1w resistor in a 0.25w resistor footprint. How? - Page 1
Forum section: electronics / beginners

author: bobbydazzler
author_rank: Regular Contributor
timestamp: April 08, 2021, 02:01:47 pm
content: I bought these resistors the other week because I needed some to just drop some voltage for a voltage regulator. I bought them based on appearance and their 1w rating(I think they look nice). Just curious about how they're capable of 1w in such a small package?They're 5% metal film resistors so are similar construction as "normal" 0.25w metal film resistors yet still fit in the same footprint, I have them pictured next to a "normal" 0.25w metal film resistor as well as a 2w metal oxide resistor(which is huge in comparison). I've stress tested them at 0.9w each soldered nearly next to each other and they ran fine, didn't melt the solder or anything so they probably were running close to their 150c max temperature.

author: mariush
author_rank: Super Contributor
timestamp: April 08, 2021, 02:58:43 pm
content: It would make way more sense to increase the heatsink of the regulator on your board.Even something as simple as a bunch of solid copper wires cut from an ethernet cable soldered to the pad will increase the dissipation area ...center a bunch of wires over the pad, solder them to it, then bend both ends of the wires and spread them a bit so they look like tiny antennas dissipating the heat.A small sheet of aluminum from a can also works, but you need to use liquid flux to add solder to the foil otherwise solder won't beat the oxidation on the can... a copper coin also works reasonably well as an improvised heatsink.Regular diodes also drop voltage and they're cheap... each 1n4001 drops around 0.8v

author: bobbydazzler
author_rank: Regular Contributor
timestamp: April 08, 2021, 03:27:32 pm
content: My post is about the small, high wattage resistors.For a heatsink on the voltage regulator I'm limited by wanting to make the pcb as small as possible so no heatsink, though I do have a bit of 26 awg single strand copper wire soldered to the metal tab of the regulator and running the length of the backside of the pcb.

author: Gyro
author_rank: Super Contributor
timestamp: April 08, 2021, 03:36:47 pm
content: Quote from: bobbydazzler on April 08, 2021, 03:27:32 pmQuite simple. They get hotter, you never get something for nothing. Their construction is such that they can sustain a higher surface temperature than bog standard metal film and carbon film ones. Usually they have a heat resisting cement covering rather than 'lacquer.Your construction needs to be able to cope with the higher operating temperature, stand-offs and spacing of sensitive parts from the (more radiant) heat source.P.S. Looking at your second photo, you'll notice that their surface area is significantly larger than that 0.25W metal film too - they just have the same useable pitch.

author: SeanB
author_rank: Super Contributor
timestamp: April 08, 2021, 04:23:34 pm
content: You also need to space them off the board a little, at least the body diameter, and preferably the same again for nearby parts, as they need the extra volume to cool. You can also ensure they are vertical, or at least able to convect air around them, as they will be the hottest point on the board. Long term they will turn that SRBP board black under them. As to the regulator simply take a small piece of copper strip and solder to the tab, then allow 2mm space and fold down the length of the regulator to increase dissipation. Even 2 tabs along the side will help a lot, just increase surface area. Yes burning off heat with a resistor is better, as running the silicon cooler helps a lot.