Hi All,

How do I determine the value of R to be used when I'm using a 9v battery to power an LED with voltage rating of 3v?

Help please.

 

Go to aac books,check ohms law.

 

Sorry, this thing some what makes me really confused.

I don't know the current, so how would i know the resistance? The only info I have is the LED's Voltage rating is 3V.

How? I know it's the most basic thing and it's a silly question. But please help understand.

Thank you.

 

Where there a current meter,there a way.

 

ok. Thank you. silly me

 

Resistance value for current limiting for an LED = (Vcc-Vf)/I

Vcc = supply voltage = 9v
Vf = LED forward voltage (anywhere between 1.8 -4.3V for different types)
I = Led current (Normally 15 or 20mA) , use as 0.02 in formula.

 

Look at our Ebook in the LED section - http://www.allaboutcircuits.com/vol_3/chpt_3/12.html

The LED will have its forward voltage (Vf) and operating current stated. Use those to figure the fixed resistor.

 

Where there a current meter,there a way.

Bad advice, loosewire!

I hope our OP did not follow it, as they probably fried their LED or meter by now.

jj_alukkas gave the proper reply. I usually spell it out a little bit more.

The basic formula is:
Rlimit >= (Vsupply - Vf_LED) / DesiredCurrent
Vsupply needs to be higher than Vf_LED, of course.

In your case, assuming your LED's Vf was measured at 20mA current:
Rlimit >= (9v - 3v) / 20mA
Rlimit >= 6v/0.02A
Rlimit >= 300 Ohms
300 Ohms is a standard value of resistance. You can use a higher value of resistance, but you should not go lower, or you risk burning out your LED.

There is a table of standard resistance values on this site:
http://www.logwell.com/tech/components/resistor_values.html

 

Thanks,Sgt. Will be more careful.

 

Thank again to all of you for helping out.

Appreciate it very much, really helped!!

 

You cannot buy a 3.0V LED. They have a range of voltage like 2.5V to 3.5V.
If you do not measure its actual voltage then your circuit must limit the current enough that a low voltage LED does not burn out and must allow enough current for a high voltage LED to be bright enough.