My 1W resistors are half as big as my 1/2W resistors - Page 1
Forum section: electronics / beginners

author: smbaker
author_rank: Regular Contributor
timestamp: December 07, 2015, 10:14:07 pm
content: Silly question of the day... I ordered some resistors from digikey. The package on the left are 1/2W and are consistent with the size of other 1/2W resistors that I have on hand. The ones on the right are 1W, but are consistent with the size of 1/4W resistors.Have resistors suddenly become smaller? or did digikey screw up? All of the 1W resistors in my order are of similar size.

author: smbaker
author_rank: Regular Contributor
timestamp: December 07, 2015, 10:18:04 pm
content: What!?!? Somehow I missed an essential episode? I'm going to have to rewatch them all now!

author: moya034
author_rank: Regular Contributor
timestamp: December 07, 2015, 10:19:26 pm
content: You could always hook it up to a constant current power supply and see how much it takes to blow it up

author: EEVblog
author_rank: Unknown
timestamp: December 07, 2015, 10:20:24 pm
content: Quote from: smbaker on December 07, 2015, 10:18:04 pmIt's actually a tutorial on resistor power dissipation.

author: smbaker
author_rank: Regular Contributor
timestamp: December 07, 2015, 10:36:13 pm
content: Quote from: EEVblog on December 07, 2015, 10:20:24 pmThanks Dave, I just finished the episode -- interesting viewing.Also, I did look up my parts on digikey, and I now realize the one on the left is carbon, whereas the one on the right was metal film. I hadn't paid attention to the construction when ordering the resistors, just went for something of the correct resistance and wattage. Now I feel like conducting some experiments with a power supply and a temperature probe.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 08, 2015, 03:42:43 am
content: Yeah, higher temperatures. You can dissipate a lot more power for a given bit of convection air, if you heat it up that much more!Metal film resistors are commonly made with an epoxy or silicone enamel that's good to modestly ridiculous temperatures (often 200C+); silicone and ceramic power resistors are normally rated to 250-300C, and ceramic / vitreous enamel types can operate at dull red glow (>500C!), being only limited by eventual oxidation of metal parts (nichrome wire within -- oxygen will diffuse into it slowly; terminals, mounting hardware, etc.), not to mention the electrical terminals coming unsoldered rather quickly!If you were expecting a resistor of a particular size, just remember, RTFDS...Tim

author: smbaker
author_rank: Regular Contributor
timestamp: December 08, 2015, 04:43:25 am
content: Quote from: T3sl4co1l on December 08, 2015, 03:42:43 amI certainly don't mind that it's smaller -- my first reaction was just that they'd shipped me the wrong resistors because they seemed so tiny.I did take one of the tiny 1W resistors and bring it up to a full watt. It managed to make it up to about 80C. I've convinced myself it is indeed a 1W resistor.

author: TimFox
author_rank: Super Contributor
timestamp: December 08, 2015, 01:40:13 pm
content: Many resistor types have different nominal power ratings for different allowable temperature rise.The best specification is a derating curve showing allowable power vs. temperature rise or ambient temperature.However, it is good practice to down-rate the power to reduce temperature and the resulting drift in resistance.

author: Richard Head
author_rank: Frequent Contributor
timestamp: December 09, 2015, 06:18:59 am
content: Just because the resistor can safely dissipate 1W doesn't mean the PC board can handle it. The heat will be conducted down the leads and heat the solder joints eventually causing the solder to "crystalise" and the epoxy board to char. For higher power you need a greater surface area ie larger resistor. I tend to use the larger bodied carbon film resistors as apposed to the PR01/PR02 metal film resistors. The carbon films are larger but don't get as hot and no funny smells are produced.

author: tszaboo
author_rank: Super Contributor
timestamp: December 09, 2015, 11:20:56 am
content: Not so long ago, I got some 3W flameproof, fusible resistor. It got me thinking, what happens if I overload them, and I needed to know the failure mode. So I connected them to a PSU, increased the voltage. I stopped at some 1000% overload, because:- It was emitting visible light, yellow- The desk below it started browning and smokeing- I wasn't really comfortable with itA rating for a 0805 resistor can be anything between 0.1W and 1W. The 0.1W will operate maybe to 125 degrees, the other one o 175.

author: Deathwish
author_rank: Unknown
timestamp: December 09, 2015, 12:50:57 pm
content: See, shoulda asked a woman, they will tell you, size doesn't count.