# Resistors for Dummies



## rhysweatherley (Feb 24, 2005)

Every now and then someone says "I want to use LED's but don't know how to calculate the resistance", inevitably followed by "You can get ones with built-in resistors at such and such a store" or "I found this handy LED calculator over here".

But calculating the resistance isn't that hard. And resistor-less LED's are going to be easier to find at your corner electronics store.

First, find out the properties of your LED's. The two values that you are interested in are the voltage and the current. For example, some of my white 5mm LED's are rated at 3.2V (volts) and 30mA (milliamps). You can get these values from the electronics store's catalogue.

Let's say we wanted to run this LED off a 9 volt battery. The LED will consume 3.2V volts, and we need a resistor to soak up the remaining 9 - 3.2 = 5.8 volts. The resistor needs to do it at 30mA (or 0.03 Amps).

According to Ohm's Law, V = IR. Here, V is 5.8 and I is 0.03. Therefore, R = 5.8 / 0.03 = 193.333 ohms. That's how big the resistor needs to be.

193.333 is an odd value, and you won't find such a resistor at the store. The next standard size up is 220 ohms. That one will work fine. The LED will merely be a little dimmer (hardly noticeable on a bright white LED).

Now, what wattage resistor do you need? Another simple equation: Power = IIR. 0.03 * 0.03 * 220 = 0.198. That's less than 0.25, so a standard 1/4 watt resistor will work. In fact, I've never found a LED circuit yet that needed more than a 1/4 watt, so I normally just ignore the power calculation.

You can overdrive a LED beyond its ratings to make it brighter if you are strobing the LED very quickly. I normally just buy a LED with a higher mcd rating (the mcd indicates how bright it is). Why risk destroying a LED by overdriving it if a higher brightness costs about the same?

Adjusting the resistance is also a good way to get different effects. Increase the resistance a bit more and it will dim the LED for a cooler glow. There's no law that says your bridge spotlights need to blind the cat at 40 paces, so drop it back a bit. It is harder to do this with LED's that have built-in resistors.

Normally you wouldn't run just one LED off a 9 volt battery. You'd run several in series, with a drop-down resistor at the end to soak up the left-over voltage. There are good electrical reasons for using series rather than parallel, but they aren't important here. Think of it this way: the more LED's you can put in series, the less resistors you need to buy.

Common red LED's are normally rated at 1.7V and 15mA. You can string up to five of these in series: 1.7 * 5 = 8.5 volts. The resistor needs to soak up the remaining 0.5 volts at 15mA. You do the math. As long as the total voltage on a string of LED's is less than the supply voltage, you're fine.

That's all there is to it. Anyone with a pocket calculator can do it, with a little bit of practice.


----------



## ModlrMike (Apr 20, 2004)

For someone who has never lit up a kit, but wanted to... this is great. Thanks Rhys


----------



## Chuck_P.R. (Jun 8, 2003)

I thought resistance was futile.


----------



## rhysweatherley (Feb 24, 2005)

Old Borg Joke: "Resistance is futile, except when powering the lights on the outside of the cube".


----------



## Zombie_61 (Apr 21, 2004)

ModlrMike said:


> For someone who has never lit up a kit, but wanted to... this is great. Thanks Rhys


Yeah. Now all I have to do is figure out what the hell all of that means! :freak:


----------



## CaptDistraction (Feb 1, 2005)

bwahahhaha, at the jokes.

This is probably the best post I've seen to date. Thanks for putting the calculations into plain English. Even though my degree is in Computer Science, I never had the oppurtunity to take Circuits 1 or 2.


----------



## DL Matthys (May 8, 2004)

Now if one is mathaimaticly lazy like me I cheat the scratch pad and got to:

http://ourworld.compuserve.com/homepages/Bill_Bowden/homepage.htm

There are many little java scipt calulators for sorting out electronics values like series resisters for use with LEDs. I go there a lot and still learn alot about hobbists circuits.


----------



## John P (Sep 1, 1999)

Jesus tap-dancing Christ, it still blows my mind. I'll just wait for someone to put out lighting kits.


----------



## Zombie_61 (Apr 21, 2004)

Tap-dancing? In _those_ sandals?


----------



## Rogue1 (Jan 3, 2000)

John P said:


> Jesus tap-dancing Christ


----------



## B.Wildered (Apr 25, 2004)

Rhysweatherly: as a former high school physics teacher, that is the best explanation I have yet read. You put things together enough for me to try LEDs myself. I especially enjoyed the comment about blinding the cat at 30 paces.
More colloquially: dude, you rock!!!


----------



## rhysweatherley (Feb 24, 2005)

Thanks for the positive feedback guys. I'm glad that the information has been useful.


----------



## Jobi Wan Kenobi (Dec 9, 2003)

Hey, that is a great explanation. Now, to help tie it all together. I wonder if anyone has a pic they could post showing how a series of leds as described above would be physically connected? I've seen pics of other folks wiring before, but they always seem to be far away or blurry. I've also seen people post wiring diagrams, and they only make my head hurt!! 


Thanks

Jobi Wan Kenobi


----------



## rhysweatherley (Feb 24, 2005)

Hi Jobi. It really depends upon the model and the person what the wiring looks like. On big models, it is almost always a rats nest, no matter how careful one is. The best thing to do is take it in simple steps. I always prototype my circuits on a breadboard first. Here's an example of two white LED's in series:










I've marked the anodes (A) and cathodes (K). The anode is the longer wire on a LED when you buy it. It should always be oriented towards the positive side of the circuit. A 100 ohm resistor was used for this test. 

You can ignore the 2K trimpot on the right. I use that for testing LED's sometimes - I hook up a LED, adjust the resistance on the trimpot for the desired brightness, and then measure the trimpot with a multimeter to determine the final resistor to use in the real circuit.

Here's another one, with two sets of two LED's, wired in parallel. This is how you create circuits with lots of static LED's. You wire as many as you can on each parallel string (up to the supply voltage), and then put the strings in parallel:











Hope that helps!


----------



## jwrjr (Oct 31, 2003)

I could show you pictures of the interior of my refit, but I'd scare some of you. If only it were as well organized as a rat's nest.


----------



## F91 (Mar 3, 2002)

The inside of my NX looks like Gen. Greivous' chest right before Obi wan 







SPOILERS!!!!





Shoots him at point blank a couple of times with a blaster.


----------



## Jabbs (Oct 14, 2003)

It helped me but what I'm about to post isn't quite what you had in mind but it made things easier for me.
This and Jwrjr to be more precise. Muchas gracias maestro!


----------

