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MOSFET Issues

Discussion in 'General Electronics Discussion' started by jereburke, Feb 12, 2018.

  1. jereburke

    jereburke

    6
    0
    Feb 12, 2018
    Hey guys, I attached a circuit schematic for my problem, it's simple but I can't seem to find the issue. I want to be able to discharge a 3.8V lithium ion battery over the resistor. I used a 1 ohm resistor so I should get ~3.8A (little lower due to the resistance of the MOSFET,) but it's limited at 2.2A and I'm not sure why. I even tried using a 0.05ohm resistor which should yield significant higher currents, but that capped at 2.7A on full duty cycle.

    Anyone know why I can't get desired currents?

    EDIT: The MOSFET for reference is a IRLZ44N (logic level)
     

    Attached Files:

  2. KMoffett

    KMoffett

    680
    65
    Jan 21, 2009
    Do you get 3.8A if you connect the battery and resistor in series, without the MOSFET? What happens to the battery voltage under the the 1Ω load?

    Ken
     
  3. jereburke

    jereburke

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    0
    Feb 12, 2018
    It will discharge at 3.8 amps if I ignore the FET
     
  4. BobK

    BobK

    7,480
    1,594
    Jan 5, 2010
    Do you have a common ground between the pulse source and the gate?

    What is the voltage of the pulses?

    Bob
     
  5. Nanren888

    Nanren888

    51
    7
    Nov 8, 2015
    "ignore"? remove?
    So you measured this value with a meter?
    This is a good battery if it does not drop at all under load.
    The pulse is referenced to the same ground?
    Seems if everything is right, then there is something else going on. Your hookup wire is ok for 4 amps?
     
  6. KMoffett

    KMoffett

    680
    65
    Jan 21, 2009
    Have you tried just driving the gate with a higher fixed Vg...like +12v?

    Ken
     
  7. Bluejets

    Bluejets

    3,091
    604
    Oct 5, 2014
    What is your definition of " full" duty cycle?
     
  8. jereburke

    jereburke

    6
    0
    Feb 12, 2018
    Basically using an arduino and writing the pin high
     
  9. jereburke

    jereburke

    6
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    Feb 12, 2018
    yes i do
     
  10. jereburke

    jereburke

    6
    0
    Feb 12, 2018
    yes remove. and i'm using 22 AWG jumper wires so it should be good for 4 amps
     
  11. jereburke

    jereburke

    6
    0
    Feb 12, 2018
    Hi Ken, I just gave it a try and still getting the limited current. Is there a chance the MOSFET isn't functioning properly?
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,080
    2,635
    Jan 21, 2010
    Is the mosfet getting hot?

    Can you read the voltage across the MOSFET while it's on with this load?

    Likewise can you measure the voltage across the battery?

    If you're doing that, you might as well measure the voltage across the load as well.

    In all cases, measure as close to the device as possible to prevent errors caused by the resistance of the leads caring the load current.
     
  13. WHONOES

    WHONOES

    586
    114
    May 20, 2017
    Try putting at least 5vdc on the gate and see what happens.
     
  14. AnalogKid

    AnalogKid

    2,234
    629
    Jun 10, 2015
    1. What is the voltage across the MOSFET drain-source pins when it is turned on by applying 12 V DC to the gate-source pins?

    2. Check your pinout. If the FET is in the circuit backwards, the battery is discharging through the body diode. This will reduce the discharge current.

    ak
     
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