Converting voltage question - please help a geek

[Fuelrod]

I'm sure this is a stupid question for you EE majors out there but please help a simple minded geek. I have a Red LED that is rated for 1.7 volts (2.4 max) and 20 mA. I would like to tap into the 3.3 voltage line on my power supply to power the LED. Is there a resistor I can buy from Radio Shack to somehow put inline with the LED to drop the voltage or am I in over my head and I should go back to playing with Legos and stay the hell away form electricity. Help.

 

[bignateyk]

 

[blahblah99]

Voltage drop across the LED is about 2.1V, so you need a resistor to drop 1.2V

Since the led can only sink about 20mA, 1.2V/20mA = 60 Ohms. Pick a resistor greater than 60 ohms but closest to it.

 

[Fuelrod]

Thanks, I'll pick one up tomorrow and give it a try.

 

[cressida]

Originally posted by: blahblah99
Voltage drop across the LED is about 2.1V, so you need a resistor to drop 1.2V

Since the led can only sink about 20mA, 1.2V/20mA = 60 Ohms. Pick a resistor greater than 60 ohms but closest to it.

uhm... would the 3.3 voltage line burn out the LED since it's maximum voltage rating is 2.4? Or do I need to review my electronics? 🙁

 

[CTho9305]

Originally posted by: SoloKid

Originally posted by: blahblah99
Voltage drop across the LED is about 2.1V, so you need a resistor to drop 1.2V

Since the led can only sink about 20mA, 1.2V/20mA = 60 Ohms. Pick a resistor greater than 60 ohms but closest to it.

uhm... would the 3.3 voltage line burn out the LED since it's maximum voltage rating is 2.4? Or do I need to review my electronics? 🙁

V = IR
a diode effectively has almost no resistance but creates a voltage drop, so without a resistor, huge amounts of current will flow if you just use 3.3v

with the resistor, you now have something that limits current, and it wont burn.

 

[TLfromAI]

Originally posted by: CTho9305

Originally posted by: SoloKid

Originally posted by: blahblah99
Voltage drop across the LED is about 2.1V, so you need a resistor to drop 1.2V

Since the led can only sink about 20mA, 1.2V/20mA = 60 Ohms. Pick a resistor greater than 60 ohms but closest to it.

uhm... would the 3.3 voltage line burn out the LED since it's maximum voltage rating is 2.4? Or do I need to review my electronics? 🙁

V = IR
a diode effectively has almost no resistance but creates a voltage drop, so without a resistor, huge amounts of current will flow if you just use 3.3v

with the resistor, you now have something that limits current, and it wont burn.

He's right...go EEs!

 

[Superdoopercooper]

Just a note.... unless your LED says it has a recommended operating current at 20mA.... I would go much less than that. Go for 10mA... .which means you'll double the resistor value someone else above calculated.

The advantage is that with 50% less current, you won't see THAT much difference in brightness (experiement to verify... ), but you'll allow your LED to live longer.

Remember... power = IV. So, you double the current you will double the power that thing has to dissipate. And you're more likely to burn out the p-n junction the higher the constant current is (shorter life span).

 

[Mday]

go to google.

search for "LED resistor calculator" or something similar. =)

O_O

you dont want to tap the 3.3 line. tap the 5V line.

Get as close to 20mA as possible. LEDs are fussy like that. And the voltage, I would recommend 2V for it.

 

[CTho9305]

Originally posted by: Mday
go to google.

search for "LED resistor calculator" or something similar. =)

O_O

you dont want to tap the 3.3 line. tap the 5V line.

Get as close to 20mA as possible. LEDs are fussy like that. And the voltage, I would recommend 2V for it.

Whats wrong with 3.3? A single LED is an insignificant load....

 

[neo4s]

LEDs can be confusing. They will drop about 1.7 volts no matter what voltage you apply to them. LEDs offer no resistance to current, so a resistor in series is always needed to limit current.

Calculate the needed resistance:
resistance = (power supply voltage - 1.7) / 0.015

(the 0.015 is 15 milliamps, 10 to 20 mA is fine)
for 3.3 volts you need a 100 ohm resistor (give or take a few ohms)
for 5 volts you need a 220 ohm resistor

Calculate the needed power disapation of the resistor:
power = (power supply voltage - 1.7) * 0.015

a little 1/4 watt resistor will work fine for 3.3 or 5 volt power supplies

The 5 volt output on the power supply is much easier to get to than the 3.3 volt. Either voltage will work just fine so Id recommend the 5 volt. LEDs have a positive and negative lead, if it dosnt work one way, just turn it around. If the LED gets hot or is glowing a weird color then check your supply voltage - make sure its not conneted to the 12 volt line. Hope this clears everything up.