mosfet problem

Discussion in 'The Projects Forum' started by Turkish, Sep 10, 2010.

  1. Turkish

    Thread Starter New Member

    May 16, 2010
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    I'm using a mosfet to switch 4 leds powerd by a 12v power supply and aarduino to switch it but no matter which combo i try i cant get it to switch its either on or off what am i doing wrong?
    mosfet to-92 http://www.st.com/stonline/books/pdf/docs/10202.pdf
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Can you post a schematic over here?
    That way we can see where the trouble lies.

    Bertus
     
  3. Turkish

    Thread Starter New Member

    May 16, 2010
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  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The main problem lies in the fact that you are using "normal" mosfets.
    Those will conduct fully when the gat voltage is 10 Volts.
    The arduino will give 5 Volts at max at its output.

    You can solve the problem by using "logic gate" mosfets.
    Those will conduct fully when the gate voltage is 5 Volts.

    Bertus
     
  5. Turkish

    Thread Starter New Member

    May 16, 2010
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  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The transistor you show is a bipolair junction transistor (BJT).

    If you want to stick with mosfets take a look at the IRL510.
    (perhaps a bit overkill, but it will give you some headroom).

    Both datasheets are attached.

    Bertus
     
  7. Turkish

    Thread Starter New Member

    May 16, 2010
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  8. wayneh

    Expert

    Sep 9, 2010
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    You COULD add a comparator (eg. LM339 available at the Shack) in between the 5v. signal and the gate of the FET. This would allow you to pull the gate up to 12v (full on) when the comparator is open ("on", input "+" voltage exceeds input "-" voltage) and down to ground when the comparator closes "off" (opposite of above), with the comparator on/off depending on the input voltage vs., say a 1v. reference.

    Of the transistors you listed, I'd pick the MPS2222A.
     
    Last edited: Sep 10, 2010
  9. Ghar

    Active Member

    Mar 8, 2010
    655
    72
    Everyone, please look at the datasheet for Rds-on.

    Rds-on = 1.8 ohms typical, 5 ohms max at Vgs = 10V, ID = 0.5A
    Rds-on = 2 ohms typical, 5.3 ohms max at Vgs = 4.5V, ID = 0.5A

    5V is entirely sufficient to get good conduction.
    Your probably lies in something else, specifically it looks like you're trying to use the NMOS on the high-side.
    This won't work with 5V, you need to switch it to low-side.

    Refer to this thread:
    http://forum.allaboutcircuits.com/showthread.php?t=42605

    Specifically post #7 and #11
     
    Last edited: Sep 10, 2010
  10. wayneh

    Expert

    Sep 9, 2010
    12,123
    3,048
    Agreed, the circuit should work at 5V, if not efficiently. The circuit supplied doesn't make it clear (a red flag) if it is wired high or low. And the problem may be even simpler: On that MOSFET device, the pins are G-S-D. As drawn, the gate is not connected to the input, hence the on-or-off behavior without switching.
     
  11. marshallf3

    Well-Known Member

    Jul 26, 2010
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    As drawn there's no common ground either.
     
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