Resistor power sizing

Thread Starter

MB107

Joined Jul 24, 2016
290
When I started working with Arduino, I purchased an assortment of 1/2W resistors. Now that I have a design nearly complete and starting to see there real power requirements, I can see that most cases my power is well under 1/8W. So I was wondering if I should be concerned about using resistors of a higher power value than required. Should I just use my 1/2W for everything or is there an advantage to getting a stock of 1/4 W and 1/8W and install them appropriately. Is there any margin of safety that should be maintained or some percentage range I should stay between.

John
 

SamR

Joined Mar 19, 2019
4,129
No, and I prefer 1/2W since the wire is a bit larger and stays in the breadboard better. Doesn't matter as long as the resistor is higher than the generated watts.
 

dl324

Joined Mar 30, 2015
14,322
Should I just use my 1/2W for everything or is there an advantage to getting a stock of 1/4 W and 1/8W and install them appropriately.
Aside from space requirements, cost, and looking silly when you use a 1/2W resistor when a lower wattage would suffice, you can use whatever you want.
Is there any margin of safety that should be maintained or some percentage range I should stay between.
Resistors can only dissipate their rated values at up to 70C.

From Yaego for metal film (I've seen the same data for carbon film):
1642531484646.png

Some people derate to 50% for temperatures below 70C, but you can see from the chart that that isn't necessary.
 

DC_Kid

Joined Feb 25, 2008
924
Do any of the resistor characteristics impact circuit operation, and will those characteristics change with temp change?

The 1/4w might get hotter on surface than the 1/2w, simply because of surface area diff. Remember, heat dissipation might be constant (like ditching 1/8w worth of joules), but surface area dictates temp for that same 1/8w worth of wasted energy.
 

SamR

Joined Mar 19, 2019
4,129
If you were building 30,000 units that only required a 1/8W resistor, that is what you would buy to reduce unit cost. But as a hobbyist you buy or use from your personal stock what works for you and does the job.
 

MrChips

Joined Oct 2, 2009
25,930
The decision is not so straight forward. When I got started in electronics many moons ago the standard stock of resistors consisted of E12 10% ½W. Now things are much more complex.

Here are some considerations:

#1 - Consider the cost, time and effort of acquiring, sorting, and storing multiple sets of resistors.
#2 - What are your requirements, through hole or SMD?
#3 - What tolerance meets your requirements, ±10%, ±5%, ±1%?
#4 - Which series should you stock, E6, E12, E24, E48, E96, E192?
#5 - And finally, what wattage?
#6 - And so, what are your typical applications?

Let's narrow it down:

#1 - Stick to one set, the recommendation as follows:
#2 - Thru-hole resistors
#3 - 5%
#4 - E12
#5 - For most applications, analog or digital, stock ¼W resistors. There is no great advantage in stocking 0.125W. If you want to build compact circuits you would go to SMD.

There will be occasions when you need to go to higher wattage such as ½W and 1W. You need to decide on these by doing the wattage calculation. Always go up to the next higher wattage. For example, if your calculated power dissipation is 0.2W go to ½W resistor. You want to go higher, not lower. There is no detriment to going higher except for the extra space required.

Instead of calculating the power dissipated, you can use this chart for a quick estimate of the resistor wattage you should use.

1642534016981.png

The chart might look complicated. Here are the simple instructions.
For example, pick a 500Ω resistor. Follow the black straight line with the 500 label.
Select the current or voltage. You only need one since Ohm's Law will dictate the other.
For example, suppose the voltage across the resistor is 12V.
Pick 12V on the voltage scale on the left and move straight across to the right until you meet the 500Ω black line.
You will be just over the ¼W green curved line. Hence you should select a ½W resistor.
 

Reloadron

Joined Jan 15, 2015
6,537
Consider the above and keep in mind. If I calculate my power dissipation will be 1/8 watt I am not going to use a 1/8 watt resistor. I will go with a larger power rating like 1/4 or even 1/2 watt.

Ron
 

LowQCab

Joined Nov 6, 2012
1,931
Consider the above and keep in mind. If I calculate my power dissipation will be 1/8 watt I am not going to use a 1/8 watt resistor. I will go with a larger power rating like 1/4 or even 1/2 watt.

Ron
.
Using over-rated Resistors helps to keep the over-all Heat down on the Board.
.
.
.
 

Reloadron

Joined Jan 15, 2015
6,537
Huh? A 1/8 W and a 1/2W resistor dissipate the same heat when used in the same circuit.

Bob
That would be true but the 1/2 watt verse 1/8 watt would be distributing the same heat over more area. The 1/2 watt has a larger surface area. Not that I ever gave it much thought but sounds logical. :)

Ron
 

MrChips

Joined Oct 2, 2009
25,930
The same amount of power is generated regardless of the wattage rating of the resistor. It is a matter of where it is generated and how it is dissipated.

There are only three ways to dissipate heat, that is, via conduction, convection, and radiation.

If the physical construction of the resistor helps to dissipate the heat more efficiently then yes, the board will run at a lower temperature.
 

Reloadron

Joined Jan 15, 2015
6,537
The same amount of power is generated regardless of the wattage rating of the resistor. It is a matter of where it is generated and how it is dissipated.

There are only three ways to dissipate heat, that is, via conduction, convection, and radiation.

If the physical construction of the resistor helps to dissipate the heat more efficiently then yes, the board will run at a lower temperature.
Has my vote. :)

Ron
 

DC_Kid

Joined Feb 25, 2008
924
Huh? A 1/8 W and a 1/2W resistor dissipate the same heat when used in the same circuit.

Edit: And, therefore, produce the same amount of heat.

Bob
True. I think that post meant to say "keep overall temp down". Same heat (joules) is going out, but since the area is bigger the temp would be lower. Component temp may be lower, but inside a box the overall joules dissipation will make the inside temp rise just the same.

Temp is a critical factor when looking at many characteristics of the component.

The other major process is temp swing. Although same joules may be going out, temp swings (cycles) is a factor for MTBF.

I have sometimes used 1/8w resistors, only because space was a concern (small board, lots of parts, etc), and did not want to go SMD.
 

LowQCab

Joined Nov 6, 2012
1,931
When the Exterior-Surface has ~4X the Surface-Area,
then 4X more heat can go directly to Convection,
instead of Heating-Up the Board.

In fact, the larger the number of Components,
and the bigger those Components are,
the more Heat can be dissipated by the whole assembly,
because there is substantially more Surface-Area than a flat PC-Board.
.
.
.
 

Thread Starter

MB107

Joined Jul 24, 2016
290
The decision is not so straight forward. When I got started in electronics many moons ago the standard stock of resistors consisted of E12 10% ½W. Now things are much more complex.

Here are some considerations:

#1 - Consider the cost, time and effort of acquiring, sorting, and storing multiple sets of resistors.
#2 - What are your requirements, through hole or SMD?
#3 - What tolerance meets your requirements, ±10%, ±5%, ±1%?
#4 - Which series should you stock, E6, E12, E24, E48, E96, E192?
#5 - And finally, what wattage?
#6 - And so, what are your typical applications?

Let's narrow it down:

#1 - Stick to one set, the recommendation as follows:
#2 - Thru-hole resistors
#3 - 5%
#4 - E12
#5 - For most applications, analog or digital, stock ¼W resistors. There is no great advantage in stocking 0.125W. If you want to build compact circuits you would go to SMD.

There will be occasions when you need to go to higher wattage such as ½W and 1W. You need to decide on these by doing the wattage calculation. Always go up to the next higher wattage. For example, if your calculated power dissipation is 0.2W go to ½W resistor. You want to go higher, not lower. There is no detriment to going higher except for the extra space required.

Instead of calculating the power dissipated, you can use this chart for a quick estimate of the resistor wattage you should use.

View attachment 258080

The chart might look complicated. Here are the simple instructions.
For example, pick a 500Ω resistor. Follow the black straight line with the 500 label.
Select the current or voltage. You only need one since Ohm's Law will dictate the other.
For example, suppose the voltage across the resistor is 12V.
Pick 12V on the voltage scale on the left and move straight across to the right until you meet the 500Ω black line.
You will be just over the ¼W green curved line. Hence you should select a ½W resistor.
Nice data. Really getting down to the nitty gritty and its a great source of information. My main concern for posting this was I wasn't sure if for example, a 10Ω 1W resistor was still 10Ω if it is only used with 0.05W. But stocking one size makes life easier and then once you have it all figured out on a bread board get something smaller if you need to conserve space. So everything I do now is through hole and I do find the 1/2 W resisters to fit better into breadboards. as far as precision, I cant even find 10% anymore and the 1% and 5% are the same price in the 1/2 W range. So I get the 1%. I have no idea about what series resistors I have but these are them:

1642557553930.png
 

MrChips

Joined Oct 2, 2009
25,930
Here is my take on this.
I grew up learning the E12 ±5% resistor code by heart. It only has three color bands plus GOLD.

1642559304380.png

So when I look at a resistor I know its value instantly.
It takes me longer to decipher a ±1% resistor code.

For breadboarding, my choice remains a set of E12 ±5% ¼W.
It's your decision to make.
 

MrChips

Joined Oct 2, 2009
25,930
When you are finished breadboarding a project, do you throw away the used resistors considering how inexpensive they are?
Not me. I put all the components back in their designated drawers to be used again.
Consider what it must be like to sort through a few hundred resistors. I have done this many times.
E12 ±5% are a lot easier to sort.
 
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