Resistor combos - RF Cafe Forums
Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would
be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate
that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views.
It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if
you would like to post something on RF Cafe's
page, please do.
Below are all of the forum threads, including all
the responses to the original posts.
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.
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
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
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
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