
Resistor combos  RF Cafe Forums


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Guest Post subject: Resistor combos Unread postPosted: Sat Jan
01, 2005 10:57 am
I need to combine a couple 1% resistors to
make a nonstandard value. For power dissipation in the resistors, is
it best to use series resistors or parallel resistors? Thanks.
:lol:
Top
garylsmith2k Post subject:
Resistor power dissapation Unread postPosted: Sun Jan 02, 2005 4:09
pm Offline Captain
Joined: Sun Oct 31, 2004 3:01 pm
Posts: 5 Location: N/A Parallel.
_________________
Cheers, Gary Smith
Top Profile
Guest Post
subject: Unread postPosted: Wed Jan 05, 2005 7:24 am
no,
Smith answer is wrong because it is incomplete.
In general it
does not matter whether you use a series or parallel circuit, but you
need to use resistors that are equal or almost equal to obtain an equal
distribution of the total power over all components. This is alo the
best strategy to obtain optimal tolerance improvement.
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Kirt Blattenberger Post subject: Unread postPosted:
Wed Jan 05, 2005 9:41 am Offline Site Admin User avatar
Joined: Sun Aug 03, 2003 2:02 pm Posts: 308 Location: Erie,
PA Total power dissipation in the series/parallel resistor combinations
is going to be the same for a given equivalent resistance. The only
difference is how the power dissipation will be distributed between
the resistors. Depending upon the combination(s), one resistor can dissipate
nearly all the power while the other(s) dissipate very little. Using
values as close to equal as possilbe will keep the power distribution
nearly equal.
Based on Ohm's Law, power dissipation is proportional
(or inversely proportional) to resistance, so in a series combination,
the power dissipation in the larger resistor will be greatest (same
I through all resistors and P=I^2*R, so larger R dissipates higher power),
and in a parallel combination the power dissipation in the smaller resistor
will be greatest (same voltage across all resistors and P=V^2/R, so
smaller R dissipates greatest power).
One last comment. With
series combinations, the closest you can get to some exact nonstandard
value is equal to the number of significant places in the nominal resistance
value and the available stanrd values. For instance, if you need exactly
37.5 ohms, then for 5% values the closest you can get with a series
combination is within +/0.5 ohms (22+15=37, 22+16=38 ). Using a parallel
combination of two 75 ohm resistors gets you right on. Theoretically,
any degree of precision can be obtained with enough parallel resistors,
but not with series.
 Kirt Blattenberger :smt024
Posted 11/12/2012



