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Resistors in Parallel..... tabulated?

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

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  1. kontiki

    kontiki Guest

    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. Jamie

    Jamie Guest

    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. Eeyore

    Eeyore Guest

    You can do this easily by using conductance values.

  4. Tom Biasi

    Tom Biasi Guest

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

  5. Chris

    Chris Guest

    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

    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).

  6. Gareth

    Gareth Guest

    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.

  7. kontiki

    kontiki Guest

    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
    I tend not to fire up the pc each time I need to do the math...
  8. Try using Excel or Google Spreadsheet.
  9. Tom Biasi

    Tom Biasi Guest

    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.

  10. John  Larkin

    John Larkin Guest

    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.

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

    Happy new year to all!
    Costas Vlachos Email:
    SPAM-TRAPPED: Please remove "-X-" before replying
  12. Tom Biasi

    Tom Biasi Guest

    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.

  13. John  Larkin

    John Larkin Guest

    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.

  14. 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

    Have a look at this:

    More specifically:

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

    Costas Vlachos Email:
    SPAM-TRAPPED: Please remove "-X-" before replying
  15. John Fields

    John Fields Guest

  16. redbelly

    redbelly Guest

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

  17. 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.

  18. 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.

  19. PeteS

    PeteS Guest

    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
    that worked (and yes, I ued a spreadsheet). Admittedly, the non-linear
    curve of an NTC rather complicated things, but having a number of
    choices in a printout might have saved some time.


  20. Dorian

    Dorian Guest

    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.

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