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How to correctly power LEDs (in the car)
#1
Some may get offended by this subject: I know how to calculate resistors for LEDs

Some do not realize is there is not 12V under your hood. When that voltage is at 12V the battery is at almost discharged. Also when system power is supplied by alternator (everytime your car is idling or running), voltage may be even higher and can go up to 15V. I had several problems with alternators in the past and added voltmeter to my dashboard to quickly spot any issues, it usually sits at 14.6V when running. When alternator regulator becomes faulty, which is a common problem with some cars, it can go even way higher. Would it be ok for thousands of dollars of electronic modules inside the car to fry just because $200 alternator fails? Including those fancy LED lights you spend whole weekend wiring up ?

Common 20mA 3mm and 5mm LEDs are enclosed inside the plastic lens and do not have very good cooling, the heat is trapped inside so when current exceeds 20mA, they will overheat internally and eventually fail. Speed of overheating depends on the current, if current is just a bit higher it may take hours to overheat, but if current is high enough, they may overheat in few milliseconds and you won't even see them flash, that how fast they fail.

[Image: LED5.jpg]


Common misconception: 3x LEDs + 330 ohm

   

A customer complained we sold him defective red and yellow LEDs, but had no problem with white.

Red and yellow usually have forward voltage around 2V, so three in series will have 6V forward voltage. When onboard power is 15V, the current through these three LEDs will be (15V-3*2V)/330 = 0.027A. What's worse, the resistor itself is dissipating (15V-6V)^2/330 = 0.2454 W, which is awfully close to 1/4W - if resistors are in enclosed space, it will be hot and this added heat will not help those LEDs, nor the resistor itself.

3x white LED + 330ohm will result in (15V-3*3.3V)/330 = 15.4mA. Whites are running fine, under the nominal current.

Plan for Worst Possible Scenario

Good designer designs his circuit for the worst possible scenario, which would be around 15V in the car. It would be also useful to have some overvoltage protection circuit that will shut off the output if it goes over the limit, some of the simplest ones may be only 3 parts. Or this zener with fuse is only two external parts. Even better, use the regulator.

Red and yellow LEDs have approx 2V forward voltage @ 20mA, so connecting 3 in series atr 15V will need (15V - 2V*3)/0.02 = 450. Closest higher is 470 ohm.

White LEDs have approx 3.3V forward voltage @ 20mA, so connecting 3 in series at 15V will need (15V - 3.3V*3)/0.02 = 255. Closest higher is 270ohm.

Disadvantage of the resistor as current limiter is red and yellow above will be running at 12.8mA and white above will be running only at 7.8mA when system voltage is 12V. With 3V voltage swing, the current changes a lot. For this purpose it may be better to use current sources like this one:

[Image: ConstantCurrent-2.gif]

or even better - voltage regulator that will keep the voltage always at 12V regardless on the input and design the resistors for exactly 12V. For higher currents and wider variations, use buck converters - some can go as high as 36V, which will also protect your LEDs from overvoltage from faulty alternator. If using linear regulators, the higher currents may thermally shut-off your regulator untill it cools down and your LEDs won't be on, but will rather flicker as the regulator is heating up and cooling down.

[Image: DCDC-XL4015E-DSUN-2.jpg]
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#2
Nobody should get offended in forums. Nobody is trying to offend anybody, sometimes things get taken the wrong way as they are intended in text.
Appreciate any advice you give Roman. Maybe basic for some but not for others. Good refresher anyways.
This is a learning Topic so it should contain how to light an led.
I would like to add lights in my car too. I wouldn't know where to tap into the power though.
If I did add lights to my car I would of never thought about the voltage swings from my battery and alternator.
Thanks Roman for this advice. And the current source circuit and buck converter idea.

Actually interior light bulb in my truck has been burnt out since I bought truck, just never got around to replacing it. This thread got me moving on it now. Measured voltage on terminals of light socket. Showed 12.4 volts with car off. When starting it, it went down to 9 volts, then to 12, then settled at 14.7 volts running.
I have a spare tiny 12V, 20W incandescent bulb. Think that would blow too easy.
Maybe with your idea Roman, I can make an led circuit and put it in there. Smile
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#3
Lots of good tips - here are a couple more to add about these battery circuits -

- check the current and voltage specs on the actual leds you have, don't go by rule of thumb for some that look similar.
- consider a clamping diode in the opposite direction since in a car you can get huge transients on the wiring if you connect somewhere other than the battery itself due to things like the starter and other motors in the car. (LEDs don't like reverse bias - ever hook one up to ac with a limit resistor ? you find that out quick.)
- many cheap charger circuits rely on the battery as a load, if you disconnect the battery or a fuse blows you can get the open circuit charger voltage across your circuit which could be way higher. plan a crowbar circuit or regulate the bad power your circuit might see.
- lead acid batteries on smart chargers get an equalize cycle every so often and its deliberately up in the 15-16v range so beware of that too.
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If things weren't meant to be modified, they would not come with wires attached.
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#4
I would say there is much smaller probability the LEDs being bad from the start than they died in the rig from over-current or a transient. It is not easy to protect sensitive electronics in the car.

You would not believe how many times I get angry emails from people trashing me that I sold them defective LEDs, and when I ask them what current or voltage/resistor they used, I get a "Huh? I just connected them to my XY battery and they do not work, send me the good ones" ... so I wrote this tutorial and will be linking to this article from my sales pages as well Smile

Jon, great tips, thank you !
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#5
Thanks Jon for those tips. Looks like you have to be really careful when wiring to a car.

I have a 78L05 5v, 100 ma voltage regulater, that can handle up to 20v input.

http://pdf1.alldatasheet.com/datasheet-p...78L05.html

Do you think that alone with resister would be good enough to light a couple of leds, or do you think I would need a heat sink or some other component to be safe?

(2016-03-25, 06:54 PM)roman Wrote: I would say there is much smaller probability the LEDs being bad from the start than they died in the rig from over-current or a transient. It is not easy to protect sensitive electronics in the car.

You would not believe how many times I get angry emails from people trashing me that I sold them defective LEDs, and when I ask them what current or voltage/resistor they used, I get a "Huh? I just connected them to my XY battery and they do not work, send me the good ones" ... I wrote this and will be linking to this article from my sales pages as well Smile

Jon, great tips, thank you !

Ignorant customers  that do not appreciate advice.  And ashamed they get mad of some defective leds, they are so cheap to buy and it is the norm to get the rare defective led.  Even knowing in these peoples case it wasnt their leds, but there lack of knowledge and ability to take advice.
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#6
78L05 are maximum 600mW (limitation of TO92 package). As long as you do not exceed 100mA and 600mW, it should work fine.

Another thing to keep in mind is that 78L05 voltage drop is at least 1.5V @ 1mA or 2V @ 70mA, so output voltage can never be as high as input. If you have 12V at the input, you can only get 10.5 at the output.
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#7
Thanks Roman for the tips on the 78L05.
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#8
Hi guys,
Even those seasoned circuiteers need to remember the basics and realize how easy it is to overdrive led's.
I was using top end LUMI leds and they can take a steady voltage and current no issue at all.
I used other product and they did burn out relatively quickly.
You can't assume anything with LED's...confirm what the spec sheet says and follow it closely.
I have not found a single LED that did not work in all the years that I have been buiding...I did have some with the flat side opposite polarity though.
Bob D
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#9
Customer informed me that he identified the issue not being the LEDs themselves but hot glue he was using.

Add that to the list of things that can damage the LED.
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