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How to choose the correct potentiometer?

Discussion in 'General Electronics Discussion' started by sniktun, Aug 21, 2018.

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  1. sniktun

    sniktun

    6
    0
    Aug 21, 2018
    Hello all,

    First time posting here, hopefully this will be an easy one for you guys. I am having some issues with a potentiometer and I am not sure why, I am hoping someone can explain what I have dont wrong and what I can do when sizing resistors in the future

    I am using an industrial computer with an analogue output (set to 10V) and a 25K potentiometer (25K Bourns 91 series) to control a fan speed (0-10Vdc) on a building site. This is so the air balancers can change the speed while I am not there. I chose a higher resistance of 25K so that there wouldn't be too much current draw and just a voltage reference point.

    What seems to be happening is when the pot is dialed down low to around the 2.5Vdc output all of a sudden it jumps to 0V. Why would this be happening?

    2.5Vdc - 10Vdc output works fine, I just cant get any control between 0Vdc-2.5Vdc. Any ideas? Also the pot is very sensitive so mabye I need one with more turns for the full range?

    Thanks,
    James
     
  2. WHONOES

    WHONOES

    746
    161
    May 20, 2017
    You could be using a Log pot when you should be using a Lin pot. Also the value seems a bit high. It depends on the input resistance of the controller you are using.
    What make and type is it?
     
  3. Audioguru

    Audioguru

    2,671
    605
    Sep 24, 2016
    Bourns have a website for their 91 series pots with big long part numbers for the type of shaft, bushing etc. and if it is linear (that you need) or audio (logarithmic).
     
  4. sniktun

    sniktun

    6
    0
    Aug 21, 2018
    I selected a linear pot, here is a screenshot of the pot specifications[​IMG]

    The 10V supply is coming from a Trend controller analogue output and its controlling a systemair mub 100 710ec fan via the pot
     
  5. sniktun

    sniktun

    6
    0
    Aug 21, 2018
    I selected a linear pot, I did try to post a comment with a screenshot of the specs but because I am a new user its waiting for a moderator to approve!

    The 10V supply is coming from a Trend controller analogue output and its controlling a systemair mub 100 710ec fan via the pot
     
  6. WHONOES

    WHONOES

    746
    161
    May 20, 2017
    The datasheet claims that the units are supplied with a pre-wired potentiometer. I would suggest you use that and follow the installation instructions. It also seems to infer that the control voltage is generated within the fan unit and that you just need to connect the pot in the correct manner.
     
  7. Robert_fay

    Robert_fay

    124
    34
    Jun 15, 2017
    Do you have a part number on the pot? What kind of current are you running through the pot? Do you have a measurement? What is the part number for the fan?

    Sorry for all the questions just trying to narrow down the issue and there is not much to go on above.
    -Robert
     
    sniktun likes this.
  8. sniktun

    sniktun

    6
    0
    Aug 21, 2018
    Yes Mfr. Part No. 91A1A-B28-E17

    Current is about 30mA

    Fan part no. is systemair mub 100 710ec
     
  9. sniktun

    sniktun

    6
    0
    Aug 21, 2018
    The fan is up in an unreachable area so I can't even do this option as a temporary unfortunately.

    The fan is controlled by our BMS system so will work from our controllers 0-10Vdc output just fine. I wanted to know why it wasn't completely working with a potentiometer I have put in. I figured that the resistance was too high for the current available from the controller output but wasn't sure
     
  10. WHONOES

    WHONOES

    746
    161
    May 20, 2017
    30mA through the pot suggest it value is 330Ω and not the 25K you originally quoted. Current thru 25K at 10V supply = 250μA.
    Another thought, do you have the pot connected correctly?
     
  11. sniktun

    sniktun

    6
    0
    Aug 21, 2018
    No sorry I mean the total current available from the controllers output is 30mA, I am not on site so can't measure the current draw through the pot. It is definately a 25K pot

    Yes I believe it is connected correctly. As I mentioned it works fine between 2.5V-10V. I have a nice controllable increase/decrease in voltage up until the point I try to go below 2.5V then it just jumps to 0V
     
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