# When looking for resistors, what is the W for?

I'm looking for a variety of resistors for my project. Since I'm so new to this, I don't have the slightest idea what I'm looking at.

Essentially, as I research ideas, I get recommendations that include "You'll need X resistors"

Since I'm looking for a variety pack so that I can have them at my disposal, I want to ensure I'm getting the right ones. I've found a pack of 1000 resistors that range from 1Ω to 10MΩ but all of them are 1/4W carbon film resistors (5% tolerance)

I know what resistance (Ω) I'm looking at, but not so much the rest of it. My "assumption" is that the "W" is "Wattage" but I'm not certain. Is there a general (rule of thumb) "W" rating that I should be looking for, and what exactly is that "W" representing?

Yes, the "W" is for Watts. The Wattage rating represents the total power (in Watts) that the resistor can safely handle without overheating and destroying itself.

You can determine the needed "W" rating by calculating the total current flow through the circuit, at a given voltage. Wattage is given by voltage (in Volts) times current (in Amperes). Use Ohm's law and your knowledge of the supplied voltage and any other resistance in the circuit to calculate the current, and then converting that into Watts is straightforward. Just watch unit conversions... eg, if you're working in milliamps or millivolts, do the necessary conversion(s) as part of your calculations.

This site has more information on the topic, that you might find useful:

http://www.csgnetwork.com/ohmslaw2.html

For a number of introductory electronic projects (the kind of things you'll find in online tutorials and intro books), the 1/4W resistors are often sufficient. Also note that you can increase the power handling capacity in a circuit by using a matrix of identical resistors. Or you can just buy resistors with a higher power rating.

• This is the pack I'm looking at. – Chase Florell Nov 28 '11 at 20:56
• That seems like a perfectly fine assortment to have handy, if you're just getting into circuit building. That said, Kellenjb did make a good point in his answer: You don't want to "push" components right to their limits all the time... so buying components with some "margin" in terms of their properties can be a good idea. The downsides to just throwing over-rated components around, however, include increased cost, and possibly physical space issues (that is, the higher power parts are usually physically larger). – mindcrime Nov 28 '11 at 21:08
• Yes, going up to 1/2W has few downsides, especially if you're just bread-boarding circuits for learning purposes. Once you start talking about mass producing an item for retail sale or something, then you have to start thinking in terms of "this part costs 2 cents more, but I need 8 million of them," etc. :-) – mindcrime Nov 28 '11 at 21:11
• @chase To follow up on mindcrime's response: the disadvantages of 1/2 W over 1/4W are ususally that 1/2W may be somewhat larger, so space constrained designs may have problems with them, and they cost slightly more. For prototyping though, both those factors tend to be negligable, so you may as well buy 1/2W resistors for the headroom. – Kevin Cathcart Nov 28 '11 at 21:15
• Don't expect 1/2W to always be more expensive then 1/4W as the economics of electronics are much more complex then that. It is the popularity of a resistor at a specific tolerance at a specific wattage that has the largest effect on price. – Kellenjb Nov 28 '11 at 21:53

W does stand for Wattage. This is the max power that the resistor is able to dissipate safely. To determine what wattage you need just take your expected voltage drop squared divided by the resistance (or any of the other power calculations).

1/4W and 1/2W are pretty common for your general purpose use and will work for most any simple breadboarding you will do, but before you hook anything up, make sure you wont kill anything.

The simplest way to be extra safe when trying to decide what resistors to use is to look at the highest voltage you are using (lets say 5v) and then look at the lowest resistance you will be using (lets say 100 ohm) and then calculate the power (in this case 1/4W). So since you will be dissipating 1/4W, you could say that you just need a 1/4W resistor, except that as with most electronics, you don't ever want to run them at their rated maximum for extended periods of time, so you would be best to go with a 1/2W or higher resistor.

In general you are better off purchasing resistors that can handle higher power dissipation then you expect to ever have.

As for the rest of the resistors name, Carbon Film is the type of material used in the resistor that actually does the resisting. There are various types that have different characteristics, but carbon film is generally just fine. The only reason you would want to consider something different is if you had some specific case, like accurate resistance across a larger temperature range, or a specific size.

The 5% tolerant is referring to the tolerance of the resistance is self. There is no way to produce resistors that are always exactly 100.00000000000000 ohms. Because of this manufactures apply a tolerance to the value. This means that when you pick up the resistor you bought and test its resistance, you should expect it to be +/- that percentage of what you actually bought. Again 5% is probably just fine for general use. 1% and better can be used if you have any specific cases that need to be accurate.

"W" represents the wattage.
Wattage is defined as the maximum power that can withstand without destroying it.
(Or) The wattage rating of a resistor tells you how many watts of power it is able to dissipate without damaging itself.

The wattage is determined by muliplying the voltage across the resistor by the current through it.
A good rule of thumb is to use a resistor with twice the wattage rating that will be dissipated in it.
Here is the link that may be usefull in understanding the wattage of resistor.