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?
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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. -
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.Comment
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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.Comment
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Re: Barely visible LED diodes.
Thanks for the help guys.
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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.Comment
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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.Comment
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Dear members,
I have a linear variable power supply with unknown brand name that I used for voltage injection.
Then it failed. I discovered that its transistors (Q1 and Q2) are shorted.
In addition, I discovered that its zener diodes are defective.
I can replace the burned transistors.
But I do not know the model of the zener diodes. the model number is not marked on the zener diodes.
Therefore, I cannot replace the zener diodes.
I do not want to test the zener diodes in a test circuit to discover their model or value.
no schematics are available... -
Elco (?) 50" Smart TV. Backlight ( BL) issue.
Single Board : TP.SK708D.PC821
I bought new BL set . - OK
Power up : Blip . Protection.
Led Tester shows current at 34.2 mA , Previously it was 42.1 mA
Solution : Change current output .
The BL IC is marked on the photo and datasheet is QOB33535 Datasheet(PDF) - Schneider Electric (alldatasheet.com)
Calculating according to the datasheet - see attached photo :
With ILed = 34.2mA , equation #1 is RFb = 400mV / 34.2 = 11.7 Ohm , and with 42.1 mA we get 9.50 Ohm . i.e I should look for 9.5 Ohm...06-09-2024, 12:25 PM -
Dear community, I want to increase the lifespan of my backlight led in my TV by decreasing the current.
However I need some help identifying the resistor I need to replace to achieve this.
Thank you!
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Hi.
Quick question.
I have a laptop that is completely dead. I opened it up and it looks like the current sense resistor is blown. However, because it's blown, the value printed on the top has disintegrated and there is no schematic available for this particular laptop (Dynabook Portege) so I'm not sure what to replace it with.
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