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Finding the right transistor

  1. Feb 25, 2013 #1
    Hi,
    I'm working with a microcontroller that outputs 5V, 25mA max from each pin. I need to source 120-150mA. I'm looking at 2n7000 MOSFET for the job. Spice simulations show the output current is 50mA when Vg = 5V (microcontroller output) and about 120mA when Vg = 5.5V. Since I'll be using over 20 pins from the microcontroller, amplifying the output for each pin is impractical. Is there another cheap and reliable MOSFET that can be substituted so this will work?

    Thanks
     
  2. jcsd
  3. Feb 25, 2013 #2

    MATLABdude

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    Science Advisor

    Are you using the transistor as a low-side switch? TO-92 packaged 2n7000s should be able to handle that load quite easily. I'd try it on a breadboard to be sure.

    Transistor as a switch (because I'm too lazy to mock it up in Circuit Lab):
    http://www.electronics-tutorials.ws/transistor/tran_7.html
     
  4. Feb 25, 2013 #3
    Is there some reason you are looking to MOSFET? It does not seem to be very high power - and for 20 outputs - that is a lot of descreet components. Did you consider a Driver IC - like this Darlington 8 Channel in one IC... https://www.sparkfun.com/products/312
     
  5. Feb 26, 2013 #4
    Thanks for the fast replies.

    The transistor is set up so that the drain is connected to a 5V supply, the source is connected to the load and the gate is connected to the microcontroller. The site you posted has interesting configurations, I'll take a look.
    The way I understand how MOSFETs work is the source current depends on how much voltage is put at the gate (in this case the voltage is .5V too low to give the 120mA).

    I didn't search around a lot for driver ICs. That IC you posted looks like it's just what I need, thanks! Do you know if there are PSpice models out there to run simulations with that device?
     
  6. Feb 26, 2013 #5
    The DS has a schematic for each stage.
     
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