and output, and may very well get zapped during such a load dump.

Our OP wants to power two LEDs. It's frequently hard enough to convince n00bs that they need a resistor to limit current through an LED, let alone active regulation. Trying to add in a pi filter and other filtering/protection means I could spend a week trying to explain it all.

If our OP were going to power a microcontroller, I would've suggested something more elaborate. In this case, it would've seemed like swatting a fly using a battleship.

A TL783 would be more appropriate. However, they can pick up an LM317 at any Radio Shack, rather than having to order it from somewhere.

5. ### loganfinlay Thread Starter New Member

Mar 31, 2010
5
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hey, thanks a lot guys. i am a "n00b" when it comes to this kind of wiring so i appreciate the help. but in case i am a complete imbecile let me make a recap of what i have understood.

i should make the serial connection and put a 62 ohm resistor on the LED that is going to receive the power connection, then connect a LM317 regulator which is inturn connected to a 1Amp fuse which connects to the 12V power source...correct?

is it ok if i use a electrical "glue"? its like a heat shrink tube but you gust paint it on.

6. ### SgtWookie Expert

Jul 17, 2007
22,182
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Here, like this:

Note that the LM317 terminal labeled "COM" is actually the "ADJ" terminal.

You can find a datasheet for the LM117/LM317 family here: http://www.national.com/mpf/LM/LM117.html
Examine the datasheet to determine the pin connections for the regulator you have.
Get familiar with reading a datasheet. They are your guide to all electronic components. The first read will be pretty tough, and you probably won't understand most of it. But read it through several times over a week or so, and things will start to "click".

No, don't use the "electrical glue" stuff. That's for circuit board repair.

Use solder, and a soldering iron. Use heat shrink tubing to insulate your connections. Note that the tab on the regulator is "live". You will need to insulate it from contacting anything metal.

Learning to solder takes some practice. Use isopropyl alcohol (90% or better) to clean your parts before soldering them. Finger oils will make soldering very difficult.

If you apply heat for too long, you will damage the parts. If you can't get a good solder joint within a few seconds, stop, and let the parts cool back down.

• ###### 12v LED Current Limiter.PNG
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7. ### loganfinlay Thread Starter New Member

Mar 31, 2010
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thanks so much. i don't know how to thank you so it registers but what ever, thanks anyway for all your help. i'm sure you will have saved me loads of time and money

8. ### SgtWookie Expert

Jul 17, 2007
22,182
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You're welcome.

One last note; do NOT use "electrical tape" as insulation. The glue will get gummy after a few months, and the tape will fall off. You will then have bare wires with power on them, floating around in an area where there are lots of opportunities for it to short out.

That is why I suggested using heat shrink tubing. That takes a bit of practice, too - but once you select the correct size for the job, and shrink it properly, it won't come off.

9. ### loganfinlay Thread Starter New Member

Mar 31, 2010
5
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ok. one last question for you.
i will tell you a little more about what i want to do. i'm hooking up 4 sets of these to the same line of power. this is actuality to make a thing called angel eyes. they are just lights that go around the headlights for daytime driving.there will be two rings on both headlights(ie. 4 rings, or two sets of two circuits of 2 LED's [8LEDs]3.3V (typical)-3.6V (max) 25mA) would it matter if i hooked all 8 LED's to the same resistor and regulator?

so your diagram would look more like it had leds that went all the way up to D8. but in one set that was d1-d4 and the other that was D5-D8. i dont know if that makes seance or its just confusing.
i probably should have specified before i asked

10. ### SgtWookie Expert

Jul 17, 2007
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OK, now you are talking about modifying external running lights.

I am sorry, we have discontinued such discussions due to safety and legal reasons.

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11. ### loganfinlay Thread Starter New Member

Mar 31, 2010
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i understand your concern, and its reasonable. but i have looked them up. they are legal, i can buy them from the dealer but its just cheaper to make. are you still willing to help me?

12. ### SgtWookie Expert

Jul 17, 2007
22,182
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As I have previously stated, we no longer support discussions of modifications of external vehicle lighting due to legal and safety reasons.

I previously believed that you simply wanted instrument panel lamps to indicate when they were on.

You must not use the circuit for exterior lighting.

We have no way of knowing if it will be in accordance with all applicable laws and regulations for the area you reside in, or other members.

We also cannot assure that the materials you use and your workmanship will be of a level appropriate for such safety-related items.

If you want these "angel eye" rings, have your automobile dealership install them.

Other than that, the only thing we can recommend is that you maintain your vehicle's exterior lighting as it was equipped when originally sold by the dealer, as at that point in time it met all applicable regulations and safety standards.

13. ### SgtWookie Expert

Jul 17, 2007
22,182
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No, I will not help you with this via E-mail, either - even if you paid me.

I am not an automotive engineer.

If you want those lights, pay the dealer to install them.
The products they sell have to be certified, and the installers are licensed/certified to perform the task. That is why it is more expensive.

Apr 5, 2008
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Hello,

SgtWookie is right.
Changing the external lightning of a vehicle must be done by autorised dealer.
This way you are sure the lightning is still legal.

Bertus