Resistor combos - RF Cafe Forums
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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.
Post subject: Resistor power dissapation
Unread postPosted: Sun Jan 02, 2005 4:09 pm
Joined: Sun Oct 31, 2004 3:01 pm
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.
Unread postPosted: Wed Jan 05, 2005 9:41 am
Joined: Sun Aug 03, 2003 2:02 pm
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 non-standard 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