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Selecting a Potentiometer

  1. Jun 28, 2016 #1
    Hi. Please could anyone advise me on selecting an appropriate Ohms and and Watts range for a Potentiometer. This is to energize the speed controller on a car heater blower motor that has lost its supply from the ECU pompously known as the Climate Control ! The blower motor is 12 Volt and fused at 30 Amps and this is the control unit on Ebay http://www.ebay.co.uk/itm/360825657777?_trksid=p2055119.m1438.l2649&ssPageName=STRK:MEBIDX:IT

    In case this link is not allowed it consists of a pair of MJ11016 transistors.

    With the cheapest of multimeters I have measured the current when I've feed 13.5 volts in to the activating terminal as a .05 amp blip to start, then motor running steady at .03 amps. With the voltage reduced (by cobbling together various resistors) to approx. 3 volts the current was a steady .01 amps. Thank You for any advice.
     
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  3. Jun 29, 2016 #2

    berkeman

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    Welcome to the PF.

    You don't usually use a potentiometer to control the speed of a motor. You use a PWM unit to control the motor speed...
     
  4. Jun 29, 2016 #3

    CWatters

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    I assume that while you made these measurements the motor was running so you could judge the effect..

    If that was the slowest you want the motor to run then that corresponds to the maximum value of resistance...

    13.5-3V = 10.5
    Rmax = V/I = 10.5/0.01 = 1K

    Was that with no resistors? eg you connected the activating terminal directly to 13.5V using the meter leads and measured 0.05/0.03 amp and the motor went at max speed? If I have understood correctly then...

    Rmin = 0.

    So you want a potentiometer that can go from 0 to 1K.

    The potentiometer will dissipate... 13.5 * 0.05 = 0.67W so pick one rated for say >1W

    If you find a cheap source perhaps order values either side of 1K. Smaller will increase the minimum speed. Larger will reduce the minimum speed. The max speed should be unaffected.

    The remaining problem is how to turn the motor off completely. If the motor turns off with the activating pin disconnected then get a potentiometer with a built in switch.

    Edit: I should add that this is based entirely on the info you posted. I've no experience of how this motor controller works. The only circuit diagram I could find online was for a 95-98 model that appeared to have 3 fixed speeds rather than continuously variable but other models may have more refined controls.
     
    Last edited: Jun 29, 2016
  5. Jun 29, 2016 #4

    CWatters

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    Perhaps..
    http://www.mouser.co.uk/ProductDetail/CTS/026TB32R102B1B1/?qs=TO73vhinuSGqRJniBflDOg%3d%3d [Broken]
     
    Last edited by a moderator: May 8, 2017
  6. Jun 29, 2016 #5
    Hi Berkeman and thanks for your advice. In this instance the purpose of the potentiometer is to replicate the input to the existing motor speed control that no longer gets an input from the ECU.
     
  7. Jun 29, 2016 #6

    berkeman

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    Oh, duh! Sorry that I misinterpreted what you posted. o0)

    BTW, here is a very good tutorial about potentiometers from Bourns: http://www.bourns.com/pdfs/trmrpmr.pdf [Broken]
     
    Last edited by a moderator: May 8, 2017
  8. Jun 29, 2016 #7

    jim hardy

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    Like Berleman , I suspect the device expects a PWM signal

    here's from the transistor datasheet
    upload_2016-6-29_14-49-49.png
    that heatsink does not look to me big enough to handle the fifty or so watts you'd dissipate with DC control
    but i could be wrong....
    and i have no idea what is the relay for, perhaps "Max Cool " fan speed ?

    feel the transistors while motor is running and turn it off before they burn your fingers.

    I think you ought to to consider a 555 timer hobby kit with a knob to provide a pwm signal to that controller.
     
  9. Jun 29, 2016 #8
    Hi CWatters and many thanks for your succinct mathematical deductions. Yes the motor was running whilst testing with the fan impeller on but in 'free air' so not under any real load.
    I did try starting it with the impeller momentarily held and 13.5 volts applied which gave a steady 2.5 amps on the meter. This was done to get an idea of the value for a supply fuse rating.
    When it was running with a 3 volt supply my thoughts indeed were " why would anyone need less speed?" so I will take your advice and try the switched 1 and 2 K ohms 2 Watts pots that I have found for the princely sum of £1.68 each inc. postage.
     
  10. Jun 29, 2016 #9
    Yes the Bourns is very good but to big a commitment of time to sort one problem on what is a mobile dog kennel !
     
    Last edited by a moderator: May 8, 2017
  11. Jun 29, 2016 #10
    It might expect a PWM signal but I'm changing its diet to a varied voltage! ... as it makes the fan run without trusting a dead computer
    I also do not know what the bobbin device is for. It reminds me of the voltage/current regulators from days of dynamos. Having partially dismantled another one of these controllers it appears that the coil is connected to the collectors and the contacts to the emitters and into some potted very small diodes which is all beyond me.
    I will check the temperature as you suggest but with a thermometer not my fingers!... as the data sheet said some horrendous temperature and the heat sink temperature sensor no longer serves a purpose it might be worth stopping it back from running at maximum . Thank you for your thoughts on this matter
     
  12. Jun 29, 2016 #11

    CWatters

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    Check if there is already a resistor in the base circuit. If not then you could/will have a problem when the new pot is set to zero ohms.
     
  13. Jun 29, 2016 #12

    jim hardy

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    I like your spirit !
    Good Luck , and keep us posted ?
    old jim
     
  14. Jun 30, 2016 #13
    Oh! Not a sign of any components with coloured bands . The terminal that I'm feeding goes directly to the base of the first transistor and a small offshoot track (approx .5 mm) into some ant sized components, mainly black with silver nos. on them. These are covered in a material similar to that what is used to protect engineering cutter bits etc. Regards.
     
  15. Jun 30, 2016 #14

    Averagesupernova

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    Then you can expect the transistor driver to fail. The reason it is PWM is so that the transistor is either completely on or completely off. Never in between.
     
  16. Jun 30, 2016 #15

    CWatters

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    In that case you need a resistor in series with the new pot to limit the base current when the pot is turned down to zero.

    Re the PWM vs DC issue...

    The problem is that any power not dissipated in the fan will be dissipated in the transistors. So they are likely to fail unless the heatsink is big enough. The heatsink is probably sized assuming PWM so the transistors may well get too hot/fail.

    You can get PWM DC motor speed controllers quite cheap on ebay (perhaps <£10). Do you know how much current the fan motor draws?
     
  17. Jun 30, 2016 #16
    As this vehicle is 16 years old I'd assumed it was prior to the use of such technology, and that all the regulation was built in with the transistors. Quite happy to install a PWM controller if this is the best option. I am doing this as a favour so I'd rather do it once.
    The fan motor current is unknown at present other than its fused at 30 amps. Will attempt to find the amp clamp tomorrow, "I might be gone some time" Best Regards
     
  18. Jul 3, 2016 #17
    Measured the current of the fan with a quality amp clamp as 20 amps with the fan installed in its airbox. This was with the engine running and a voltage of 14.4 volts to the fan and feed to the activating terminal of the regulator, which is now drawing 0.08 amps. Have looked at Ebay offerings and it seems these type of controllers http://www.ebay.co.uk/itm/New-12-50...d-Control-PWM-HHO-RC-Controller-/400891849379 would replace/bypass the existing fan speed regulator. Figures for the Khz I assume is 200 not the 12000 on this sellers page !
    As the existing regulator is mounted with its heatsink fins in the forced airflow it makes me wonder how one of these replacement controllers would fare situated behind the dash panel/fascia in still air.
    Will look further into Jim Hardy's suggestion of a 555 kit when I return p.m. but what I've seen so far is a bit daunting.
     
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