MOSFET Issues

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

  1. jereburke

    jereburke

    Joined:
    Feb 12, 2018
    Messages:
    6
    Likes Received:
    0
    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:

    jereburke, Feb 12, 2018
    #1
    1. Advertisements

  2. jereburke

    KMoffett

    Joined:
    Jan 21, 2009
    Messages:
    659
    Likes Received:
    63
    Location:
    Minnesota USA
    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
     
    KMoffett, Feb 12, 2018
    #2
    1. Advertisements

  3. jereburke

    jereburke

    Joined:
    Feb 12, 2018
    Messages:
    6
    Likes Received:
    0
    It will discharge at 3.8 amps if I ignore the FET
     
    jereburke, Feb 12, 2018
    #3
  4. jereburke

    BobK

    Joined:
    Jan 5, 2010
    Messages:
    6,855
    Likes Received:
    1,401
    Location:
    Worcester, MA
    Do you have a common ground between the pulse source and the gate?

    What is the voltage of the pulses?

    Bob
     
    BobK, Feb 12, 2018
    #4
  5. jereburke

    Nanren888

    Joined:
    Nov 8, 2015
    Messages:
    21
    Likes Received:
    2
    "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?
     
    Nanren888, Feb 12, 2018
    #5
  6. jereburke

    KMoffett

    Joined:
    Jan 21, 2009
    Messages:
    659
    Likes Received:
    63
    Location:
    Minnesota USA
    Have you tried just driving the gate with a higher fixed Vg...like +12v?

    Ken
     
    KMoffett, Feb 12, 2018
    #6
  7. jereburke

    Bluejets

    Joined:
    Oct 5, 2014
    Messages:
    2,235
    Likes Received:
    445
    What is your definition of " full" duty cycle?
     
    Bluejets, Feb 12, 2018
    #7
  8. jereburke

    jereburke

    Joined:
    Feb 12, 2018
    Messages:
    6
    Likes Received:
    0
    Basically using an arduino and writing the pin high
     
    jereburke, Feb 13, 2018
    #8
  9. jereburke

    jereburke

    Joined:
    Feb 12, 2018
    Messages:
    6
    Likes Received:
    0
    yes i do
     
    jereburke, Feb 13, 2018
    #9
  10. jereburke

    jereburke

    Joined:
    Feb 12, 2018
    Messages:
    6
    Likes Received:
    0
    yes remove. and i'm using 22 AWG jumper wires so it should be good for 4 amps
     
    jereburke, Feb 13, 2018
    #10
  11. jereburke

    jereburke

    Joined:
    Feb 12, 2018
    Messages:
    6
    Likes Received:
    0
    Hi Ken, I just gave it a try and still getting the limited current. Is there a chance the MOSFET isn't functioning properly?
     
    jereburke, Feb 13, 2018
    #11
  12. jereburke

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Joined:
    Jan 21, 2010
    Messages:
    23,541
    Likes Received:
    2,179
    Location:
    Mid way between Beijing and the Ronne Ice Shelf
    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.
     
    (*steve*), Feb 13, 2018
    #12
  13. jereburke

    WHONOES

    Joined:
    May 20, 2017
    Messages:
    108
    Likes Received:
    15
    Location:
    Oxfordshire
    Try putting at least 5vdc on the gate and see what happens.
     
    WHONOES, Feb 13, 2018
    #13
  14. jereburke

    AnalogKid

    Joined:
    Jun 10, 2015
    Messages:
    1,996
    Likes Received:
    562
    Location:
    Ohio - GMT-5
    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
     
    AnalogKid, Feb 13, 2018
    #14
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.