Have resistor - want to know wattage?  - Page 1
Forum section: electronics / beginners

author: bitman
author_rank: Unknown
timestamp: March 09, 2018, 12:08:22 am
content: 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?

author: IanB
author_rank: Super Contributor
timestamp: March 09, 2018, 12:21:42 am
content: 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.

author: amspire
author_rank: Super Contributor
timestamp: March 09, 2018, 12:28:22 am
content: Quote from: bitman on March 09, 2018, 12:08:22 amYou 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.

author: Brumby
author_rank: Unknown
timestamp: March 09, 2018, 03:03:09 am
content: Quote from: amspire on March 09, 2018, 12:28:22 amThat is if it is from a circuit.If it's loose, then we are back to the guessing game - or a temperature rise experiment.

author: amspire
author_rank: Super Contributor
timestamp: March 09, 2018, 03:12:24 am
content: Quote from: Brumby on March 09, 2018, 03:03:09 am

author: bitman
author_rank: Unknown
timestamp: March 09, 2018, 05:31:19 pm
content: 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 newAgain, thanks for the replies.

author: TimFox
author_rank: Super Contributor
timestamp: March 09, 2018, 05:36:23 pm
content: 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.

author: Old Printer
author_rank: Frequent Contributor
timestamp: March 09, 2018, 06:30:03 pm
content: 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.

author: Burczyk
author_rank: Contributor
timestamp: March 11, 2018, 05:47:17 am
content: 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.Not to mention the leads were corroded, and the paint was coming off the resistor's body.