# The importance of wattage in resistors

#### droggie

Joined Oct 21, 2012
137
Recently I noticed that there are smalls resistors and larger resistors of the same value. So a couple of questions

What are the watt values from smallest to largest? 1/4...1 Watt

What is the significance of the resistor wattage and how important is it?

How can you determine the appropriate resistor wattage for the circuit?

And finally and how is it indicated on a schematic? I only see the ohm value labelled on the resistor in schematics.

My circuits are generally small; typically experimental LED circuits.

Thanks!

• Geek_404

#### ErnieM

Joined Apr 24, 2011
8,196
Wattage may be more importaint then resistance. Resistors can range in wattage from nearly zero to... tens of thousands of watts. Without looking hard I found resistors on Digikey for sale range from .0125 to 80 watts.

Wattage means how much power the physical resistor is rated to handle. The rating is always for given conditions, such as the ambient temperature, or when mounted on a heat sink.

You determine the power in a resistor by computing the voltage across times the current thru it. Usually a safety factor is also used, so the rating is at least twice the expected power. Such derating helps make reliable circuits.

Power usually doesn't make it onto the schematic. If it does it is only shown for a few exceptional resistors. Usually it is only noted in the bill of materials. If you wish to note it just put the rating followed by W for watts.

• PackratKing and droggie

#### MKCheruvu

Joined Nov 20, 2012
30
A resistor being a dissipative element produces heat which inturn raises its Body Temperature .
The physically mid point of the Resistor normally gets elevated to maximum temperture and can be a Hot spot.
Resistor temperature depends upon heat transfer thru - conduction thru leads, by radiation into surroundings . This can be further enhanced by adding a heat sink of large surface area prefferably with Black colour.

• PackratKing

#### #12

Joined Nov 30, 2010
18,223
Not much left to say here. The formula is P=IE and it's called Watt's Law. You spend most of your time figuring out how many ohms are needed, but before you build it, you must always run the wattage math for every part to make sure it doesn't start smoking.

• PackratKing

#### MrChips

Joined Oct 2, 2009
24,610
What everyone says is correct. But no one wants to stock up on a whole range of resistance values times a whole range of wattages.

So we tend to stick with a wattage that will suite 99% of our needs and that happens to be 1/4 watt resistors. For their physical size, these will suite most circuits without the need to calculate the wattage for every resistor and presuming we are not into SMD resistors.

Experience will tell you when you need to step back and do the power calculation.

And when do you have to do that?

When:

1) The voltage across the resistor is very high, say greater than 24V @ 10mA
2) The anticipated current is very high, exceeding 50mA @ 5V
3) The resistance is low, less than 100Ω @ 5V

Note than in all of these cases we have approached the 1/4W limit and allowed no room of safety. We would want to switch to 1/2W resistors even before we get to this point.

If it would make it easier for you, I can prepare a chart that will show when it is ok to use 1/4W resistors and when you should move up to 1/2W and 1W.

Last edited:
• #### WBahn

Joined Mar 31, 2012
26,398
The only thing I would note is that the power dissipated in the resistor is a function of time, meaning that, in general, it is constantly changing instead of being a single number. When dealing with the heat problem, it takes time to cause a change in temperature. So a high power (much higher than the wattage rating) that only lasts for a very short time but that spends most of its time well below the wattage rating might be just fine and putting a resistor that is rated for the "peak power" would be overkill, expensive, introduce additional parasitic effects, and eat up space -- but doing so would ensure that you don't overheat the resistor. On the other hand, sizing the resistor to just handle the average power of a long period of time might let the resistor heat up too much from time to time and cause problems, including of the magic-smoke-and-flame kind. What you, ideally, need to do is look at how much energy is stored in the part in a manner similar to looking at charge stored in a capacitor by doing a thermal simulation. If you don't have a simulator that can perform such a simulation, you can actually set up a circuit (normally in a simulation, but you can do it physically as well) where the voltage at a certain node is an estimate of the temperature of the component. But usually you would only go to this kind of trouble when it really mattered (and experience is how you learn to recognize with it really matters, usually gained piecemeal as a consequence of all the times you failed to recognize that it really mattered). So in the end you develop a feel for when it is safe to go with a standard rating, when you need to bump up the safety factor, and when you need to really look at the details.

#### droggie

Joined Oct 21, 2012
137

I've noticed that the most commonly used resistors are the 1/4 watt size. If I were to use a 1 Watt resistor in replace of a 1/4Watt would it affect the overall performance of the circuit or anything else?

#### mcgyvr

Joined Oct 15, 2009
5,394

I've noticed that the most commonly used resistors are the 1/4 watt size. If I were to use a 1 Watt resistor in replace of a 1/4Watt would it affect the overall performance of the circuit or anything else?
In general no. Going larger will only hurt your available physical space.
Also most people take the calculated wattage and multiply it by 2 or even 3 as a "safety factor" So if you calculate a 1/8W resistor get/use a 1/4W. (this might have been covered already but I didn't read all the other posts)

#### MrChips

Joined Oct 2, 2009
24,610
There is another consideration to think about.
The leads on the resistor tend to be fatter as the wattage goes up.
You may not be able to insert the higher wattage resistor into the holes of your PCB or breadboard.

#### inkyvoyd

Joined Dec 6, 2011
25
Question, it is practical to connect two resistors in parallel (or three) that are twice (or three times) the resistance of the intended original? I mean, if one needed a 1/2 watt 1k ohm resistor, could they simply connect 2 2k ohm 1/4w resistors in parallel? Or is this a bad idea?

#### MrChips

Joined Oct 2, 2009
24,610
Nothing wrong with that. In fact, it is even better because two resistors dissipate the heat better than one resistor.

#### JMac3108

Joined Aug 16, 2010
348
Yes, its fine to parallel resistors to get the desired power rating. Just remember to take into account the tolerance of the resistors and the fact that they won't share current perfectly due to this. However, as a practical matter you should be oversizing your resistors anyway so that this isn't really a problem.

The typical rule of thumb is to use at least twice the power rating that you need.

Oh, and you can put resistors in series to get a higher power rating too.

Here's a trick I've used in the lab when I'm in a hurry and need a power resistor in a value I don't have ... Lets say I need 100ohms in 1W and all I have is 100ohms in 1/4W. If I get four of the 1/4W 100 ohm resistors and put 2 in series, then put that in parallel with two more, the result is 100 ohms at 1W! Works great, fast, and no calculator required #### MrChips

Joined Oct 2, 2009
24,610
There are three parameters, current I through the resistor, voltage V across the resistor and R, resistance.

These are related by Ohm's Law, V = I x R.
You have to know any two of the three parameters.

Select any two parameters on the chart to locate the approximate wattage.

Note that the wattage lines show the power dissipation. Always derate your resistor by going to a higher wattage rating. For example, if your power dissipation falls in between the 1/8W and 1/4W band, select a 1/2W resistor.

Example 1:

100Ω resistor needs to pass 35mA. This puts us around the 1/8W line. Choose a 1/4W resistor.

The 100Ω resistor needs to pass 40 to 50mA. Choose a 1/2W resistor.

Example 2:

1kΩ 1/4W resistor is ok if the voltage across the resistor is less than 12V.

If your power dissipation falls under the 1/8W curve, then 1/4W resistors would be fine. Last edited:

#### #12

Joined Nov 30, 2010
18,223
One way to make it easier is to calculate a limit based on your Vcc. If I have a 5 volt circuit and 1/4 watt resistors, I calculate for 1/8th watt: R= E^2/P and see that anything 200 ohms and above will not have trouble in this circuit. Then I only check resistors under 200 ohms for whether they are in a situation that will save them from smoking. For instance, I just did a design with a 160 ohm resistor on a 5 volt supply, but it only did pulses that were 12.5 milliseconds long and could not do more than 4 pulses in a row, then wait for most of a minute. It will survive.