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need basic schematic for a 12v gradual power up for small water pump

Discussion in 'General Electronics Discussion' started by CamaroMan, Aug 12, 2017.

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  1. CamaroMan


    Aug 12, 2017
    hi everyone - new here so howdy!

    I have very small electronic knowledge so please forgive me. I have soldered many years and know what current is -

    I have a small transfer pump we hook up to a car battery. alot like this unit but without the power button -

    Ours just whirs full speed instantly! It just goes on, it pumps ALOT though, only issue is it eats thru impellers! at $15 a pop I know what the issue is, the immediate spin up of the motor (its VERY powerful) is too much for the little soft rubber impeller and it keeps shearing the center piece loose making it just spin and not pump. One poor chap on amazon was on his 5th impeller already!

    Anyway - I was wondering if there was a little schematic I could wire up that would make it power up over say 3 seconds - almost like stepping gradually on a sewing machine pedal -

    I dont know what these things draw amp wise but im sure 10a wd prob be plenty - and could I wire it up in series to the positive lead?
  2. Bluejets


    Oct 5, 2014
    The link you supplied says the unit is only suitable for diesel fuel, hence I would say the sheering of the impeller.

    Wattage output is , according to the link, 175W so you are way out on the estimated current draw.
    (more like 15 to 20A)

    You could try one of the pwm speed controllers off Ebay but that would make it necessary to manually operate every time and most of the more economical ones only go to 10A.

    An alternative would be to try it on a 6 volt battery or put a 12v 100w auto headlight in series with the pump.
  3. duke37


    Jan 9, 2011
    You could use a capacitor and resistor to charge it slowly, then feed this voltage into a FET to gradually turn it on. If you get it right, the FET would dissipate little power when fully on since low resistance ones are available
  4. Alec_t


    Jul 7, 2015
    Methinks the FET would turn on fairly abruptly, so the current would not change smoothly over 3 sec. A few extra components could help to linearise the current ramp, but doing so would increase the heating of the FET.
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Torque is proportional to current. A low value resistor in series with the motor (sufficient to drop maybe 1V under normal operating conditions) may be sufficient to reduce the starting torque. You need to be careful that it doesn't reduce it so much that the pump won't start.

    (actually I like the headlight in series thing, but the cold resistance of the bulb will allow the torque t start very high before dropping it and then allowing it to rise again).

    If it really does draw 10A, then a 0.1Ω 20W resistor might do the trick. However, resistors of this power rating are not trivially cheap, so trying various values won't be economically viable.

    Do you have a multimeter? can you measure the resistance of the motor? And then, what is the value shown on your meter when the probes are shorted?
  6. CamaroMan


    Aug 12, 2017
    thanks guys - i found this and ordered it: at least it goes to 20a and is somewhat water (and spark!) proof. as for the shearing, the rubber seems to not react at all to the gas (plus I do flush it). I jb welded it and it held for about 5 hours - I think the force of the motor is just too much for the little rubber impeller, esp when primed - the spin up / torque on these things is quite insane!

    Anyway thanks for the replies! sure do appreciate it - ill have to manually crank it up to full speed, plus at least I have an ff switch now as well instead of popping it off the battery causing a spark each time -

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