Barely visible LED diodes.

[Arc_Jester]

I have a couple of led diodes I want to use in a project but when I tested them they went bright for a second then faded, they are really dim now and I was wondering can they be fixed or are they dead?

[mariushm]

Re: Barely visible LED diodes.

How did you test them?

IF you didn't use a resistor to limit the current or use the diode measurement function of your multimeter, you probably damaged or killed them.

LED brightness varies with the current, once the voltage goes above the minimum voltage required by the diode to conduct current, you need to limit the current going through the led, otherwise it overheats and dies.


So you put a resistor in series with the led to limit the current.

Here's how to figure out what resistors to use.

Voltage = Current x Resistance

Let's say you have a red led which typically has an advertised voltage of 2v and a recommended maximum current of 10mA (0.01A).
You want to test it with 5v and a current of 5 mA , in which case:

Voltage (5v) - LED Voltage (2v) = Current (0.05A) x Resistance

So you need a resistor with a value of around R = (5v - 2v ) / 0.05 = 3 / 0.05 = 60 ohm

Smaller value would mean a bit more current than 5mA, larger value makes led dimmer.

[Arc_Jester]

Re: Barely visible LED diodes.

-snip-

I used a multimeter but no resistor, I reduced the current and the leds are working now. Thanks for the help.

[EFC]

Re: Barely visible LED diodes.

How did you test them?

IF you didn't use a resistor to limit the current or use the diode measurement function of your multimeter, you probably damaged or killed them.

LED brightness varies with the current, once the voltage goes above the minimum voltage required by the diode to conduct current, you need to limit the current going through the led, otherwise it overheats and dies.


So you put a resistor in series with the led to limit the current.

Here's how to figure out what resistors to use.

Voltage = Current x Resistance

Let's say you have a red led which typically has an advertised voltage of 2v and a recommended maximum current of 10mA (0.01A).
You want to test it with 5v and a current of 5 mA , in which case:

Voltage (5v) - LED Voltage (2v) = Current (0.05A) x Resistance

So you need a resistor with a value of around R = (5v - 2v ) / 0.05 = 3 / 0.05 = 60 ohm

Smaller value would mean a bit more current than 5mA, larger value makes led dimmer.

Good explanation but any chance that 5mA should have been 0.005 (which would have resulted in 600 ohms)?

[LLLlllou]

Re: Barely visible LED diodes.

Good explanation but any chance that 5mA should have been 0.005 (which would have resulted in 600 ohms)?

LOL, yeah, .05 is FIFTY milliamps, not five. DAM YOU MATH, WHY DO YOU HAVE TO BE SO UNFORGIVING!?!?!?!??!

[SteveNielsen]

Re: Barely visible LED diodes.

You can avoid the math altogether using a variable resistor in series; adjust to the current the LED is happily lighting with and check the resistance the vr is at.

[mariushm]

Re: Barely visible LED diodes.

Correct, it was 0.005A so the resistor value was 600 ohm. Short story, a 330-470 ohm resistor should work for testing most small leds on the market.

// As usual, that's what happens when someone types posts at stupid hours like 4:30 in the morning.

[Arc_Jester]

Re: Barely visible LED diodes.

Thanks for the help guys.

[eccerr0r]

Re: Barely visible LED diodes.

I'd not use a variable resistor by itself because it could be adjusted into the "LED killer" range... At least have a 150 ohm resistor in series with a ~2K ohm variable resistor max if you want an adjustable brightness test jig for typical 20mA LEDs with a 5V supply. It will underdrive some LEDs but should be safe for most cases.

Granted, most people want the LED to be as bright as possible anyway, so it's best to just do the math.

[SteveNielsen]

Re: Barely visible LED diodes.

It should be obvious to have the VR at max resistance to begin with at this point in the discussion. For those who have regular encounters with Murphey's Law adding the 150 ohm resistor will certainly help fool-proof it. I'm careful and use a 5K trimpot as a practical test. No dead LEDs yet.

There are online LED resistor calulators for the math, too. I also use a transitor tester to show the actual forward voltage drop of the individual LED. The math gets me real close but practical testing lets me hone in on the highest resistance that will produce sufficient illumination.

[ben7]

Re: Barely visible LED diodes.

You can avoid the math altogether using a variable resistor in series; adjust to the current the LED is happily lighting with and check the resistance the vr is at.

Until you turn it too far .... eh, been there done that

I was too slow to catch the magic smoke, if I did then I could have put it back in xD