

Resistor Combos  RF Cafe Forums

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:
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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.
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Cheers, Gary Smith
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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



