Mosfet not turning off

Discussion in 'Embedded Systems and Microcontrollers' started by soderdaen, Nov 26, 2015.

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  1. soderdaen

    Thread Starter New Member

    Nov 21, 2015
    Hey guys,

    I am working on a project which is a load controller for a water turbine in stand-alone operation.
    When the regular load is to low or the river is to fast, there is a power surplus which has to be dissipated. That's where the lamp comes in.
    The microcontroller realizes that too much power is available and gives a signal to the mosfet to switch on. The power supply is realized by a capacitive power supply.

    The problem is following: When I give a high signal (5V) to the Mosfet and it turns on the way it should. But if the signal is low (0V) the mosfet does not turn off. Instead the lamp is shining less bright. Even if put out the cable from the µC to the mosfet and hold it to the source/ground the light doesn't turn off.
    When I put the grounds of the mosfet (at source and the ground at the 1k resistor) to a separate "ground 2" the mosfet shows the same behaviour BUT now I can remove the signal from the µC and the lamp is totally off.
    The mosfet is working properly by the way. When test it all alone with a different ground it is working.

    I think it has something to do with the grounds but when I measure the signal based on the ground with a differential probe on the oscilloscope, then there is the right signal at the mosfet.

    My question is: Why does the mosfet behave like this and how can I make it work properly?

    Sorry for my grammar, I am from germany :p
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    Try connecting R8 (1k, but could be higher) directly between the gate and source of the FET. Ideally you should have a single-point ground so that high current via the FET drain and source doesn't cause 'ground bounce' which would change the Vgs voltage.
  3. ian field

    Distinguished Member

    Oct 27, 2012
    Did you actually verify by measurement that Vgs is going sufficiently below Vgs-thr?

    If you picked a MOSFET with very low RDSon, it'll conduct a fair bit until you get Vgs well down on VGSthr.
  4. wayneh


    Sep 9, 2010
    "Ground" is not always ground if current is flowing. Check to be sure you have good wiring that can carry the currents involved.
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
    Replace the fet with a Thyristor, that will switch off automatically as the voltage goes to zero.


    beachten Sie , transformatorlosen Netzteilen sind gefährlich !
    Last edited: Nov 26, 2015
  6. ScottWang


    Aug 23, 2012
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