[sdiy] Moving to Surface Mount - lots of questions

[Neil Johnson]

> Is there any problem with the lower power rating of the 0603 package?
> I can get 1206 resistors that will take 1/4 Watt (with a little
> hunting, as some are only rated at 1/8W), but 0805s are only good for
> 1/10W and the 0603 typically will only take 1/16W. Might this
> sometimes be a problem with the +/-15V supplies and 10V signals we
> commonly use? I'm assuming the real solution is to go through the
> circuit and use 1206 (or even 1210) packages for the few areas that
> would need them, but how often would you think this comes up?

Lets do a little bit of maths.  We'll add in a bit of margin to be on 
the safe side.  And in both cases lets assume a maximum of 15V RMS.

P = V^2 / R

So,

R = V^2 / P

For 0805, lets say 1/12W.  Then Rmin = (15 * 15) * 12 = 2k7.
For 0603, lets say 1/20W.  Then Rmin = (15 * 15) * 20 = 4k5.

Higher-valued resistors will dissipate less power for a given applied 
voltage.

For 30V RMS quadruple the minimum resistor values (10k8 and 18k 
respectively).

Cheers,
Neil
-- 
www.njohnson.co.uk