# EEVblog Electronics Community Forum

## Electronics => Beginners => Topic started by: bitman on March 09, 2018, 11:08:22 am

Title: Have resistor - want to know wattage?
Post by: bitman on March 09, 2018, 11:08:22 am
I'm not sure if what I have is a 1/4W, 1/2W or 3/4W resistor. How can I tell? Thru-hole resistors are tough - not only are the colors impossible to read for those of us who are color-challenged, but I cannot find anything that indicates their wattage?
Title: Re: Have resistor - want to know wattage?
Post by: IanB on March 09, 2018, 11:21:42 am
Buy a sample of 1/4 W, 1/2 W and 3/4 W resistors and compare for size.

Usually you can eyeball this from experience: 1/8 W are tiny, 1/4 W are small, 1/2 W are notably larger, 3/4 W are quite chunky.

Or do a test: feed 1/4 W into a test resistor and see how warm it gets. If it doesn't get too hot to touch it is probably OK.
Title: Re: Have resistor - want to know wattage?
Post by: amspire on March 09, 2018, 11:28:22 am
I'm not sure if what I have is a 1/4W, 1/2W or 3/4W resistor. How can I tell? Thru-hole resistors are tough - not only are the colors impossible to read for those of us who are color-challenged, but I cannot find anything that indicates their wattage?
You can guess from the size, but is only a guess. I have seen very small 1/2W resistors and big 1/4W resistors.

Most of the time, you can work it out from the circuit power supply voltage and the resistance value. If the resistor is 1K and the supply is 12V, then the maximum power that can go through that resistor is 122/1K = 0.144W. If it turns out the resistor only has 1V across it, the maximum power is 1mW. The majority of resistors on a typical circuit board are usually running very much below their power rating.
Title: Re: Have resistor - want to know wattage?
Post by: Brumby on March 09, 2018, 02:03:09 pm
Most of the time, you can work it out from the circuit power supply voltage and the resistance value. If the resistor is 1K and the supply is 12V, then the maximum power that can go through that resistor is 122/1K = 0.144W. If it turns out the resistor only has 1V across it, the maximum power is 1mW. The majority of resistors on a typical circuit board are usually running very much below their power rating.
That is if it is from a circuit.

If it's loose, then we are back to the guessing game - or a temperature rise experiment.
Title: Re: Have resistor - want to know wattage?
Post by: amspire on March 09, 2018, 02:12:24 pm
If it's loose, then we are back to the guessing game - or a temperature rise experiment.
Probably best to assume the worse. The most common resistors are probably 6-7mm long. Just assume they are 1/4W.

If you are running an unknown resistor to the point it gets hot to touch, just get a bigger resistor.

Title: Re: Have resistor - want to know wattage?
Post by: bitman on March 10, 2018, 04:31:19 am
Thanks all - I luckily found the old invoice for the resistor pack so I got the data I wanted.
In regard to size, these are 1/4W resistors but are larger and have much thicker "pins" (thru hole resistors) than my old 1/4W has that I've used for bread-boards for years. I bought this package to add more resistors for the values I used the most, but that was a while ago so I forgot. Bottom line is, size definitely did not work to judge the wattage of the resistor.

I guess the best/only way is to test it. That seems extreme. Maybe this is one advantage SMD has - at least I have to look up the datasheet and I'll see the wattage there? Well, if you can read the number etc. on the component that is.

So I learned something new :) Again, thanks for the replies.
Title: Re: Have resistor - want to know wattage?
Post by: TimFox on March 10, 2018, 04:36:23 am
Back in the days of Allen-Bradley carbon composition, phenolic cased resistors, we all learned to tell the wattage from the size.
Since then, as both technology and marketing have advanced, there is a trend to put higher wattage ratings on smaller packages.
Some of this is due to higher temperature rating, due to better materials, others is merely increasing the temperature-rise rating on existing designs to less conservative values.
Title: Re: Have resistor - want to know wattage?
Post by: Old Printer on March 10, 2018, 05:30:03 am
I recently cracked open an old scope from the early 1970's. Most of the resistors are of the Allen Bradley style, and the sizes are pretty obvious, 1/4 watt - 1/2 watt 4/3 watt etc. The edges are very sharp as well. Modern resistors are more rounded on the corners and look as if they have been molded, then spray coated with a colored enamel, and as a group appear about 20% smaller.
Title: Re: Have resistor - want to know wattage?
Post by: Burczyk on March 11, 2018, 04:47:17 pm
I have some 1/8 W resistors that are the same size as 1/4 W, but they have thinner leads. If you dont know the wattage of your resistor, try comparing their lead thickness.
However, this method wont work on chinese resistors. I bought some 1/4 W from aliex, and their leads were thicker than my 1 W resistors.  :palm:  Not to mention the leads were corroded, and the paint was coming off the resistor's body.