# question about resistor size verses wattage rating



## Lincoln (Jun 2, 2008)

I ordered some resistors from Newark/Element 14 and they arrived today. They are all packaged as 1 watt resistors (and I had ordered all 1 watt resistors)......but some of them are the size of 1/4 and 1/2 watt resistors while others look 
like they actually are 1 watt resistors.

Do I believe what's printed on the pick tags? I can see a newbie parts picker making a mistake or not knowing the difference. I'm a little nervous to install these "1/4 watt" looking resistors into a circuit that requires 1/2 watt or better.
Google seems to say 1 watt should all be a certain physical size but I'd really like to hear it from the experts here.

(I trust you guys more than I trust the internet)

Thank you
Dave2


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## greco (Jul 15, 2007)

I haven't bought a lot of resistors, so please DO NOT trust my knowledge/experience at all.

However, I remember being surprised about the variations in sizes (given the same wattage value) when I was shopping at the electronics store. 
Could they be made of different materials?...that would be my guess.

I am interested to learn the answer to your question.

Cheers

Dave


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## dtsaudio (Apr 15, 2009)

This is something you will not get a straight answer for from either manufacturers or distributors.
However the wattage rating of a resistor is based on the material, the maximum withstand voltage and the "de-rating" that the manufacturer uses. In one instance a resistor was rated at 1 watt, however that rating was maximum 1000V. A similar valued resistor from a different manufacturer was maximum 300V. The 300V one was 1/4 of the size. I've also seen resistors rated at 25 watt but only if mounted on heatsinks. Without the heatsink the wattage was 5 watts.
The de-rating is usually based on the ambient temperature. Rated at 25 C a 2 watt resistor might be 1 watt at 35 C.
Manufacturing techniques over the years and different materials have also contributed to resistors getting smaller. You will find metal film are typically smaller than carbon, which are smaller than wirewound.

If I don't want to get up in all these specs I check physical size, and usually go up in wattage. I also have resistor types I use all the time and know just how big they are and what they'll take.

It used to be simple. Not anymore.


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## Lincoln (Jun 2, 2008)

dtsaudio said:


> This is something you will not get a straight answer for from either manufacturers or distributors.
> It used to be simple. Not anymore.


Oh great, I've stumbled into another grey area. :smile-new:

What I did was I ordered more resistors the same values (they are cheap) and I'll see when they get here if look the same as the last batch I got.
That should eliminate the picker error factor.

Am I being silly to want a 1 watt reisistor that looks like a 1 watt resistor?


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## greco (Jul 15, 2007)

dtsaudio said:


> It used to be simple. Not anymore.


Having so many choices can make life complicated. It is a double-edged sword.

I prefer simplicity...but that is likely my age showing...LOL

@ Lincoln.... Good luck! Don't let the "grey areas" bug you too much.

Cheers

Dave


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## jb welder (Sep 14, 2010)

Lincoln said:


> Am I being silly to want a 1 watt reisistor that looks like a 1 watt resistor?


The key to that question is a "when" :smile-new:. I'm astounded by the small size per watt of modern resistors. What I think a 1/2 or 1 or 2W resistor looks like has no bearing on modern components. The material type (ex. metal oxide) has a big impact.
As you are dealing with a major supplier, there should be exact mfgr part numbers for the brand of resistors you ordered (ex. Vishay). Go to the mfgr web site and look up the specs for the exact part #. They should show the actual physical size.


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## Wild Bill (May 3, 2006)

jb welder said:


> The key to that question is a "when" :smile-new:. I'm astounded by the small size per watt of modern resistors. What I think a 1/2 or 1 or 2W resistor looks like has no bearing on modern components. The material type (ex. metal oxide) has a big impact.
> As you are dealing with a major supplier, there should be exact mfgr part numbers for the brand of resistors you ordered (ex. Vishay). Go to the mfgr web site and look up the specs for the exact part #. They should show the actual physical size.


Yep, what JB said!

I sold these things long enough to know what is going on. Back when carbon comps were all you could buy the sizes of resistors and their wattage ratings were obvious. That hasn't been true for over 40 years!

In the early 60's we saw the introduction of carbon film resistors. They were only slightly smaller than carbon comps for the same wattage, if at all. Then in the late 70's we started selling metal film resistors from Phillips. They were less than half the size of the carbon films for the same wattage rating.

JB's suggestion is exactly right. Look up the mfgr's part number and specs on their website. You can believe it. Otherwise they would not be in business long!

Wild Bill/Busen Amps


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## Lincoln (Jun 2, 2008)

dtsaudio said:


> If I don't want to get up in all these specs I check physical size





jb welder said:


> The key to that question is a "when" :smile-new:. I'm astounded by the small size per watt of modern resistors. What I think a 1/2 or 1 or 2W resistor looks like has no bearing on modern components. The material type (ex. metal oxide) has a big impact.
> As you are dealing with a major supplier, there should be exact mfgr part numbers for the brand of resistors you ordered (ex. Vishay). Go to the mfgr web site and look up the specs for the exact part #. They should show the actual physical size.





Wild Bill said:


> JB's suggestion is exactly right. Look up the mfgr's part number and specs on their website. You can believe it. Otherwise they would not be in business long!
> 
> Wild Bill/Busen Amps


Thanks guys. I never thought to look up the dimensions in the specs :sSig_DOH:
That will solve all my issues (resistor issues that is)


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## dtsaudio (Apr 15, 2009)

What Bill said.
As an example BC, Yageo, and Panasonic all make resistors in 1,2, and 3 watt. The 1 watt are the size of old carbon comp 1/4 watt. The two watt are the size of 1/2 watt CC. 
If you go with Vishay Dale resistors, you're on your own. They make 1/2 watt that are the size of 1/8 and several lines that are all different.

You ordered from Newark I see. They have the data sheets available right on site, so no need to go to the manufacturers website. Mouser and Digikey do the same thing.


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## Lincoln (Jun 2, 2008)

I ran a few of the numbers and sure enough, they resistors I bought are listed as being 6mm long and 2mm diameter. That's pretty dam small. (1/4" long x 3/32 diam.)

I guess what threw me off so bad was I've been buying resistors from Antique electronics and some off ebay and they've always been what I would consider normal size. 

I learned something again today.


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