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what transistor

Discussion in 'General Electronics Discussion' started by fastpcuser, Sep 3, 2012.

  1. fastpcuser

    fastpcuser

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    Apr 8, 2012
    I have a water pump with DC motor but I don't have a data sheet and couldn't find one. So I measured amps with multimeter and when it is running it uses around .6A continuously and when I try to stall it it uses 3.6 Amps, so my questions is should I use transistor which is rated above 3.6A or one with 1A should do it. Also I'm using the transistor as switch for relay.

    Thanks
     
  2. wingnut

    wingnut

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    Aug 9, 2012
    I have used TIP 122's and TIP 127's in the past to drive relays and motors as they are high power Darlington pairs and come with their own protective diode. If you use a relay, then that would mean no spike across the transistor. Just protect the base of the TIP with a 500 ohm resistor.
     
  3. fastpcuser

    fastpcuser

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    Apr 8, 2012
    Anything with smaller footprint (running out of space) ? I tried to use 2N2222 but it is rated only for .6A max and I think it started causing some problems. What I'm trying to do is auto top off water controller. Everything was working fine for about a week then I started having issues with relay not turning or for some reason the whole thing would turn off but it may be little different problems somewhere...
     
  4. wingnut

    wingnut

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    Aug 9, 2012
    The 2n222 as you say is a low power transistor, not really designed to drive much. Maybe after a week, the 2n222 overheated.
     
  5. wingnut

    wingnut

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    Aug 9, 2012
    How much water are we talking about in a day or a week? One litre, or 100 gallons? Just curious :)
     
  6. fastpcuser

    fastpcuser

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    Apr 8, 2012
    its for aquarium so it is about 4 liters a day, but it fills it when the level drops so maybe 250ml in few hours and it ony stays on for couple sec ( maybe 10-15)
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
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    Jan 21, 2010
    The other thing you'll need is a diode across the motor. If you don't have one the high voltages generated by the motor can damage the transistor quite quickly.
     
  8. fastpcuser

    fastpcuser

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    Apr 8, 2012
    Transistor is driving relay and I have diode between the transistor and the other end of the relay coil
     
  9. davenn

    davenn Moderator

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    1,745
    Sep 5, 2009
    no, thats not correct, and not what steve said ;)
    you need the diode across the relay, like this.....

    [​IMG]

    Ignore the voltages etc just note the wiring of the diode, relay coil and transistor :)

    cheers
    Dave
     

    Attached Files:

  10. Harald Kapp

    Harald Kapp Moderator Moderator

    9,137
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    Nov 17, 2011
    Can you post a schematic of your circuit? To me it looks like we are talking at cross-purposes. Does the transistor drive the relay, then the relay drives the motor? Or does the transistor drive relay and motor? Or do you have two transistors, one for the relay, one for the motor?
     
  11. wingnut

    wingnut

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    Aug 9, 2012
    The vessel you pump the water out of would also have to be replenished on a regular basis - not so?

    I have a similar need to yours and two mechanical rather than electronic solutions have come to my mind.

    1) To install a square bucket (like a toilet cistern) with a float valve like they used in old toilets. You can take a pipe from this to your aquarium and never have to worry again.

    2) The second idea I am toying with is to build a water dispenser similar to those used in bird cages but bigger (like an upside down bottle filled with water in a bowl). If the mouth of the bottle dries, more water runs out. The water reservoir could be far from your tank with the opening of the pipe at the level you want it in your aquarium.

    At any rate I am interested to know how you keep the vessel you pump water from, filled with water :)
     
  12. fastpcuser

    fastpcuser

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    Apr 8, 2012
     
  13. fastpcuser

    fastpcuser

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    Apr 8, 2012
    wingnut:

    I made and stand which looks like piece of furniture and put 12G tank inside. it last for 7-10 days they I refill it manually when I do my water chage. Also my Ato has 20 sec timeout on it so if it runs too long its either Float bad float switch or empty reservoir. after 20 sec I get error Led blinking and I know somethings is wrong. the pump won't turn on untill I hit reset button on the ATO.controller
     
  14. Harald Kapp

    Harald Kapp Moderator Moderator

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    1,846
    Nov 17, 2011
    In that case the transistor rating depends only on the relay, not on the motor.
    The relay, in turn, must be rated so it can turn on and off the motor.

    The diode in dave's schematic is correct. When turning off the relay, it will dissipate the energy that is stored in the relay coil.
    You should add a second diode (likewise catode to plus, anode to -) across the motor terminals. Otherwise the energy stored in the motor's coil will lead to arcing when the relay contacts open.
    Alternatively you can use a snubber circuit.
     
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