Ohm's Law

[Fear.Sin]

I have been fooling around with electronics and bread board type of stuff and never really understood ohms law. I use calculators to get the resistors I need and I read the wikipedia description of ohms law and I think I am thick headed or something because I still don't understand it.

If anyone has any articles or if they want to describe it for me themselves that would be great. I know there are some electronic experts here so I expect someone should be able to explain it in lamins terms.

Thanks in advance!

[Helix666]

Ohm's Law.



V
-------
I x R


(Voltage is equal to current divided by resistance.)

for example, if you know the current (in Amps) and the voltage (in Volts. ;) ), you can work out the resistance (in Ohms) of the circuit.

e.g.
If 25V at 5A is flowing through a circuit:

R = V / A

R = 25 / 5

R = 5

The total resistance of the circuit is 5 Ohms.


I think... someone correct me if I'm wrong. I'm writing this from my rather sketchy class notes.

[Luke122]

If you are looking for a quick formula to calculate resistors, V=I*R. V=voltage of the source, I = required current, R = Resistance.

R = I/V

I=V/R

V=I*R

For example, if you have 12v source, and you need 30mA for an LED, then:

R = 12v/.03 (30mA = 0.03)
R = 400 ohms

Beyond single LED calculations, I need a calculator. Heres my favorite one online. (http://led.linear1.org/led.wiz)

*Edit: Haha.. Helix beat me to it!*

[xRyokenx]

Thanks guys, I'm now having flashbacks from Physics... :mad:

Heh heh... I actually want to take that again now, I should see if my old teacher isn't too busy to give me some basic instruction. It was an interesting class and when I had Chem. with him it was awesome but depression really did effect his teaching skills... huh.

Math based Science does kick ass though, it adds a theme to the math and makes it interesting.

[Fear.Sin]

Alright so I have the simplest equation but I still don't understand the princible of ohms law.

So I tried using the calculator listed to figure out how the equation works and cannot get it (or I am not trying hard enough. At work :P)

I put in the following in the calculator:

Voltage - 5v
Forward voltage - 3.3v
mW - .20
Number of LEDs - 1

Resistor = 100 ohms

I tried dividing 5v by both the forward voltage and mW and neither come to 100ohms.

I am guessing that ohms are the counter current to lower the voltage to what you need for your circuit. Lets say you have a 12v source and each LED needs 4v. Why do you still need the resistor in that scenario?

Yea I am a pain I know, but I want to know how the law applies to the different scenarios.

Thanks for the replies and the great calculator!

[crenn]

LEDs can only handle small amounts of current.

Do you mean mA instead of mW? Because they're different things.

Rule for LEDs (I think)

R=(VSource - (n*VLED))/I



Where:
n=Number of LEDs
VLED=Forward voltage of the LED
I=Required current of the LED
VSource=Voltage of the source

Providing the Source voltage is larger than the LED voltage, you should be fine.

EDIT: btw, no LEDs I've seen have a forward voltage of 4v.

[Fear.Sin]

The source I have is using mA. I am getting the info from an electronic site that sells LEDs specifically.

http://www.lsdiodes.com/shop/index.php?main_page=product_info&cPath=1&products_id=16

So you think maybe that is what was wrong with me trying to recreate the equation the calculator is using?

.................:bunny:

Is the only way to figure out the required voltage source of any particular piece of electronics is to get it from the manufacturers site or look it up online? Is there any way to test parts for the required voltage?

[crenn]

It's best to use a tech sheet.... but if that doesn't work, a 2x1.5V (3 if that doesn't work) button batteries can be used to determine what the forward voltage is. Make sure you have your voltmeter across the resistor.