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PWM - Potentiometer ohm importance...

Discussion in 'General Electronics Discussion' started by New Bee, Aug 4, 2018.

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  1. New Bee

    New Bee

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    Aug 4, 2018
    Hi guys can anyone help me please, I have scoured the internet trying to find an answer to this question but I can't find an answer anywhere

    Does potentiometer ohm rating matter in a PWM circuit? Is it still acting as a resistor? or is it just using the wiper setting of the pot to adjust the duty cycle setting?

    I'm talking about this board

    https://m.ebay.co.uk/itm/Universal-...Regulator-Controller-Switch-S8W2/282658184072

    I'm new in electronics hence my name can anyone please help me, thanks
     
  2. Minder

    Minder

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    Apr 24, 2015
    Like any component in an electronic circuit, the value is dependent on the function it does in that particular design.
    In this case a typical value could be 100k linear pot.
    What is the reason for the question?
    M.
     
  3. dave9

    dave9

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    Mar 5, 2017
    In a SMPS, anything that adjusts the output is adjusting the duty cycle. If you want a better answer, you need to get the controller chip part # and look at the datasheet to see if it is on a control line or the feedback loop.

    Are you just trying to learn how these work in general or do you need to modify it for a particular reason (which would be helpful to mention) ?
     
  4. New Bee

    New Bee

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    Aug 4, 2018
    The current pot has very limited range control on my intended motor it's speed control all seems to happen in a little part of a turn, so I was wondering if I replaced the potentiometer with one of a different value, smaller probably (current 100k) then would it give more control over the speed? Would it detriment the PWM in any way or is it just there to adjust the duty cycle so value doesn't matter?

    Anyway I was thinking of replacing with a 10 turn wirebound pot for greater control hence why I'm asking if pot value matters as there are multiple ohm choices

    https://m.ebay.co.uk/itm/3590S-Prec...5K10K20K50K100K-Ohm-Pot-10Turn-U/192078162105

    Would changing the pot work or is it just not a great pwm?
    Thanks
     
  5. kellys_eye

    kellys_eye

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    Jun 25, 2010
    It could be that you have been supplied with a logarithmic track potentiometer instead of a linear track version.

    Can you see any identification marks, numbers, codes on the pot you are using?
     
  6. New Bee

    New Bee

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    Aug 4, 2018
    Just W.L on the metal
    And b100k on the pcb
     
  7. WHONOES

    WHONOES

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    May 20, 2017
    It is difficult to tell fro the pictures in your link, but it is probably based on a 555 timer and if so the value of the pot is quite important. Using a smaller value would increase an already high switching speed.
    There also seems to be a lot of bits, which probably includes a voltage regulator, on the board for what is a fairly simple circuit.
     
  8. Alec_t

    Alec_t

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    804
    Jul 7, 2015
    That marking could indicate either a linear or logarithmic taper pot, depending on manufacturer and country of origin. You probably need a linear taper pot.
     
  9. kellys_eye

    kellys_eye

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    Jun 25, 2010
    ^ concur.... annoyingly the North American marking differ from the European marking in that they are 'reversed' i.e. European has A for logarithmic and B for linear whilst N.A is the opposite!!!

    It 'sounds' like you're using a log pot - simple matter to get a replacement (linear) which should fix your problem.
     
  10. Minder

    Minder

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    Apr 24, 2015
    The one I picked up is very similar and mine has a 100k linear pot.
    Place the pot slider in the centre and measure to each end, that would tell you.
    Also are you using a power supply at the voltage rating of the motor? With a PWM drive you normally would be at least 10% higher voltage than the motor rating.
    M.
     
    Last edited: Aug 4, 2018
  11. New Bee

    New Bee

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    Aug 4, 2018
    Using 12.6v adapter (waiting on a 12v 18650 battery pack) 12v motor, I purchased one of those 10 turn wirewound pots in 100k size so I'll see how that goes, not sure if the current one is or isn't linear as I've not tested it yet but either way itself or the pwm board will need replacing anyway as somethings not right. Hopefully this will fix things thanks for all the helpful answers everyone
     
  12. Minder

    Minder

    3,103
    664
    Apr 24, 2015
    Did you try an automotive lamp? See if the same results?
    M.
     
  13. WHONOES

    WHONOES

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    May 20, 2017
    Had a quick look at the one you bought, and its switching speed is 25KHz which is a bit fast.
    I run mine at between 50 and 300Hz. You might expect the motor to make a bit of a racket at that frequency but mine is OK but, having said that, mine are fairly low power 50 to100W or so. But I do get good control.
     
  14. New Bee

    New Bee

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    Aug 4, 2018
    Could i ask what your running yours on?
     
  15. Bluejets

    Bluejets

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    Oct 5, 2014
    In the first link to Ebay, the photo of the pot on it's own shows B10K written on front of the bakolite insulator opposite the 3 terminals.
     
  16. New Bee

    New Bee

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    Aug 4, 2018
    Random link I found quickly not the actual one but is "identical"
     
  17. WHONOES

    WHONOES

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    May 20, 2017
    Currently using it for my hand held 12V mini drill which is often to fast for some of the jobs I do. It runs at about 250Hz if I remember correctly.
     
  18. FuZZ1L0G1C

    FuZZ1L0G1C

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    Mar 25, 2014
    New Bee, a quick 'n dirty way of checking your pot value, and if its Linear or Logarithmic can be done thus:
    Value of pot:
    With ohms on highest scale, measure end to end tag (outer two solder tags).
    Lower ohms scale on meter until you get a reading.
    The nearest standard value will be the pots rated resistance value.
    LOG vs LIN:
    From the wiper to each of the end tags, check ohms while turning the pot fully CW and ACW.
    A LIN taper will gradually cruise from minimum to maximum resistance.
    A LOG taper will leap after turning a few degrees (on a standard 300o travel).
     
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