LEDs for Beginners

Step 4 Resistors

I opened up the assortment pack to find that resistors aren't labeled with what value they are. The pack said it contained a whole bunch of different resistors from 100 ohms to 1 Meg ohm so I set out to see what was what. When I poked around online I found that all resistors have a coding system on them that tells you what value they are.

Here are two pages which explain in depth about how to calculate resistor values.

Do it yourself
or
Have it done for you

I'll go through the examples of how I calculated the values myself in the next few steps when I start wiring up my LEDs.

For the time being I just admired their little colored stripes and moved on to trying to get just one LED to light up.

Comments

[bobbubbles]

iv only got a couple of 4.7 kohms resistors
could i use this??
(sry couldent upload image its yellow violet red gold)

[grizzly g]

first of all what do resistors do? and how do you know like which goes to which bulb and everything?

[CertifyThis]

A resistor is anything that electricity can not travel through easily. When electricity is forced through a resistor, often the energy in the electricity is changed into another form of energy, such as light or heat. The reason a light bulb glows is that electricity is forced through tungsten, which is a resistor. The energy is released as light and heat. A conductor is the opposite of a resistor. Electricity travels easily and efficiently through a conductor, with almost no other energy released as it passes. I got this information off the webiste www.qrg.northwestern.edu

[vile]

Do you lose power or generate heat with these resistors?

[porcupinemamma]

One Easter I bought light -up Bunny ear headbands for all the guests and a couple caught on fire! No one was hurt thankfully, and we still recall "the hot" time we all had . Do you think the head bands would have had LEDs in them?

[marselsipod]

No, as LED's are more conservative and relaese less heat then conventional bulbs, which grow very hot. It was either a short ciruit( some wires touching that arnt suposed to touch, heating up and sparking) or a conventional lightbulb.

[grizzly g]

ok thank you that helps alot

[DJTOONJR]

lol god dammit! i had 5 red LED lights connected to a 9.6v attery.. u know the ones u use for an r/c car.. lol and right wen i connected it they burned out on me.. now i ponly have 3 LED lights.. =(

[sivion44]

Don't use 9V!

[MindsEye69]

One thing that most of these LED tutorials neglect to mention is the WATTAGE of the resistor, which can not be discerned from the color bars, at least not as far as what I can tell...

Let me give an example:

I am building an 8 led strip, parallel connected with a 9 volt power supply.

My reasoning for Parallel connection is that I want to deliver the same power to every LED and avoid a lot of complicated mathematics and resistors everywhere, as well as the benefit of not having one blown LED cripple the entire string.

I am using 8 5mm Ultrawhite LED 30mA 3.5 Volt

A quick trip to this page: metku.net/index.html Scrolling down to the LED's in Parallel calculator, I enter my numbers:

9v power supply
3.5 volt drop across the LEDs
Desired LED current 30mA
8 LEDs connected

Gives me a result of:

1) Calculated Limiting Resistor
 22.92 OHMS
2) Nearest higher rated 10% resistor
 27 Ohm
which is a Red/Purple/Black/Gold (10% tolerance) banded resistor
3) Calculated Resistor Wattage
 1.32 WATTS
4) Safe pick is a resistor with power rating of
 2.2 WATTS

The important information is the 2nd and 4th whereas the 1st is an EXACT value which leades to the 2nd which is the NEAREST manufactured resistor capable of handling my demands.
The 3rd vale again is the EXACT value in my circuit, and the 4th is the SAFEST higher valued resistor that I can buy that will not lead to heat problems or heavily loaded resistor. (Hope that is not confusing)

In front of me on my desk is a strip of resistors. Indeed there is a 27 Ohm resistor in that pack, Red/Purple/Black/Gold SO am I good to go?

NO.

Closer examination of the package the resistors came in show me that they are 0.25 Watt 10% Tolerance.

My calculation above shows clearly that I need 2.2 WATTS for my "safest pick" Which means I need to go back to the store and buy the proper resistor, OR I will need 9 of the ones I have which is doable, but not ideal.

The next question that I see a lot but not many are answering, is "Do I need a resistor on every leg. The answer to that is no, a single resistor of the proper value on the + side before the led chain will ensure that the power to all legs of the chain is already resisted and the same for every LED in the chain.

I am also a noob to this type of thing, but had this spelled out for me very clearly by a dear friend, and have passed the information on to you exactly as I have recieved it in the hopes it will help someone else.

Enjoy and Good Luck!

///MindsEye

[burwuro]

Thankyou very much! you have no idea how much that helped a newbie like me!

[legless]

The reason you need a resistor with a higher power rating is that in your example you are using only one resistor. It must be able to dissipate the full power in your circuit. Your calculations show that a 1W resistor won't be enough but you'd probably get by with a 2W one. If you put a resistor on each LED you will be able to use your much smaller 1/4W resistors and some cases 1/8W resistors.

Physical size might be an issue - the bigger the power rating the bigger the resistor. Some larger resistors can give off quite a lot of heat and may even need to be fitted with a heat sink to help keep it cool. Unlikely in most electronic circuits though.

Also note that the power rating of a resistor is usually quoted at 25°C so higher operating temperatures will lower this rating.

[Entropy512]

Also, for various reasons covered above, multiple parallel LEDs off of a single resistor is not a good idea. "as well as the benefit of not having one blown LED cripple the entire string." - If you run multiple parallel LEDs off of a single resistor, this is what will happen. In fact, unlike the series arrangement where the blown LED will cause the other LEDs to go out (but not get damaged), a blown LED will cause the other LEDs to get driven harder. They'll get brighter, but unless you rectify the situation soon, this will cause another LED to burn out. You wind up with a cascading failure where all LEDs become permanently damaged. The chances of an LED blowing when driven properly are quite slim. (However, depending on the environment you use them in, the leads might corrode. This is a problem with the cheaper Christmas light strings for example.)

[geekman101]

thats appsolutly correct so if you put a 1k resistor on every positive lead that won't happen.

[Virtuous]

Now this is a pretty useful page....i was trying to figure out what color combinations equaled what

[chadd_from_pa]

Yes, but sometimes it is easy to confuse colors, especially in low lighting. The best way to determine resistor value is to measure the resistor with an ohm meter. You can purchase an ohm meter at radio shack as a part of a volt meter for $10.00 or so. If you need information about using resistors in LED circuits, try http://www.lunaraccents.com/design-LED-circuits-design.html

[hydrorunner]

Those pretty little colored stripes are used to indicate what value they are, i beleive. -.-