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Charger using mosfet not working, help please

  1. May 5, 2014 #1
    Long story short, I have a circuit board with I programmed smart charger on it. The microcontroller is supposed to put out 5v to trigger an enhancement type n-channel mosfet which simply connects 15v to my battery.
    But only 3v is coming out of the mosfet.
    It is an irl510, what is going on?
     
  2. jcsd
  3. May 5, 2014 #2
    Post a schematic. At the moment we don't have enough info to analyze your issue.
     
  4. May 5, 2014 #3

    meBigGuy

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    Gold Member

    If you are trying to do high side switching, then you have a source follower.

    You need to do low side switching, or use a two stage switch (nmos driving pmos).
     
  5. May 13, 2014 #4
    sorry I took so long
    this is the jist of it:


    my computer is not leting me share the diagram I just made, all it is is 15v put on the mosfets drain, then the gate is triggered by a 5v output from a picaxe, and the source of the mosfet goes to the v+ of the battery to charge. is there something obvious i am saying which could be what is making it not work?
     
  6. May 13, 2014 #5

    Baluncore

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    The N channel enhancement mode mosfet source cannot be a higher voltage than the gate when on.
    You must ground the source and charge the battery between the drain and +15V supply.
     
  7. May 14, 2014 #6

    meBigGuy

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    Obviously you did not understand what I posted. What you are doing can be called high side switching. That is, you are trying to switch the high side of the power supply on and off. For that you need a pmos and you pull down the gate to turn on the pmos. But, the gate needs to go to 15V to turn off, so you add a pullup resistor. But, the 15V would damage the uP, so you add an nmos switch to pull down the gate.

    The way you are connected is as a pmos source follower. The source can never go higher than the gate.

    The alternative is to do low side switching with an nmos. That is, you switch the connection to ground. That is the simplest.
     
  8. May 17, 2014 #7
    I just hooked up the mosfet to switch the connection between the grounds of both battery and power supply.

    I have the gate connected to the picaxe, the drain connected to the battery's negative, and the source connected to the power supply negative.

    To test it I connected my volt meter between the drain and the power supply 15v.
    With no gate voltage I am reading pretty much full power on my meter. I even switched out the mosfet and it was still happeneing. Why!?
     
  9. May 17, 2014 #8

    Baluncore

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    You are measuring voltage not power. You should put some load between the drain and +15V, (across the meter) to get a correct reading. Try using a resistor of about 1k.

    Check you have the pinout correct for the mosfet.
     
  10. May 18, 2014 #9
    My pin out is correct, I put the 1k in,

    For some reason with nothing on the gate of the mosfet, I am reading 15v between 15v and drain.
     
  11. May 18, 2014 #10

    Baluncore

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    The gate should be connected to the source to turn off current through the MOSFET, that will result in power being removed from the drain load. Connect the gate to the source and check your measurement.

    When the gate is connected to a positive voltage relative to the source, the MOSFET will turn on and the voltage will appear across the load between drain and +15V.

    You must not let the gate float. If the gate floats it will pick up electrical noise from the local AC mains which is then rectified by the internal protection diodes to develop an electrostatic charge on the gate. It will probably turn the MOSFET on and give you the 15V across the load.
     
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