Ohms Law Help Needed

[bobisculous]

Ok, I have done few mods without the help of a good buddy of mine. He went off to college a few days ago so now I am off to fend for myself in mods. I just cant figure out the ohms law thing completely. I am currently in Physics now, but we don’t start electricity till the end of the year. So I hopefully I can get this stuff down pat with the help of ya’ll. Remember those goofy little foam discs shooters that are battery operated. You load the discs into the top, turn it on, fan spins, pull trigger and out shoots a disc, well, I have one, and got the bright idea to modify it with at least 4 white LEDs and if possible, 4 reds for night vision. I would then Somehow rig up the on/off switch to differ between red and white, and even though the trigger has nothing to do with the electronics, I would find a way to make it the Momentary switch for it. So, here’s where I need your help. I try calculations and they make no sense to me(as far as resistors). The gun uses two AAs, so 3 volts. I am up for some decent overdriving cause I will get plenty of LEDs from somewhere around here(probably besthongkong.com) so if and when one goes out, I will somehow make it easily changeable. The LEDs will be protected decently by the gun so I may only glue down the resistors and not LEDs. If someone doesn’t mind, do a small explanation on how I figure out resistance and what not. Here are the LEDs I am thinking on getting, for the white... BestHongKong and for the red LEDs... Red LEDs . This isnt meant to be a great mod so 'good' LEDs just do not matter. Give me any help you can and any other input you may have, if you dont mind. (This is just meant to be a goofy mod cause I am currently bored of school already /ubbthreads/images/graemlins/icon3.gif. Thanks
Cameron

 

[gadget_lover]

You can look in the Batteries Included forum for the CPF university thread, or follow these simmple formula.

P= power in watts
I = current in amps
R = resistance in ohms
E = voltage in volts.
Resistors in series add up their resistance.
Resistors in parallel divide the resistance.


Given: LEDs want a paricular current, not a particular voltage. They are generally rated with something like 30ma at 2.2 volts.

Given: a circuit with a 6 volt battery, a resistor and an LED will result in the resistor consuming part of the power (expressed as a voltage drop) and the LED will consume the rest. How much each will consume depends on the resistance of both parts.

You always start somewhere. Assume you want 30ma (.030 amps) of current per LED and 4 LEDS. If the 4 LEDS are in parallel the total current will be 120 ma (4 * .03) at 2.2 volts. If they are wired serial, the current will be 30 ma but will need 8.8 volts.

So, Ohms law: E=I*R

This also means that I=E/R and that R=E/I. You can substitute E, I and R in interesting ways; I=(I*R)/R but they are less helpful.

You can use the power formula P=I*E to determine the proper SIZE of resistors. Just as a 100 watt light is hotter than a 5 watt night light, a resistor consuming 1 watt will be a lot hotter than one consuming 1/8th of a watt.

So let us apply this.
We have a 3 volt power supply (a battery) and want to run 4 LEDs at 30 ma each. The documentation for the LED says 30 ma at 2.2 volts. so we need to 1) wire them in parallel to have enough voltage and 2) we need to get rid of .8 volts.

The formula to solve for is what resistor will drop .8 volts at 30 ma. We know the E and I, so the formula would be R=E/I, or R= .8/.03 = 26 ohms

So you have a 3 volt battery connected to a resistor. That resistor is connected to one leg of each LED. The other leg of each led goes back to the battery. The resistor is dropping .8 volts at 30 ma, so you need a resistor able to consume .024 watts.


Does that help?

Daniel

Disclaimer; I'm tired and may have made a mistake in any of the calculations or formulas. The 30ma @ 2.2 volts is made up and does not apply to any LED.

 

[bobisculous]

Ok, I am kinda getting it. That was alot of information that I just read and need to digest it. I will tell you what I do figure out though. /ubbthreads/images/graemlins/smile.gif But in the mean time, thanks you /ubbthreads/images/graemlins/clap.gif
Cameron

 

[greenLED]

Check out jtice's LED Pro. I found it really useful. G'luck!

 

[bobisculous]

greenLED. I actually have that program on my PC. Its just that I dont know how to use it. I dont know enough about circuits and all that to understand what all of it means. Thats what I am now studying up on.
Cameron

 

[andrewwynn]

the last LED project i used, i used a pair of AAAA batteries in series and ran 4 LEDs in parallel with no resisstance at all... the internal resistance of the batteries apparently was enough to keep the current level, though likely a little over-drive plently low enough that they just ran a little bright... If you have enough to spare risking letting out some magic smoke, i'd actually try to wire up 4 or 5 in parallel and see if they can deal with the overdrive... i'd measure what current goes through the array..

Caveat... LEDs w/o that series resistor don't often draw the same current you may notice a difference in brightness.

-awr