Need a component with opposite polarity of a mosfet

Discussion in 'General Electronics Chat' started by palerider2029, Mar 18, 2015.

  1. palerider2029

    Thread Starter New Member

    Mar 16, 2015
    18
    0
    This should be a total newb question.

    Okay I am working with a 3-pin mosfet.
    If + voltage is supplied. Negative (input) is allowed to pass.

    However, I need the opposite.

    +3v input
    +3v ouput
    100-300ma Current
    If - is supplied output is allowed. If no negative is present no output.

    Thanks!
     
    Last edited: Mar 18, 2015
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,532
    2,369
    What is the circuit configuration you are using this in?
    A P channel version needed maybe?
    Max.
     
    Roderick Young likes this.
  3. BobTPH

    Active Member

    Jun 5, 2013
    782
    114
    It is called a P-channel MOSFET.

    The source connects to a positive voltage. If the gate is negative compared to the source by some threshold voltage, it conducts from source to drain.

    Bob
     
    Roderick Young likes this.
  4. palerider2029

    Thread Starter New Member

    Mar 16, 2015
    18
    0
    How does this differ from the other mosfet? What channel is it?

    Thanks
     
  5. palerider2029

    Thread Starter New Member

    Mar 16, 2015
    18
    0
    So the one I am using is likely an N channel.

    Can I think of it as N channel. negative input/ouput
    P Channel positive input/ouput

    ?
     
  6. Reloadron

    Active Member

    Jan 15, 2015
    963
    232
    Start by reading this link. Without knowing you application it is about impossible to answer your questions. The link will explain P and N channel MOSFETs.

    Ron
     
    Roderick Young likes this.
  7. BobTPH

    Active Member

    Jun 5, 2013
    782
    114
    Typically, an N channel is used as a switch between ground and the load.
    P channel is used as a switch between a positive supply and the load.

    However, N channel MOSFETs are cheaper for the same performance and are sometimes used to switch the positive supply to the load. But in this case, the gate voltage has to be higher than the positive supply, which adds a lot of complexity.

    Bob
     
  8. palerider2029

    Thread Starter New Member

    Mar 16, 2015
    18
    0
    Let me explain fully what my issue is.

    I've got a bulb housing that is already grounded.

    My rotary switch has a negative pole.

    There is no way to turn the bulb on and off with the rotary switch. unless that negative pole on the switch can switch on the + supply to the bulb, since the bulb is always getting ground from its housing regardless of rotary switch position.

    Was hoping to find something that could allow me to control this bulb with my rotary switch.
    Prefer something in a small package like the TO-92-3
     
  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,797
    1,103
    Sounds like a P-channel MOSFET will do the job.
    What is the supply voltage?
    What are the current/wattage/voltage ratings of the bulb?
     
  10. palerider2029

    Thread Starter New Member

    Mar 16, 2015
    18
    0
    The supply voltage is 3v. The bulb is rated for 3v, and it draws less than 200ma of current.
     
  11. palerider2029

    Thread Starter New Member

    Mar 16, 2015
    18
    0
  12. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    could use a pnp transistor, with the switch grounding the base and the lamp connected to the emitter.,
     
  13. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,532
    2,369
    Also akin to the NPN PNP bipolar versions.
    That low a voltage maybe use bi-polar?
    Max.
     
  14. palerider2029

    Thread Starter New Member

    Mar 16, 2015
    18
    0
    Okay, I'm searching pnp transistors on Digikey, but the value selections are beyond my knowledge.

    I hate to ask, but if someone with experience could post a direct link to a TO-92-3 package pnp transistor and a p-channel mosfet that would likely work, with characteristics suited for 3v and less than 200ma of current. I'll just buy them and see what works best. Only way I'll learn is to get experience with a lot of this stuff.

    Thanks
     
  15. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,980
    744
    ZVP3306A, 2n5401 , to92 pin out
     
  16. palerider2029

    Thread Starter New Member

    Mar 16, 2015
    18
    0
    Both out of stock at digikey.
     
  17. Reloadron

    Active Member

    Jan 15, 2015
    963
    232
    2N2907 Should also work. Just about any common PNP low power TO92 switching transistor should work as long as it can handle your bulb current.

    Ron
     
    Roderick Young likes this.
  18. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,797
    1,103
    The bulb current at power-on is much higher than when the filament has reached running temperature. Is 200mA the 'cold' current or the 'hot' current?
     
  19. Roderick Young

    Member

    Feb 22, 2015
    408
    168
    And the old standby, the 2N3906.

    EDIT: I take it back, see Gopher's post below.
     
    Last edited: Mar 19, 2015
  20. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,030
    3,799
    The OP need a 200-300 mA, the 2N3906 will only do 100 -150 mA.

    If you use a PNP instead of a MOSFET, the PNP will use up 0.6 volts and leave only 2.4 volts of your 3.0 volts. If you are ordering from Digikey anyhow, look for a mosfet with a low "on" voltage. Here is a p-channel mosfet with 1.5 volt gate voltage, it will handle a few amps and less than 0 .1 ohm ON resistance.

    http://www.digikey.com/product-detail/en/AO3435/785-1544-1-ND/3621481

    All for less than $0.50
     
Loading...