Resistor selection

Discussion in 'General Electronics Chat' started by hunterage2000, May 6, 2012.

  1. hunterage2000

    Thread Starter Active Member

    May 2, 2010
    400
    0
    Hi,

    When designing circuits, what are the selected values of the resistors based on?

    In opamp design I always seem to see examples with 100kohm resistors. Is there a selection criteria for the selected values?
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
  3. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Lots of things come into play. Input impedance, noise immunity and power consumtion are the most obvious in simple gain stages. With active filters etc. the resistor values affect the capacitor and values, so for example you would need huge capacitors if you used low resistors.

    But many times the actual value in non-critical designs is just the designers favourite value. (or the one that is cheapest or he has it in stock :))
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    More on what Kubeek said, the 10's multiples tend to be in everyones box.

    In many circuits the exact value is extremely non critical. It is the ratio's that are important, for voltage dividers.

    I have a list of the common 5% part I use. There are many good references out there that expand on this, including Vol. V - References on top of this page.

    1. 10
    2. 11
    3. 12
    4. 13
    5. 15
    6. 16
    7. 18
    8. 20
    9. 22
    10. 24
    11. 27
    12. 30
    13. 33
    14. 36
    15. 39
    16. 43
    17. 47
    18. 51
    19. 56
    20. 62
    21. 68
    22. 75
    23. 82
    24. 91

    You can get these parts from 1/8W to 2W. I made a full kit with ¼W parts, that being extremely common.

    Resistor Parts Storage
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,360
    I conduct a digital electronics laboratory.
    Here are the resistor values I stock for students to choose:

    100 220 470
    1000 2200 4700
    10k 22k 47k
    100k 220k 470k
    1M 10M

    These are 1/4W, 5% tolerance resistors. Rarely do students need any other value.

    In my personal stock I use the following values in each decade (five decades from 10Ω to 910kΩ, plus 1M, 10M, 22MΩ). These 1/4W 5% resistors will cover all the needs of a hobbyist:

    10
    22
    33
    47
    56
    68
    75
    82
    91

    I rarely use 1% resistors.
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    Same here. If you look closely you will find the values lean toward logarithmic value spread.

    I went a bit crazy with my kit because I could.
     
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    With USPS shipping from Minnesota to New York being just a few dollars for 2-3 day delivery I don't keep inventory beyond the leftovers of previous orders. I always over order resistors as they are quite cheap for the SMD types I use (0805 parts are quite handy on 0.1" space hole proto boards).

    Values just above 1.0 are closely spaced, keep this in mind if selecting parallel combos or such and you need a find adjustment.

    Bob Pease always pointed out certain popular values (1K, 10K and the like) are the first ones to go out of stock. Better pick something more obscure (1.01K, 9.9K) in your circuit, especially if the exact value is not critical.
     
  8. MBVet05

    New Member

    Jul 21, 2011
    27
    1
    I use many values for many applications when modifying circuit boards. Some very common resistors are 0603 size surface mount. We use a lot of the 10k and 0 ohm resistor values when testing circuitry. You can find a huge selection, including assortments, at Digikey.
     
  9. #12

    Expert

    Nov 30, 2010
    16,278
    6,789
    Mr Chips' method is called a 1-2-5 sequence. My oscilloscope has the same sequence on the voltage selection switch. It works well enough because each value is about 2 or 2 1/2 times the previous value.

    Edit: the next post describes what we call the Chyna Syndrome ;)
     
    Last edited: May 6, 2012
    shortbus likes this.
  10. Lundwall_Paul

    Member

    Oct 18, 2011
    220
    19
    If you’re into analog circuits like I am you will need a good assortment. Do not do what I did and order kits and assortments from EBay. I ordered what “5%” and they had the gold tolerance band but they were more like 20%.
     
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