LED's burned out - SRT Forums

JAMESJACOBS333 · 07-25-2006, 10:34 PM

I added LED's to my hvac control head, and switches. I soldered all connections and used the resistors recommended by an LED calculator and by the how-to in the sticky section. My problem is that I keep burning out LED's. The LED's are 3.4-3.8 foward voltage 30mA foward current. The Resistors are 330 ohm .5 watt 5%. I have tried placing them on both the positive and the negative side of the circuit but I am still losing LED's. Any suggestions???

gargoile · 07-25-2006, 10:48 PM

go with a higher resistor.

dommo_g · 07-26-2006, 12:17 AM

Just buy the LEDs with built in resistors made specifically for 12v applications. Wester Auto, 4 pack for less than $10.

SLO-SRT4 · 07-26-2006, 06:45 AM

or order some from autolumination.com I think I used the #74's and they work well in the hvac with no problems.

JAMESJACOBS333 · 07-26-2006, 08:40 AM

I am going to try a higher resistor hopefully it won't dim the output. Should I just increase the ohms value or the watts? The reason I did't go with LED's with built in resistors is so I would have more versatility with placement other areas in the car..

URKRZE2 · 07-26-2006, 01:08 PM

James,

You need to increase the resistance value.
I think you will be fine using 1/2 watt resistors.

I think you were running them too close to their maximum allowable current.
On a 12V system the led would be pulling 26mA of current at 3.4V or 24.8mA at 3.8V with 330 ohm resistors.

I would recommend trying to keep the maximum current draw to 20mA.
You should use a resistor between 410 to 430 ohms.
This will keep you in the 20mA range.
You may not like how much dimmer they've become, but they will last longer.

fivetoez · 07-27-2006, 06:04 AM

the ONLY way to burn out an L.e.d. is to add too much voltage!

URKRZE2 · 07-29-2006, 12:07 PM

fivetoez,

Your statement is false.

If I kept the voltage at a constant 12V and slowly lowered the value of the current limiting resistor, I would eventually burn out the led (if you have means to do this go ahead and try it). How would this be possible if "the ONLY way to burn out an L.e.d. is to add too much voltage!"

As long as you are using the correct value resistor, it doesn't matter what voltage he is using. It is excessive current flowing through the led's that is burning them out prematurely. Voltage is just the electromotive force pushing the current through a resistance. For I=current, V=voltage, and R=resistance, use ohms law. V=I*R, I=V/R, R=V/I.

As you can see current and resistance are inversly proportional to each other. That means with voltage staying constant, increasing the resistance will decrease the current flow. As you can also see the voltage and current have a directly proportional relationship. That means as you increase voltage, current will increase provided you do not change the resistance.

The voltage in his situation is constant (12V from the battery), so the only 2 things that will effect his led are current and resistance. He can control the amount of current the led is seeing by the value of the current limiting resistor he is using. I gave him values the will keep the current close to 20mA. He may not like the brightness (dimmer), but it will certainly last longer.
One more thing that needs to be taken into account is heat, which is measured in watts and is expaning ohms law with the following:

P=V*I, where P=power, V=voltage, I=current.

This is where he has to choose the correct wattage rating for the resistor.
If he does not, it will not be able to dissapate the heat being generated and will eventually open.

These are the formulas I used for his dilema:

Vss-Vled/I=R

Vss=source voltage (12V)
Vled=led voltage (3.4 or 3.8, provided that these voltage drops are accurate)
I=current (I used 20mA)
R=resistance

For the wattage rating of the resistor, use the formula from above P=V*I

Try using these formulas with any voltage you want, as long as you use the proper current limiting resistor, the led will be fine.

I'm sorry to be long winded about this, but I dislike people making statements that some know to be false and then do not provide proof to back it up.

I am not ripping on you fivetoez, perhaps you have also been misled.

I just want to try and clear up any confusion this might create for JAMES.

JAMESJACOBS333 · 08-01-2006, 10:05 PM

Thanks for all the good info. I went with a 470 ohm .5 watt and it didn't seem to make much a difference in the brightness. It was the next size up that was available locally. Thanks again...

URKRZE2 · 08-02-2006, 10:33 PM

Your welcome for the info.
I gave your dilema a little more thought and need to know if the led's are wired in series, or parallel?
What I mean is are all the led's sharing the same resistor, or does each led have it's own resistor?
That could be another can of worms to be opened.
For quick reference, check out this site.

http://www.quickar.com/noqbestledcalc.htm

JAMESJACOBS333 · 08-03-2006, 10:04 AM

I wired all the LED's parallel. They all have their own resistor. I am considering some series circuits in other places but I know the resistor value will be lower for a series circuit...

JAMESJACOBS333 · 08-07-2006, 09:04 PM

Well even with the higher 470 ohm resistor I am still losing LED's. Any other ideas? I am getting kind of tired of soldering maybe the LED's I got were poor quality? Or maybe they are taking to much heat wile soldering on the resistors.Hell maybe the resistors are failing. I will ohm one once I take everything apart again.Any ideas would be helpful. Thanks...

URKRZE2 · 08-07-2006, 10:15 PM

Are you sure about the specs on the forward voltage for the led's?
Is that an operating range or a maximum value?
We can make this quicker, who is the manufacturer of the led's you are using, do the have a spec sheet available on the web?
If they do, post a link here so I can look at what you are using.

I would like to eliminate the the possibility of operating the led's out of tolerance before we address soldering practices.

JAMESJACOBS333 · 08-13-2006, 12:34 PM

The specs are:
Wavelength: 465~470nm
Foward voltage: 3.2~3.8v
Reversevoltage: 5~6v
Reverse current: <30uA

Maximum continuous current: 30mA
Maximum peak forward current: 75mA
Maximum power dissipation: 80mW

This is all the info I have thanks again for your help. I am going to check for any faulty connections. The only thing else I can think to mention is I am loosing the ones in the control head only the doors and defrost switches are fine. Maybe there is some kind of voltage spike in the circuit or maybe there is just a bad connection..

nolimits · 08-27-2006, 04:13 PM

Very odd, I use 330ohm resistors for my Blue LED's in the HVAC and never had a problem. I alternate between 1/4 watt and 1/2 watt resistors and those dont seem to make a difference either. For the Red LED in the HVAC i used a 470ohm 1/4watt resistor then switched to a 270ohm 1/2 watt resistor with no difference. But then again, I rocking alot more LED's in my hvac then other peoples, i have 6 in strategic locations compared to 3.