Could someone please explain to me on how to use a resistor?

littlemal · Post by **littlemal** » Tue Feb 21, 2006 10:02 pm

I've been researching on electronics and all that and i want to know how to calculate how strong of a resistor i need(for say an LED). i know all about Ohms law and everything but i can't find out how you would calculate a correct resistor. For example: I have a 9 volt battery and i want to connect it to a LED but i don't know the current. How would you calculate for a correct resistor. Any help would be appreciated!!!

daguuy · Post by **daguuy** » Tue Feb 21, 2006 10:35 pm

i don't know how to calculate it but if you have some junk with a small pot on it you could turn that to the right brightness and measure that. or you could find some place that includes resistors with their LEDs

S q u e e ! · Post by **S q u e e !** » Wed Feb 22, 2006 10:19 am

Firstly, you'd have to find the current of the 9-volt using a multimeter.

Then, using the Ohm's Law Triangle where:

E=I x R

E is EMF or Voltage, I is current (in Amps), and R is resistance in Ω.
Of course, to find out Ω, you use the equation:

R=V/I

It's quite simple really.

**EDIT**

I found this, it should help visualize.

The VIR triangle

Ohm's Law triangle. You can use the VIR triangle to help you remember the three versions of Ohm's Law. Write down V, I and R in a triangle like the one in below.

* To calculate voltage, V: put your finger over V,
this leaves you with I R, so the equation is V = I Ã— R
* To calculate current, I: put your finger over I,
this leaves you with V over R, so the equation is I = V/R
* To calculate resistance, R: put your finger over R,
this leaves you with V over I, so the equation is R = V/I

The Triangle

littlemal · Post by **littlemal** » Wed Feb 22, 2006 5:44 pm

OH so you find the current using a resistor and then plug it into the equation?? So it would be like: R= 9volts/ te current found with a multimeter. Is this how you do it???

S q u e e ! · Post by **S q u e e !** » Wed Feb 22, 2006 6:33 pm

Yes it is.

littlemal · Post by **littlemal** » Wed Feb 22, 2006 9:15 pm

Wait a minute though, don't you have to know how much current the component can take before it fries

Website · Post by **Life of Brian** » Fri Feb 24, 2006 12:02 am

This is what I use: http://metku.net/?sect=view&n=1&path=mo ... /index_eng

littlemal · Post by **littlemal** » Fri Feb 24, 2006 11:19 pm

So don't you have to find the current the component can take before it fries though. (Sorry if I'm making anyone frustrated, it's just I came across the whole idea of resistance and this has just stumped me)

(Thanks for the link JoeDog but what does the desired resistance mean??)

littlemal · Post by **littlemal** » Sun Feb 26, 2006 10:42 am

(I'm still trying to figure out how to quote correctly) but anyway When you do the R=V/I would that be the resistance of the whole circuit? If so, how would you calculate a resistor for an LED or another component out of the cicuit resistance??

littlemal · Post by **littlemal** » Wed Mar 01, 2006 9:32 pm

Ok heres what i'm trying to figure out. In the formula R=V/I. R= The resistance your trying to find, and V= the supply voltage. But does I = the current that comes from the supply voltage or does it mean how much current the LED can take before blowing up?? (Any help would be appreciated

)

S q u e e ! · Post by **S q u e e !** » Thu Mar 02, 2006 10:03 am

I believe it's the max current the LED can handle, but you should get a second opinion.

Post by **Gamelver** » Thu Mar 02, 2006 2:11 pm

yes, "I" in that equation would be the max current the LEDs draw. if you've got 4 in parallel, it's probably 80mA.

littlemal · Post by **littlemal** » Thu Mar 02, 2006 9:08 pm

And then the number you get would be the number of ohms or the rating on the resistor you need to use?

A.J. Franzman · Post by **A.J. Franzman** » Fri Mar 03, 2006 3:13 am

Geez, you guys are so goofed up it's not even funny! (Except for joedog86 and his link.)

To calculate the value of a resistor to use in series with an LED, you need to know 3 things:

1. The voltage of the supply, Vs (volts).
2. The forward voltage drop of the LED, Vf (volts).
3. The forward current of the LED, If (amperes).

If If is in milliamperes (mA), divide by 1000 to convert to amperes (A). Thus, 75 mA = 0.075 A.

To determine the resistor value, subtract the LED voltage from the supply voltage to get the voltage across the resistor, Vr. (Vs - Vf = Vr).

Now use the "Ohm's Law" equation, converted to R = V / I. Or in this case, R = Vr / If.

Hypothetical example:
If the supply is 9 volts, the LED's forward current is 20 mA and its forward voltage drop is 3.6 volts, this gives:
9.0 - 3.6 = 5.4 volts across the resistor
R = 5.4 / 0.020
R = 270 Ω

This just happens to be exactly a standard value. However, if you used the max allowable LED current for If, and had different figures in the equation which did not result in a standard resistor value, you must go to a higher value to prevent shortening the LED's lifespan. The next common standard value is usually "close enough" - don't bother trying to put several resistors in series to make exactly the calculated value; it isn't worth the trouble.

Since littlemal didn't give us the voltage or current for his LED, we don't have enough info to calculate the resistor for him.

A.J. Franzman · Post by **A.J. Franzman** » Fri Mar 03, 2006 3:26 am

S q u e e ! wrote:Firstly, you'd have to find the current of the 9-volt using a multimeter.

If you do that, you'll likely blow the fuse in the meter (assuming it has one, and your 9-volt battery can put out much more than half an amp into a dead short).

You don't need to know the supply current. You need the specified "typical" or "maximum" continuous forward current of the LED. This is not something you can measure, unless you're scavenging the LED from some other working, properly-designed circuit. This value is found in documentation from the LED maker or seller.