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Useing a mosfet to trigger solenoids. HELP!

Discussion in 'General Electronics Discussion' started by RatShack, Apr 6, 2011.

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  1. RatShack

    RatShack

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    Aug 11, 2009
    I am trying to build a circuit that activates some solenoids (4 in total) (2) 3 ohm solenoids and (2) 22 ohm solenoids.Solenoids are grounded to the structure.and have a positive input wire. I am planning on doing this with a Pic Chip, I Read that useing a mosfet would allow a faster switching then a standard Darlington setup. But i have never used mosfets before. does the coil resistance of the solenoid play a factor in what mosfet i use? The output of a pic is 5 volts and the voltage needed to power the solenoids is 14 volts. can someone please explain to me how i would do this. I am sorry for such a newbie question but i am still learning about all this stuff and Would like to learn as much as i can before i go to college for electronics engineering.

    Thank you!
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Essentially you would use the PIC to drive an NPN transistor to give you an open collector output. This output would be connected to the gate of a P channel mosfet used as a high-side switch. Finally a resistor is connected between the gate and the +ve (load) power supply to turn the mosfet off. Because it's a P channel mosfet, the source is also connected to +ve and the load is connected between drain and ground.
     
  3. RatShack

    RatShack

    6
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    Aug 11, 2009
    steve is there a way i can eliminate using a NPN entirely? Couldent i just keep the Output high and a pullup resistor to keep the MOSFEt turned off then bring it low to turn it on?
    Would like to keep parts count to a minimum but still have something reliable and that has extremly fast switching time.
     
  4. RatShack

    RatShack

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    Aug 11, 2009
    or is there a better alternative entirely?
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    The problem with eliminating the NPN transistor is that the protection diodes on the microcontroller will conduct to try to limit the output from exceeding Vcc.

    Whilst a small current here is not awfully damaging in and of itself, it does have a number of other drawbacks.

    Firstly, some CMOS chips can be driven into latchup which will short the power supply through your chip. This is *very* bad.

    Secondly, the output voltage will be clamped to a diode drop above the uC's Vcc which is no great improvement.

    If your uC has open collector (or open drain) outputs then you may be able to get away with this. You could also check the specs of the device and see if the outputs can be pulled above Vcc without problems.
     
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