[sdiy] Moving to Surface Mount - lots of questions
chris at chrismusic.de
Thu Dec 8 10:42:52 CET 2011
On Wed, 7 Dec 2011 15:13:10 -0500 (EST) Harry Bissell <harrybissell at wowway.com> wrote:
> I think the best approach is to assume you have the full supply across the resistor (ie 30V)
> and calculate the resistance value that would exceed the power rating. 30V is REALLY worst case, most are
> more like 15V max.
> If the resistance is above about 10K you are OK with a 1/10 watt package. Most of our values (summing resistors
> etc are way larger than that, others have way lower voltages across them...
> Panasonic has 1/10W resistors in 0603. I'd select larger packages for more power. You can usually hose a larger
> resistor onto a smaller footprint... or use double the value and stack two vertically if you find you goofed...
It's true that you can stack them nicely, I did it for precision
resistance ratios (5k vs. 10k using two paralleled 10k's for 5k).
But you wouldn't expect the lower resistor to experience some
significant air flow to get the 1/10W away, don't you? Ok, it's not
looking like fat wire-wound stuff, but in the end it's all about heat,
just on a smaller scale.
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