# Resistors in Parallel..... tabulated?

Discussion in 'Electronic Basics' started by kontiki, Jan 1, 2007.

1. ### kontikiGuest

Hi, does anyone know where a reference table of parallel resistors
could be found?The table is very useful for quick accurate resultant
resistances (to two places of decimal), the tables I've got are from
Practical Electronics Jan. 1990? and have faded to the point of being
unusable.Most non-standard resistance values are easily ascertained
e.g.for 17 ohms->75//22
quickly and easily!
with thanks,

2. ### JamieGuest

Hmm, what's wrong with using the basic math ?
R1 = 100; R2 = 200;
R1||R2 = (R2 * R1)/(R1+R2) = 1/(1/R1+1/R2) = 66.6666666

You know, if you can do basic programming hacking, you could
print your self out a new sheet.

3. ### EeyoreGuest

You can do this easily by using conductance values.

Graham

4. ### Tom BiasiGuest

With all due respect, if you are going to work with such things it would
behoove you to learn the simple math.

Tom

5. ### ChrisGuest

Hi, Kontiki. I guess having a visual reference of 5% parallels isn't
such a bad idea. Back issues of Practical Electronics are available
at many libraries. You might want to just copy the relevant page for a
dime.

But if I might make a suggestion, how about just putting together a
spreadsheet? If your table fit on a magazine page, you could probably
backfill and print it out in less time than it would take to go to the
library, get the reference, make a copy, and drive back (especially if
you're not located nearby).

Cheers
Chris

6. ### GarethGuest

Is there really any point in doing this?

The standard resistor values were chosen with that spacing because the
value of the resistor is not exact. For example an 'ordinary' resistor
would typically have a tolerance of 5%, so a resistor marked 100 ohms
could actually have a value between 95 and 105 ohms. There is therefore
little point in paralleling up resistors to give an exact value when the
actual value of the resistors you are using up is not accurately known.

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7. ### kontikiGuest

thank you for your replies....the "math" is fine,....
lets say I want to shunt a 120k res. to get a
15.6k how long does it take to find the required value? a table is much
faster ,and if you are limited by values "at hand" again an alternative
pair is easily/quickly chosen.This is useful for establishing the
practical effects of temperature compensating resistors and
thermistors.
I tend not to fire up the pc each time I need to do the math...

9. ### Tom BiasiGuest

Sounds like you may have missed the point. If you plan on doing this sort of
thing often you really should learn the calculation. It only takes a few
seconds to make the calculation, you don't need to 'fire up' the pc.
Reminds me of the checkout girl in the local market; if the machine doesn't
tell her how much change to give me she is totally stuck.

Tom

10. ### John LarkinGuest

But such a table could save a lot of work. Especially if you highlight
(or only include) the lines associated with the values that you
*have*. If you have a target value in mind, you can make a quick
visual scan to locate the pairs that could be paralleled to get close.
There's no "simple math" that's equivalent.

I've been meaning to write a couple of programs that would look at
what I have available in stock and come up with combinations that gave
me close to a desired ratio, like for setting opamp gains or voltage
regulator things. Some day, maybe.

John

11. ### Costas VlachosGuest

Hehehe... They usually get even more stuck when you pay using the
"minimal coin change" method!

--
Happy new year to all!
Costas
_________________________________________________
Costas Vlachos Email:

12. ### Tom BiasiGuest

Are you kidding? She will break down and cry, then call the manager who will
use a handheld calculator.
My point was: Charts are an aid, not a method.

Regards,
Tom

13. ### John LarkinGuest

Given a list of available resistance values, it doesn't take "a few
seconds" to evaluate possible paralleled pairs that would hit some
target to some tolerance. It could take an hour.

John

14. ### Costas VlachosGuest

Looks like the best thing here would be a program that would apply an
optimisation algorithm and come up with the best pair/pairs for a given
spec.

Have a look at this:

http://www.miscel.dk/MiscEl/miscel.html

More specifically:

http://www.miscel.dk/MiscEl/miscelSeriesParallel.html
http://www.miscel.dk/MiscEl/miscelPreferedComponents.html

Haven't tried it, but looks like it could do the job. It's free too.

--
Regards,
Costas
_________________________________________________
Costas Vlachos Email:

16. ### redbellyGuest

If you make a printout of your spreadsheet, you will not have to fire
up the computer to look at it.

Mark

17. ### Roger DewhurstGuest

Construct your own using a simple spreadsheet such as those that come
bundled with Windows. Put one set of resistances from top left to bottom
left and a similar set from top left to top right. The equation in each
rectangle is the same.

R

18. ### Roger DewhurstGuest

If he puts his available resistors across the top and down the side of a
spreadsheet and then copies the fundamental equation to all the rectangles
the job should not take an hour, unless of course he has a lot of resistors.

R

19. ### PeteSGuest

Something like that would have come in quite handy when I was setting
the quiescent point and slope for an NTC in a battery charger a while back.

As it was, it took me about an hour or so to get standard values on hand
curve of an NTC rather complicated things, but having a number of
choices in a printout might have saved some time.

Cheers

PeteS

20. ### DorianGuest

Here's an interesting technique for finding parallel resistor values that I
found in a 1961 issue of "Electronics World".

Take the desired resistor value and multiply by any number to get R1. So if
we desire a value of 500 ohms we can multiply 500 ohms by 3 to get our R1
value of 1500 ohms. To get the value of R2 divide the R1 resistor value we
calculated, in this case 1500 ohms, by 1 less than the multiplier number we
used to get R1. Our multiplier number was 3 we will now divide 1500 ohms by
1 less than 3, or 2, to get 750 ohms. These two resistor values 1500 ohms
and 750 ohms will give you the required value of 500 ohms when connected in
parallel. This simple method can be done by most people without a
calculator. This works with any set of multiplier and divider numbers you
choose and the numbers don't have to be whole numbers.

If you need more than two resistor values to get your parallel value you can
still use this method. For example let's say I need a total resistance value
of 25 ohms. I first pick a number that when multiplied by my required
resistance gives me a value I have in stock, in this case I'll pick 6
because 6 x 25 ohms gives me 150 ohms for R1. 150 ohms divided by 5 (1 less
than the multiplier value of 6) gives me 30 ohms for R2. If I don't have a
30 ohm resistor in stock I can take my original divider number and split it
up into two new numbers that add up to the original divider number. In this
case I can use 2 and 3 since they add up to 5. 150 divided by 2 = 75 ohms
and 150 divided by 3 = 50 ohms. If I have these two resistor values I'm
finished. If we do the math we see that 150 ohms in parallel with 50 ohms
and 75 ohms = 25 ohms.

Whether you end up using this method or not it still provides an interesting
insight into the math behind the parallel resistor formula.

Hope at least some of you find this useful or interesting.

Dorian