​ ​ ​ ​ High current digital pot - Page 3
Page 3 of 3 FirstFirst 1 2 3
Results 21 to 30 of 30

Thread: High current digital pot

  1. #21
    Senior Member
    Join Date
    Feb 2012
    Location
    London
    Posts
    2,627

    Default

    Hi,

    In the old days, the "E12" resistor series used to be called the "Preferred" resistance range, which is quite an appropriate name. Each value is about 20% larger than the previous, i.e. 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82, 100 etc. and can continue through many decades (i.e. up to 1 Mohm and above).

    It dates from when resistors were made by mixing up a concoction with carbon powder, moulded, and baked in an oven. Then the value of each "resistor" was measured and put in a "bucket" appropriate to its value (give or take 10%). Now, with Carbon (or Metal) Film technology it's possible to manufacture whatever value is required to much higher accuracy, but the basic principle of "percentages" is still very relevant. Generally, you are likely to find that, for example, a 1.2 ohm resistor (which your switching box wouldn't have) to be more useful than 99 ohms (if it has 100 ohms anyway).

    Now, there is the E24 series which "fills in the gaps", i.e. 11, 13, 16, 20, 24, 30, 36, 43, 51, 62, 75, 91 and even the E96 series of "1% tolerance" values. A "resistor box" which provides the Exx series is likely to be more useful than a linear range. Perhaps as many as E96 in the middle of the range (say 100 ohms - 100 kohms), but there are practical limitations with "accurate" values below 10 ohms or above 1 Mohms, so E12 or E6 values may be sufficient in those regions. Even the E3 series (10, 22, 47, 100) can be sufficient for capacitors and inductors, etc..

    Of course you don't need to buy one resistor of every value, as hippy indicated earlier, it's possible to combine resistors in series or parallel (not quite as easy to calculate) for example 20k = 10k + 10k and 11k = 22k in parallel with 22k, etc. A good way to use the computational capabilities of a PICaxe.

    Cheers, Alan.

  2. #22
    Senior Member
    Join Date
    Apr 2013
    Location
    north dakota
    Posts
    260
    Blog Entries
    2

    Default

    Hi Alan another great piece of information ...okay so to get resistors 1ohm to 128 ohm for the use of using one 595 chip
    I think I may have to parallel only a few to get within close range ...I never heard of E96 series ... just a thought since
    it is "1%" tolerance so if I parallel say two 64ohm resistors to get 32 ohm would the tolerance drop to .5%? I have a lot of reading
    to do still ...like example--> I build this best/worst digital pot box with a picaxe 20m2,74ch595,relays,resistors..etc..then when i add my LCD
    16X2 I will still have to take forum night classes on how to read the ohms from the picaxe to display nicely on the screen.

    Just a simple quick question to over/under educated professors here
    if resistor ohms are changed by temperature ...then how does the computers on
    planet mars rovers stay working in extreme weather changes? (you can say its over my head)

  3. #23
    Technical Support
    Join Date
    Jan 1970
    Location
    UK
    Posts
    23,635

    Default

    Quote Originally Posted by newplumber View Post
    it is "1%" tolerance so if I parallel say two 64ohm resistors to get 32 ohm would the tolerance drop to .5%?
    If one has two perfect 100R resistors in parallel the resistance would be 50R. Using 100R +/-1% resistors could be ...

    101R | 101R = 50.5
    101R | 99R = 49.995, ~50.0
    99R | 99R = 49.499, ~49.5

    So that desired 50 can be anywhere between 49.5 and 50.5 which still works out at 50 +/-1%

    Quote Originally Posted by newplumber View Post
    if resistor ohms are changed by temperature ...then how does the computers on planet mars rovers stay working in extreme weather changes?
    It is probably a factor of a number of things; designing so resistor changes don't have significant effect on operation, using resistors which are more immune to temperature change, keeping the internal temperature more constant.

    A resistor NASA is using on a Mars Rover is likely a lot more expensive than what most people would be using.

  4. #24
    Senior Member
    Join Date
    Apr 2013
    Location
    north dakota
    Posts
    260
    Blog Entries
    2

    Default

    Thanks Hippy .. I think all my resistors are +/- 5% (gold) so i think I will order some 1% from digi-key which love the lighting shipping
    and i am sure the designing (rover) is way more time/thought then anything else on earth. I think some day for kicks I am going to
    apply at NASA and see what my chances are...maybe they don't know water runs down hill

  5. #25
    Technical Support
    Join Date
    Jan 1970
    Location
    UK
    Posts
    23,635

    Default

    I am not sure if 1% are really much better than 5%, especially as resistor value increases. The top-end 32768R resistor could be out by 327R at 1% and 1638R at 5%.

    If resistors were selected to be at or below their nominal values, and what values each had was stored, it would be possible to select the closest desired result. For example, wanting a 32768R might mean turning on that 32768R plus 220R plus 8R.

    You won't have a full 65,536 selection of resistances, but you will possibly have more accuracy through the range.

    Also how many 5% resistors would be outside what the 1% would deliver ? You might be better off with an accurate resistor marked 5% than an inaccurate resistor marked 1%.

  6. #26
    Senior Member
    Join Date
    Feb 2012
    Location
    London
    Posts
    2,627

    Default

    Hi,

    Yes, in general, combining resistors in series or parallel won't affect their overall tolerance very much.

    Again, "in the old days", when resistors were manufactured, measured and then marked accordingly, those that were very close to a "preferred" value would be marked as close tolerance (perhaps 2% with a red band) and sold at a higher price. So if you bought an "ordinary" resistor (probably a 10%, silver band) there was a strong chance that it would NOT be the marked value but too high or too low by several percent.

    But nowadays, when resistors can be manufactured to a specific value, there's a much better chance that you will get one close to the marked resistance. But still there must be some manufacturing tolerances (errors in measurement, etc.). If you take two similar resistors off the same "band", then they were probably manufactured by the same machine at the same time, so they will probably have similar errors. Therefore, you might be "unlucky" and get two resistors that are both near the edge of their tolerance band. But if you combine two resistors from different sources, there is a much lower risk that both will be "bad", so combining resistors may actually improve their overall tolerance.

    If you needed, say, 11 kohms, there are several possibilities. Two 22k in parallel will give 11k but there's the risk they will have a similar tolerance offset, or 10k + 1K should give the same value, but the 10k will predominate. It will contribute most to the tolerance (so the 1k could be a cheapie) but also dissipate most of the power (heat), so combining resistors with similar values can be preferable. I can't think of any other combinations that would (theoretically) give exactly 11k, but several give just 100 ohms off, e.g. 2.7k + 8.2K = 10.9k, or 3.6k + 7.5k = 11.1k, etc., should such values be required.

    Cheers, Alan.

  7. #27
    Technical Support
    Join Date
    Jan 1970
    Location
    UK
    Posts
    23,635

    Default

    I suppose one question worth asking is what exactly you are trying to achieve.

    As a 'for fun' or 'learning' project, a 'proof of concept' or 'novelty' it probably won't really matter what is used.

    If you were going for something which could deliver a certified highly accurate binary selectable resistance you would probably be talking a very expensive construction with laser trimmed resistors and would also have to take into account relay contact resistances.

  8. #28
    Senior Member
    Join Date
    Apr 2013
    Location
    north dakota
    Posts
    260
    Blog Entries
    2

    Default

    Hi
    My achievement is to have a LCD and show ohms starting at 0 ..(or with relay contacts close to 0) then run up to 255 just be reading ohms in
    a resistor ladder... now after you pros gave me a lot of great information so with that, I plan on still building my relay potentiometer and see how close
    at room temp I can have it count...I don't know much about resistors (except value and wattage) and its interesting to learn from yous. I started this project
    to see if its possible to use in different applications...like example ...my welder but not possible for me ...or a older treadmill motor driver...which again seems
    to much homework and plus there is way better ways for that in different forums on here...so overall I decided to just make a ohm reading display from my resistor ladder.
    and of course its always for fun learning 30% of a ohm at a time. I don't own any 1% tolerance so it would be interesting for me to see if there is any if at all real change with 1% vrs 5% using resistors up to
    128 ohm. And always thanks for the resistor info. simple question
    If I have say 1/4 watt resistor at 100 ohm with a tolerance of 1% or even 5% will the resistor ohm value change using different wattage like
    .125 watt = 99.5
    .150 watt = 99.9 even if the ambient temperature doesn't change?

    btw Alan I like your "in the old days" because for me it makes it clearer a lot of times how/why something is made better now days or current times

    and hippy ..lol no I don't need no certified accuracy ... it way funner using dollar price items

  9. #29
    Senior Member
    Join Date
    Jan 1970
    Location
    Colorado USA
    Posts
    3,039

    Default

    Not sure what your question is... ideally a resistor has no change with temperature. In practice some have very small temperature coefficient of resistance and some pretty large... this latter type will then change quite a bit when dissipating more power and hence heating up. An interesting example of a large coefficient of resistance comes with the tungsten filament light bulb. About 8x change cold to bright temperature... ;-0 You can find practical coefficients for substances like coat hanger / baling wire [iron] copper wire [copper] on the Internet. Special alloys like constantan are close to zero change with temperature...

  10. #30
    Senior Member
    Join Date
    Apr 2013
    Location
    north dakota
    Posts
    260
    Blog Entries
    2

    Default

    this latter type will then change quite a bit when dissipating more power and hence heating up
    thanks premelec...you answered my question

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •