# Determine resistance value

Discussion in 'The Projects Forum' started by FND, Sep 7, 2009.

1. ### FND Thread Starter Member

Apr 2, 2009
16
0
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?

2. ### loosewire AAC Fanatic!

Apr 25, 2008
1,584
435
Go to aac books,check ohms law.

3. ### FND Thread Starter Member

Apr 2, 2009
16
0
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.

4. ### loosewire AAC Fanatic!

Apr 25, 2008
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Where there a current meter,there a way.

5. ### FND Thread Starter Member

Apr 2, 2009
16
0
ok. Thank you. silly me

6. ### jj_alukkas Distinguished Member

Jan 8, 2009
751
5
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.

Apr 20, 2004
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8. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728

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

9. ### loosewire AAC Fanatic!

Apr 25, 2008
1,584
435
Thanks,Sgt. Will be more careful.

10. ### FND Thread Starter Member

Apr 2, 2009
16
0
Thank again to all of you for helping out.

Appreciate it very much, really helped!!

11. ### Audioguru New Member

Dec 20, 2007
9,411
896
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.