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7.2 V 3A Transistor Switching

Discussion in 'General Electronics Discussion' started by tiwari.sachin, Nov 28, 2010.

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  1. tiwari.sachin

    tiwari.sachin

    4
    0
    Nov 28, 2010
    Hello

    I have a device (a printer) which will work on 7.2V and requires about 2.5 amps. I need to design a circuit that will supply the voltage only when required. Hence i decided to use a transistor for the switching. I am not too sure which transistor to use. Can anybody kindly suggest me the option for the same. Or for that matter, will a simple switching circuit actually work for this configuration?

    How about using TIP31.

    Datasheet of TIP31 : http://www.fairchildsemi.com/ds/TI/TIP31A.pdf

    I need to switch it via controller which works on 3.3V. ie. High will be 3.3V max.

    Your advice will be very helpful.

    Regards
    Sachin
     
  2. tiwari.sachin

    tiwari.sachin

    4
    0
    Nov 28, 2010
    Attached is the schematic that i am using now
     

    Attached Files:

  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,838
    Jan 21, 2010
    You may need to look at the specs for that transistor (I cannot find them) as I doubt that you will get sufficient base drive to switch the current you require unless the voltage at the input is significantly higher than 7.2 volts (at 3.3 volts it won't work at all).

    You may be better off googling for a high side switch using a PNP transistor or a P-Channel mosfet.

    Note that with these options the control input will be a current source as opposed to it being a current sink. This may cause issues depending on what is producing the control signal.

    If you use P Channel or PNP transistor that I suggest, you can switch this on and off using an npn transistor which will then be compatible with your active high 3.3v signal.

    edit: this thread at SparkFun provides the answer.
     
    Last edited: Nov 28, 2010
  4. tiwari.sachin

    tiwari.sachin

    4
    0
    Nov 28, 2010
    So i should have a MOSFET that will drive this voltage while the MOSFET is actually turned ON using a NPN tansistor.

    I hope i am getting this right
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,838
    Jan 21, 2010
    I'd do something like this:

    [​IMG]

    With the input open or held low, the transistor is off and any gate charge is led away by the 100k resistor. This holds (or turns) the mosfet OFF.

    When the input rises above about 1 volt, the transistor turns on and drives the gate of the mosfet negative with respect to the drain, turning it on.

    Choose a mosfet that is capable of carrying 5 to 10 Amps (depending on the nature of your load) and with an Rds(on) well under an ohm (go for 0.1 ohms or less) at a gate voltage of -5V. Here is one option.

    The transistor can be any general purpose small signal npn transistor (a BC547 would be appropriate).

    Depending on how rapidly you're turning it on and off, you may need to consider heatsinking, or allow larger gate currents to speed the mosfet's transition through its saturation region.
     

    Attached Files:

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