Ohms Law

[biferi]

I have a Power Source of 6 Volts and a LED that needs 3.5 Volts.
So I know my Voltage Drop across my Resister will need to be 2.5 Volts.

So now all I know is my Power Source is 6V and my LED needs 3.5V and my Voltage Drop will have to be 2.5V.

How do I find the Resistance of my Resister I need I do not know the Current??

 

[dougy83]

You need to know the current to work out the resistance. If the current is 20mA, then the resistance would be 2.5/.02 = 125 ohms

 

[audioguru]

If your LED is 5mm in diameter then its recommended current is 20mA for it to be bright. Less current will make it dimmer.

 

[biferi]

Well I have 4 Dabble A Batteries in a Pack that connects them in Series so I know the Batteries are 1.5 Volts each.
So my Battery Pack is giving me 6 Volts.

And I know my Led needs 3.5 Volts to work so my Voltage Drop must be 2.5 Volts.

And I can not find the Resistor I need without knowing the Current?

What if I just conect the 2 Leads coming off my Battery Pack to one another. And as the Current is moving through I usee my Voltmeter to see how many mA are moving.

Then I can go workout the Resistance I need?

 

[KeepItSimpleStupid]

Single LED's are basically current devices and usually operate between 1 and 50 mA. That's the ball park. Try a resistor for 1 mA, 5 mA, 10 ma, 20 mA or use a fixed resistor + a variable one. Make it so the adjust-ability is between 0.5 mA to 50 mA, Check the brightness of the LED.

There is a very wide range where the LED will operate.

Your LED probably isn't red. My guess is that it's a white LED?

 

[crutschow]

......................................
And I can not find the Resistor I need without knowing the Current?

No.

What if I just conect the 2 Leads coming off my Battery Pack to one another. And as the Current is moving through I usee my Voltmeter to see how many mA are moving.

Then I can go workout the Resistance I need?

No. The short circuit current of you battery pack will likely be very large and has nothing to do with the current the LED needs.

 

[misterT]

And I know my Led needs 3.5 Volts to work so my Voltage Drop must be 2.5 Volts.

Yes. Now you know that the voltage across the resistor is 2.5 volts.
Now you need to decide how much current you want to flow through the resistor (same current flows through the LED).

As dougy83 already mentioned "If the current is 20mA, then the resistance would be 2.5/0.02 = 125 ohms"
20mA is usually the upper limit for leds, so I would use a resistor of 150 ohms or 180 ohms.

In another words. If you choose a 180 ohms resistor, And you know that the voltage across it is 2.5 V, then the current through it is:
2.5/180 = 0.014 A
(14mA)
And, because the resistor and the LED are in series, that same current flows through the LED.

 

[audioguru]

Please post a video of your 6V battery being destroyed when you short together its two wires.