LEDs keep stopping working

[gtobeck]

Hi in my car i have an LED setup (the purpose isnt really relevant). The LED works fine for 5 mins but after that they stop working.

The positve leg has a 510 ohm resistor on it and is then connected to a 12V power supply.
The negative leg is attached to a wire which goes to negative under certain conditions, all other times it is 12V+. I added a diode to this wire but that still did not help.

My theory is that perhaps the 12V wired to the negative leg is too much for the LED, it is rated at 5V max reverse DC voltage. If this is the case then how do i fix this?

Any help would be very much appreciated!!!

Thanks

 

[audioguru]

The anode of the LED connects to +12V (?) through a 510 ohm resistor. The cathode connects to 0V or to +12V so the LED is never reversed. Then why worry about it being reversed?

When the catrhode of the LED connects to +12V like the anode is then the LED will be off.

The resistor is connected to a 12V power supply? Not to the car's +12V like the cathode is sometimes?

In the beginning of a sentence you have a single LED. At the end of the same sentence you have more than one LED. I don't know how many LEDs nor their forward voltage rating. Do you?

 

[gtobeck]

ok, i dont know too much about electronics so i just assumed it was as good as reversed.

the resistor is connected to a wire which powers the heater/fan controls for my car. the cathode is connected to a wire which runs from the cars computer to a solenoid in the engine bay. So both wires are connected to the cars normal power supply.

I have one led only, i have just had to replace it a couple of times!!!!

the DC forward voltage rating is 3.3 to 3.6, condition "IF=20mA" - thats what the sheet says...

hope that clears things up for you!

 

[audioguru]

The fan and the solenoid are both inductors. When power to an inductor is turned off, it produces a voltage spike of a few hundred volts that is killing your LED.
The 510 ohm resistor would be needed to be replaced with two 270 ohm resistors in series and in series with your LED. Then an 9.1V zener diode can be connected across the LED and the resistor connected to it to limit its current to (9.1V-3.5V}/270 ohms= 21mA. It is good to connect a diode in series to stop reverse voltage spikes from the inductors.

 

[gtobeck]

ahhh great! what is the reasoning behind replacing the resistors?

so do u mean i should connect the zener diode in parallel with the resistors and the led?

it has been suggested to me that i should put a diode in parallel accross the led with the stripe facing the +12V side, would this work as it would be a bit easier to do?

 

[audioguru]

ahhh great! what is the reasoning behind replacing the resistors?

So that a zener diode can clamp the voltage across the LED and one of the two resistors. Then the other resistor gets the hundreds of volts inductive spike instead of the LED.

so do u mean i should connect the zener diode in parallel with the resistors and the led?

"The 510 ohm resistor would be needed to be replaced with two 270 ohm resistors in series and in series with your LED. Then an 9.1V zener diode can be connected across the LED and the resistor connected to it ..."

it has been suggested to me that i should put a diode in parallel across the led with the stripe facing the +12V side, would this work as it would be a bit easier to do?

The zener diode will do it.

 

[eblc1388]

Just connects a current meter across the LED. If the meter reads more than 27mA( 13.8V divided by 510 ohm), then you might have used the wrong resistor that you think is 510 Ohm.

Also have you tested your LED before you fitted it to your car? You might not need to use current as high as 20mA because If=20mA could be the maximum rating of the LED at room temperature.

 

[Hero999]

Then the other resistor gets the hundreds of volts inductive spike instead of the LED.

Which wouldn't damage the resistor at all, even if it's voltage rating is exceded for a short perioud of time no damge will result, it takes a lot of energy to damage a low value resistor. A diode in reverse parallel with the LED is all that's required.

EDIT:
You might be able to use a red LED because the reverse avalanche breakdown isn't distructive. When I was a newb I connected a red LED in series with a 10k resistor to the mains and it worked for a while until the resistor over heated, more recently I did the same with a 7.5W reistor and a blue LED and it was distroyed immediately.

There again it depends on the current consumption of the blower, if the current is too high then even a red LED will be killed by the peak current.