mosfet gate pinout..help

Discussion in 'General Electronics Chat' started by hertz.12, Sep 4, 2015.

  1. hertz.12

    Thread Starter Member

    Apr 25, 2015
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    hi
    i have problem to find the mosfet gate pinout....i have two mosfets..irf540n and irf3205..both n-channel ..ok so the datasheet say the gate pin is the first pin left..for both mosfets..but when i connect the the middle pin(drain) to volt supply and the right pin(source) to the load..the load turn on!with out any volt to the gate>>the gate pulldown to ground by 1kohm..ok both mosfets are new and it is first time use..so i tried this..connect the supply to first pin and the the third pin to the load..ok the load still off so i connect 5v to the middle pin and the load turn on!!! on both mosfets!!!
    i am confuse...realy
    are both datasheet wrong??????????
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    If the gate is floating (unconnected), the inherent gate capacitance may well charge up through leakage currents or interference pickup and hence turn the FET on. If the gate has a 1k pull-down resistor then this shouldn't happen. Both datasheets are correct.
    The best way to switch the load is to connect it between drain and +V, connect source to ground and apply the control voltage between gate and ground. If your load is a LED, don't forget to include a current-limiting resistor in series with it.
     
  3. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Are you absolutely certain of the battery polarity? Can you post an actual picture of the experiment shown on the left image (mosfet.jpg)?

    John
     
  4. pwdixon

    Member

    Oct 11, 2012
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    You have the bulb in the source line to battery negative, it'll work if you put the bulb in the drain line to the positive supply.
     
  5. ScottWang

    Moderator

    Aug 23, 2012
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    N type mosfet testing circuit.

    [​IMG]
     
    Roderick Young likes this.
  6. hertz.12

    Thread Starter Member

    Apr 25, 2015
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    thanks all.. i will put the load between drain and +V..and the source to ground..i hope it will wrok
     
  7. ScottWang

    Moderator

    Aug 23, 2012
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    Your two mosfet are all belong to Vgs=10V when Rds(on), but you can try to use 5V.
     
  8. hertz.12

    Thread Starter Member

    Apr 25, 2015
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    ok..they look like damaged mosfets! i will buy another

    thanks all
     
  9. ScottWang

    Moderator

    Aug 23, 2012
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    If you have the power supply then can using Vdd=10V, or using +12V of the old computer power supply, but you need to increase the values of R3, you can also to buy a 9V cookies battery.
     
  10. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Remember that MOSFETs are static-sensitive.
     
  11. Roderick Young

    Member

    Feb 22, 2015
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    I wouldn't be so quick to discard the MOSFET as damaged, especially for the purposes of a simple experiment.

    Most likely, you have source and drain reversed. Take a look at the circuit Scott Wang posted in reply #5, and make sure things are connected that way. The vast majority of power MOSFETs have a body diode embedded in them, as shown in the picture. That means that if you put the n-channel MOSFET in a circuit where the source is a higher voltage than the drain, the device will conduct.

    EDIT: I just looked at your original circuit. Looks all right (the picture on the left). Was the MOSFET bought new, or did it come from an unknown source? One quick (but not 100%) check for whether the thing is burned out is to remove the 1k resistor, and measure between gate and source with an ohmmeter. If you see anything other than infinite, the MOSFET probably burned in service. A better test is to connect gate to source, then measure between drain and source both polarities with an ohmmeter. One way should be infinite, the other way should conduct.
     
    Last edited: Sep 5, 2015
  12. hertz.12

    Thread Starter Member

    Apr 25, 2015
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    i am sure the source = 0v and the drain = 5.2v.....the gate pull down to 0v by 1kohm ..and still the load conduct! i tried the circuit Scott Wang posted and same results
     
  13. Roderick Young

    Member

    Feb 22, 2015
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    Ok, sounds like it's burned out. Sorry.
     
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