AC/DC adapter, modifying car air pump and understanding dc motors

Discussion in 'The Projects Forum' started by jamalAbadin, Feb 22, 2016.

  1. jamalAbadin

    Thread Starter New Member

    Feb 22, 2016
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    Hello and thank you for helping me out with my doubts.

    I've been trying to convert a small air pump from working with a car lighter to working with an AC/DC adapter, for home use.

    The pump is identical to this one but is rated for 300 psi instead of 250: http://www.aliexpress.com/item/250PS...050035638.html

    The pump has no amperage information, it only states that it's 12V DC.
    I've searched around and some of this pumps say that the car lighter should be of 15 amps or higher(??). I think that's a very high current value for a small pump.

    I've opened it up and it has a DC motor that moves a piston. When the piston comes down, air enters through a small hole, and when it goes up it exits through the air hose (which has the pressure gauge attached).

    I've tried wiring the motor with an adapter that's rated for 12V, but can only provide 500mA.When I turned it on, the motor ran really slow and almost no air came out of the pump.

    My questions are:
    • I have other adapters. None of them is rated for 12V, but they have bigger current capacity than the one I've tried. So, will another adapter with higher or lower voltage work with this pump? I'd be much appreciated if someone could give me a basic explanation of how different voltages and amperage values affect the working of the motor. By that I mean if it would work with lower/higher voltage, and if it would require the same amount of amps at a different voltage.

    • How does the motor draw more or less current? Does it have a certain speed at which it runs, and if it runs slower than that speed, it pulls out more amps? And if so, does changing the voltage affect that standard speed?

    • This is kind of a different question. The air pump is a bit loud, I deduce that most of it's noise is a result of vibration, so would rubber padding at strategic places make the pump quieter?

    • The pump will be used only to fill up inflatables and basketballs, as opposed to car tires. Will that diminish the current needed by the pump? If so, how many amps do you think it needs.
    Once again, thank you for all your help.

    P.S. I can make a list of the other adapters I have in case that helps.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    A DC motor operated with full voltage draws maximum current at switch on until revs are attained to generate a BEMF DC that opposes the supply voltage, so minimum current at this point until load reduces the BEMF.
    Did you test the current on a automotive battery direct and check the current?
    A piston compressor is under maximum load until air is drawn.
    In many noise-sensitive applications, rubber mounts or feet are fitted.
    Max.
     
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  3. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    The specs actually tell you the power requirement: 90W which is 7.5A @12V but the starting current will be much more than this.

    The most effective way to make it quieter is to fit it in an enclosure but you must also maintain ventilation.
     
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  4. debe

    Well-Known Member

    Sep 21, 2010
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    In the specifications it says 90Watts, that's 7.5Amps on 12V DC. The current draw increases with the pressure required. Your power supply will need to be 12v dc at around 10 amps to be on the safe side.
     
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  5. jamalAbadin

    Thread Starter New Member

    Feb 22, 2016
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    Thank you all for your replies.

    My pump is equal to the one on the link, but I don't know if it's the same.
    Replying to Max, I did not measure the current while plugged to the car. Is it possible to determine that current by measuring the resistance of the motor while it's at rest?

    Also, my problem is I don't have another 12V adapter and so was wondering if I could use an adapter with a different voltage that would also require less current.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    A nice source of 12vdc compressors is if you have wrecker near you, the automatic suspension leveler compressor in most modern cars is very quiet in operation.
    Max.
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    No, the resistance of the motor at rest is just the initial high current until rpm is attained, (see the BEMF reference), also it is a bad way to measure it, it is more exact to use a low voltage supply and and a ammeter with a locked rotor condition to measure current and work from there for armature resistance .
    Any adapter has to be able to handle this initial current , however.
    Max.
     
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  8. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Have you got an old computer psu to hand, they give out 12v @ 20amps..
     
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  9. alfacliff

    Well-Known Member

    Dec 13, 2013
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    if you have a 12 volt battery charger rated at 9 amps or more, use it. the motor dosnt require filtered dc or regulation in the air pump.
     
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  10. jamalAbadin

    Thread Starter New Member

    Feb 22, 2016
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    Unfortunately no, only an old laptop torn apart, but i Have no idea how to use the power source from that one. Has for the battery charger I'll look around, but are battery chargers rated to 9A?
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Generally higher than that, unless a controlled trickle charger?
    Max.
     
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  12. alfacliff

    Well-Known Member

    Dec 13, 2013
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    a car battery charger, not a laptop charger.
     
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  13. jamalAbadin

    Thread Starter New Member

    Feb 22, 2016
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    Nop, no car battery charger. Any other ideas on things that have 12v adapters?
     
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