Locking potentiometer? Recessed?

Discussion in 'General Electronics Chat' started by james211, Jul 6, 2014.

  1. james211

    Thread Starter Member

    May 29, 2012
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    So my current project needs to have a PWM controller to adjust the speed of some peristaltic dosing pumps. Its important that the controller doesn't get bumped by accident. So I was wondering if there is such a thing as a locking potentiometer?

    So I was thinking of buying something like this to controll the speed.

    And then replacing the potentiometer with something like this. I'm not sure if that would work, this is more of an example of what else I'm thinking if there isn't such a thing as a locking potentiometer.

    If you have any other ideas please let me know. The motors that run these pumps are 12v and draw about 200mA at full speed. Not sure if that changes once they start drawing liquid or not though.

    Just looking at pictures, I've attached a few things that might work physically, but not necessarily correct for a replacement potentiometer. That's why I'm here, asking for a correct option.
     
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  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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  3. james211

    Thread Starter Member

    May 29, 2012
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    Would any of the ones I mentioned work or no? Is a 10k pot big enough?

    Thanks for your quick reply!
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    ErnieM likes this.
  5. Brevor

    Active Member

    Apr 9, 2011
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    The PC mount pot you link to would work but it is not made for frequent adjustment. If you will need to change the pump speed on a regular basis something like max linked to would be better.
     
  6. james211

    Thread Starter Member

    May 29, 2012
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    It shouldn't need to be adjusted very often at all, not even once a month possibly not even every 6months.

    I like the bourns but I'm a bit confused..is it just the knob or does it include the potentiometer as well?

    Given the specs I provided in post 1, will a 10k pot be good enough?
     
  7. MrChips

    Moderator

    Oct 2, 2009
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    The 10-turn dial is just the knob and dial. You fasten it to the shaft of the pot like so:

    [​IMG]

    If you are just going to adjust it once and leave it alone, a 10-turn trimpot would be more suitable, with wire connections or PCB mounted:

    [​IMG]

    [​IMG]

    [​IMG]
     
  8. wmodavis

    Well-Known Member

    Oct 23, 2010
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  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Yes it will. Pumped volume will change with viscosity, temperature, head (pressure levels to/from the pump), and with tube issues like tubes softening over time, reducing drag, both short term (as the tube warms up) and longer term (as the tube ages).

    You will also get other issues like volume per rev changing with speed!

    I have some experience designing accurate industrial equipment using peristaltic pumps.

    How accurate does this need to be? What kind of flow rate?

    I think if you just expect to use a DC motor and a trimpot controlling PWM duty cycle to get accurate dosing you will be very disappointed.
     
  10. james211

    Thread Starter Member

    May 29, 2012
    210
    2
    Hey RB,

    Thank you for chiming in. The project is a 2 or 3 channel peristaltic dosing pump for reef aquariums. I purchased this small micro controller called a spark core from kickstarter a few months back and decided it was perfect for this project since its completely wireless. Anyway, I posted my project on one my local reef forums and it sparked a lot of attention. My personal one uses two high quality AC pumps that even after a year and a half of use individually (before the project) still pump at exactly 1.6mL/min.

    Now, to since there was so much interest in this project I wanted to make a version that was even more affordable, and the easiest way to do that was with dc pumps. I agree with you that they will most likely not be nearly as accurate. I did find a guy who wrote some calibration software for an arduino version. I haven't had a chance to test it yet as I don't have all of my pieces yet. However, he said as long as you do it once every 4-6 months its quite accurate.

    If you have any suggestions based on what I've told you please let me know. I think one of the hardest parts is finding quality DC peristaltic pumps that are reasonable. I have made it very clear to people online that you literally get what you pay for with these pumps.

    These are the pumps that I've found:

    http://www.williamson-shop.co.uk/100-series-with-dc-powered-motors-3586-p.asp

    http://www.aptinstruments.com/Merch...e=AI&Product_Code=SP100VO&Category_Code=SP100

    If you have any suggestions, advice - ANYTHING that can make this project better I welcome your opinion. The chemicals that these dose need to be some what accurate, but if they are off by 1 or 2 mL each time its not the end of the world.

    Thank you!
     
  11. MrChips

    Moderator

    Oct 2, 2009
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    I assume that a flow rate of 10ml/min ± 1 or 2 ml/min represents ± 10-20% variation.

    If that is the case I think a simple trimpot should work.
     
  12. james211

    Thread Starter Member

    May 29, 2012
    210
    2
    That's correct MrChips. Obviously the closer the better. Any specific trimpot suggestions? I did order a bourns turn counting dial, but if there are higher quality pots that I should consider please let me know.

    Also, are these sort of PWM controllers good enough to use? If you have a better suggestion please let me know.
    http://www.amazon.com/RioRand-trade...m_sbs_t_4?ie=UTF8&refRID=13QQ326ZRSDJ1DDF4NBZ

    Provided each motor tests out at the same pump rate, I should I only need one to set the speed for each motor. Keep in mind that only one pump will be running at a time, never more than one. If it were me personally I would probably have one for each pump, but to keep the cost low, I think most people will only want one controller.
     
  13. MrChips

    Moderator

    Oct 2, 2009
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    Based on what I understand you are trying to achieve, I would measure the value of the pot supplied with that controller and replace it with a 10-turn trimmer soldered directly to the board. You want a trimpot that has an easily accessible trim adjustment. There is a special screwdriver tool to use for this that prevents the driver from slipping off the trimmer screw head.

    [​IMG]

    [​IMG]
     
  14. james211

    Thread Starter Member

    May 29, 2012
    210
    2
    Ok, I'm good with that. But what if we wanted to use the bourns turn counting dials? Do you have a suggestion for a pot that would work with that?
     
  15. MrChips

    Moderator

    Oct 2, 2009
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    Bourns 10-turn pot and dial gives you excellent front panel control if that is what you desire. The combo will substantially add to the overall cost.
     
  16. vk6zgo

    Active Member

    Jul 21, 2012
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  17. james211

    Thread Starter Member

    May 29, 2012
    210
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    Last edited: Jul 7, 2014
  18. james211

    Thread Starter Member

    May 29, 2012
    210
    2
    I looked into these and it seems I could find them in 250mW & 500mW, that seems a bit low no?

    I guess what I don't know is, is the wattage for the pot based on the motor or the PWM controller?
     
    Last edited: Jul 7, 2014
  19. MrChips

    Moderator

    Oct 2, 2009
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    The pot is most likely used to control a timing parameter in the circuit.
    250mW or 500mW should be fine.
     
  20. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Thank you for providing more info on the project, that ALWAYS helps us to suggest solutions. :)


    That's the difference, the AC ones use an AC synchronous motor that turns at an exact fixed RPM regardless of load variances. Then there is little else to go wrong apart from tube softening and ageing etc.

    Providing fixed PWM to a DC motor pump won't give you a locked speed RPM. It will still vary based on PSU voltage (which you can fix with a voltage regulator) and with load, which unfortunately varies with temperature and head issues etc.

    Have you considered other options? You can get small cheap synchronous AC rotary liquid pumps (standard aquarium liquid pumps) and these will rotate at an exact fixed RPM at their rotary impeller. But they pump too much volume per minute.

    If you have a fixed head distance, you can use your microcontroller to run the AC pump for a few seconds each hour etc, and that programmed time gives you a very exact dosing. I think that would be cheaper, more accurate, and more reliable.
     
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