Stir plate potentiometer way too hot

Discussion in 'Homebrewing' started by corbmoster, Aug 12, 2015.

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  1. corbmoster

    corbmoster Pundit (848) Dec 15, 2014 Texas
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    Here is a pic I took while building my stir plate. When I used it earlier this week, the potentiometer control knob got so freaking hot! Like if I held on to it for more than a second, it would have burned me. It did work correctly. I would have thought it would have spun a bigger vortex into the starter (1750 ml), but not so much. Just a dimple. Though that was enough, in 24 hrs it was done.

    Specifics (left to right) : Power supply = 9v 650 mA. Goes to toggle switch. Goes to potentiometer 25 Ohm 3 Watt. Goes to 120mm computer case fan. Unfortunately I am not provided the draw info of the fan. It is my understanding that fans draw about 1 - 2.5 watts and about 0.13 - 0.25 Amps.

    So am I missing something? I don't see why it should be getting so hot.

    [​IMG]
     
    #1 corbmoster, Aug 12, 2015
  2. corbmoster

    corbmoster Pundit (848) Dec 15, 2014 Texas
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    I think perhaps I should have wired this as a rheostat, and not a potentiometer. This would be done by removing the white wire from the far left post on the pot while leaving it connected directly to the fan. Which would probably give me a faster RPM speed. But, it would not come to a stop if I turn the dial all the way down either (I think) (which I wanted for having to re-set a thrown stir bar).

    (post #3 here diagrams what I was trying to say about wiring at a rheostat instead of a pot)
     
    #2 corbmoster, Aug 12, 2015
  3. PortLargo

    PortLargo Pooh-Bah (1,831) Oct 19, 2012 Florida
    Pooh-Bah

    You're potentiometer is hot because it's supplying a load to reduce the voltage to the fan, that's its job. My guess is you have an econ-model, that would explain the poor heat dissipation.

    I had a similar cheapy DIY'er ... it would spin and the yeast didn't grumble that much. Then the pot stopped potting, followed by fan not fanning. Replaced with the basic model from Stirstarters and it is 1,000% better: puts a decent vortex in a full 3L flask, quiet as a church-house mouse, and the pot serves as on/off switch ... plus it looks good and the yeast are crazy about it.

    If you really want to rock, get a Stirstarters stirplate and scavenge the pot ... oh wait, there may be an easier way ...
     
    #3 PortLargo, Aug 12, 2015
    Last edited: Aug 12, 2015
  4. billandsuz

    billandsuz Pooh-Bah (2,097) Sep 1, 2004 New York
    Pooh-Bah

    you do need to match the power supply to the rheostat. the device dumps energy in the form of heat, so expect it to become hot. if you overload the device with too much juice you need to run it on the lowest setting and dump a lot of energy. the wall adapter supplies constant current, the rheostat accepts the high energy input and outputs low energy. the energy has to go somewhere.

    find the dude/lady at Radio Shack and get the proper rheostat. they are cheap. a lot cheaper than buying a new wall wort. but you may have a 6 volt adapter around that can be sacrificed. i had to go through one rheostat to get it right.

    there must be at least a few electronic wizards on this board somewhere, so hopefully the electrical engineering homebrewer can dial in the perfect combination for your adapter.
    Cheers.
     
    #4 billandsuz, Aug 12, 2015
  5. GreenKrusty101

    GreenKrusty101 Initiate (0) Dec 4, 2008 Nevada

    Use your own power...stir plates are over-rated for home brewers : )
     
    #5 GreenKrusty101, Aug 13, 2015
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  6. Beer_Baron123

    Beer_Baron123 Initiate (0) Apr 19, 2015 Canada (ON)

    This! My water bottle on the counter that I shake everytime I walk past has yet to disappoint me.
     
    #6 Beer_Baron123, Aug 13, 2015
    boothbeer, bushycook and GreenKrusty101 like this.
  7. ventura78

    ventura78 Pundit (972) Nov 22, 2003 Massachusetts

    I agree with GreenKrustry101 and beer baron 123, I make 3 gallon starters for big batches and just shake them when I walk by. When I use wyeast 3522 I shake it less, that yeast is nuts enough on its own.
     
    #7 ventura78, Aug 13, 2015
  8. billandsuz

    billandsuz Pooh-Bah (2,097) Sep 1, 2004 New York
    Pooh-Bah

    i think you guys are really missing the point of homebrewing. of course there is an easier way. but more home built stuff = better brewery. better brewery = happy homebrewer. at least i think so.
    Cheers.
     
    #8 billandsuz, Aug 13, 2015
    corbmoster likes this.
  9. corbmoster

    corbmoster Pundit (848) Dec 15, 2014 Texas
    Trader

    ^ This.
     
    #9 corbmoster, Aug 13, 2015
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  10. PortLargo

    PortLargo Pooh-Bah (1,831) Oct 19, 2012 Florida
    Pooh-Bah

    Can't say I agree with your circuit analysis here. The transformer is supplying a constant 9 volts, the current is determined by the resistance of the load (pot/fan). This is identical to a home circuit. Voltage is a unit of electrical potential ... a potentiometer meters this potential.

    For the OP: this is a long read but explains it in detail (the juicy part is the middle third)
    http://www3.eng.cam.ac.uk/DesignOffice/mdp/electric_web/DC/DC_6.html.

    Your real problem is you threw together some parts with mediocre results (same as I built mine). For instance you have a 9v transformer but almost all computer fans are 12v ...there's a 25% loss of speed. Still guessing your pot is a cheapo and/or is mismatched. It must drop the voltage to vary the fan speed and sounds like it's doing this poorly. My homemade stirplate crapped out after about 18 months, combination of cheap and unbalanced parts.

    The details to correctly size all the components are in the link ... or live with what you have ... or go commercial. Don't listen to the naysayers, stirplates kick ass.
     
    #10 PortLargo, Aug 13, 2015
  11. ChuckHardslab

    ChuckHardslab Maven (1,251) Jan 25, 2012 Texas

    I am unsure of your circuit. why do you have all 3 posts wired on your potentiometer. Usually the 2 end posts act as a fixed resistor and the middle post is the variable resistor. Wired the way you have done it there is a constant load/voltage drop applied across the pot that will dissipate the energy in the form of heat. Pull the white wire off the pot and just leave it hooked up to the fan. Leave the black wires configured like you have them. That will be your variable resistor wired in series. That should work correctly and keep your pot from getting so hot.
     
    #11 ChuckHardslab, Aug 13, 2015
  12. corbmoster

    corbmoster Pundit (848) Dec 15, 2014 Texas
    Trader

    Also stir plates saves time. My last couple shaking starters took 2-3 days. A stir plate starter will be done in less than 24. I've hear some people say 16 hrs.

    At any rate, yes, it should get a little hot. But I don't think it should be getting as hot as it is.

    @PortLargo : I'll take a look at your link tomorrow when I have time. Thanks in advance. I don't think the pot is a cheapo, I'm sure it is made to spec. I think that if it was wired as a rheostat, their would be less resistance, and thus less heat.

    If your read doesn't help, and my rheostat idea doesn't work, I'll use a digital solution.
     
    #12 corbmoster, Aug 13, 2015
  13. corbmoster

    corbmoster Pundit (848) Dec 15, 2014 Texas
    Trader

    You just described what I was asking if I should be doing.

    (post #2)
     
    #13 corbmoster, Aug 13, 2015
  14. ChuckHardslab

    ChuckHardslab Maven (1,251) Jan 25, 2012 Texas

    It should work just fine and is a very quick mod.
     
    #14 ChuckHardslab, Aug 13, 2015
  15. GreenKrusty101

    GreenKrusty101 Initiate (0) Dec 4, 2008 Nevada

    So do AK-47s...doesn't mean you need one to make excellent homebrew., IMHO : )
     
    #15 GreenKrusty101, Aug 13, 2015
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  16. corbmoster

    corbmoster Pundit (848) Dec 15, 2014 Texas
    Trader

    True. But it is a significant improvement. AK is to SKS as stir plates are to shaking.
     
    #16 corbmoster, Aug 13, 2015
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  17. billandsuz

    billandsuz Pooh-Bah (2,097) Sep 1, 2004 New York
    Pooh-Bah

    amps watts current volts. whose to say?

     
    #17 billandsuz, Aug 13, 2015
    corbmoster and inchrisin like this.
  18. CurtFromHershey

    CurtFromHershey Initiate (0) Oct 4, 2012 Minnesota

    That's a pretty nice looking setup you got, so you may not want to, but I'm pretty sure a few small holes drilled into the wood would keep the unit itself a bit cooler.
     
    #18 CurtFromHershey, Aug 13, 2015
  19. corbmoster

    corbmoster Pundit (848) Dec 15, 2014 Texas
    Trader

    That occurred to me as well, and I shimmed the open by 0.25 inches to allow air flow. It didn't make a difference. I'll try removing the white wire later.
     
    #19 corbmoster, Aug 13, 2015
  20. corbmoster

    corbmoster Pundit (848) Dec 15, 2014 Texas
    Trader

    Results and discussion:
    I removed the white wire from the pot and secured the end with heat shrink to insulate. Function test was positive. As I suggested, it does spin at the lowest speed. However, it does not have enough force to turn a bar magnet resting on the lid. It was allowed to run all night at it's highest speed, without stirring anything. Pot knob was cool to the touch after 9 hours of continual use. I'm not sure if the top speed was any faster. And I don't really have a way to quantify that (I guess I could, but it's not worth it). The next time I need a starter, I'll make an identical one and see if the vortex is any bigger. I considered getting a 12 V DC power supply, but since 9V did a good enough job, I'll just leave well enough alone. Since there is no more high heat being produced, I think I might trim down the fan blades to extend the life span (and probably increase the speed) of the fan.

    Thanks for everyone's input.
    [​IMG]
     
    #20 corbmoster, Aug 14, 2015
    Last edited: Aug 14, 2015
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