I was wondering if someone could help. I have a blue led in circuit. Running across it is 2.84 vdc but it doesnt light. When i take it out of circuit and test it with diode test it lights. Ive checked and it wired in correct way in circuit + and - correctly. I have a 100 ohm resistor before the led.
You need to check the datasheet of your blue LED but a lot of the blue ones require over 3 volts to conduct.
Blue LEDs can also have a fairly sharp on versus off voltage point. For example your blue LED might appear off at 2.84 volts and bright at 3 volts.
What current are you expecting to run your LED at?
Suppose 10mA. That would require 1V across 100 ohms, so your supply to the LED should be at least 4V assuming a general 3V for the LED. That should be enough to light it, or is your supply voltage 2.84V?
Some LEDs light at 3-3.4V, and may need more. In general it is a good idea to double the expected operating voltage and drop the LED equivalent voltage across the series resistor.
Suppose you only need 5mA of current, then your power supply would be 6V with a 600 ohm series resistor.
Is this a power LED?
More efficient methods of driving LEDs at higher power usually need a switch mode supply.
Suppose 10mA. That would require 1V across 100 ohms, so your supply to the LED should be at least 4V assuming a general 3V for the LED. That should be enough to light it, or is your supply voltage 2.84V?
Some LEDs light at 3-3.4V, and may need more. In general it is a good idea to double the expected operating voltage and drop the LED equivalent voltage across the series resistor.
Suppose you only need 5mA of current, then your power supply would be 6V with a 600 ohm series resistor.
Is this a power LED?
More efficient methods of driving LEDs at higher power usually need a switch mode supply.
The power supply is supplying 6.5 volts dc. Im a beginner in electronics so abit lost on what resistor i used use…
With your 6.5VDC supply and allowing 3VDC for the LED to operate, and with an LED current of 5mA (plenty for a Blue LED, otherwise it will be too bright) - this means the dropping resistor needs to drop the other 3.5 vDC. Ohms Law states: R = E/I, so 3.5V divided by 5mA = 700 ohms, nearest common value being 680ohms quarter watt.
If you're using the LED as an indicator (e.g., for power), you may want to use a larger resistor. I don't like overly-bright LEDs on my front panels. In that role, I usually set my blue LEDs at about 1.5 mA. In your case, that would require a resistor of about 2400 ohms. A 2700 ohm resistor would be fine.
Regards.
Regards.
Maybe your resistor is actually 100k or something.
Measure it and make sure the value is correct.
But it should be more like a few hundred ohms.
Measure it and make sure the value is correct.
But it should be more like a few hundred ohms.
Yeh, adjust the dropping resistor value to give you a comfortable LED brightness. That will also depend on the quality of the LED - some are a light more efficient than others.
My general rule of thumb with blue LED?
From 12V I use 5k1.
From 120V, I use 100k
From 320V, I use 1M.
From 6.5V, I'd start with 1k or 2k2... At full current, a blue LED is bright enough to land a plane with! (ok maybe not but it's bright!)
From 12V I use 5k1.
From 120V, I use 100k
From 320V, I use 1M.
From 6.5V, I'd start with 1k or 2k2... At full current, a blue LED is bright enough to land a plane with! (ok maybe not but it's bright!)
Yeah - a blue GaN led will practically put your eye out if you push 10 mA through it. !-2 mA is more than enough for pilot light duty. I prefer the blue from the old SiC leds that had a brief vogue before GaN took over. The color is more a "sky blue" and less irritating to my eyes. The voltage drop is higher, and you need some serious current, even for pilot light duty.
Mine must be GaN. 320µA and it's plenty bright! 1mW of power... The Green one is powered from 12V through 5k1 I think,
I replaced the grain of wheat bulb for the high beam indicator in my old car with a blue LED. Even 1M was blinding - I think I went with 10M.
I replaced the grain of wheat bulb for the high beam indicator in my old car with a blue LED. Even 1M was blinding - I think I went with 10M.
