MOSFET appears off but something is wrong

Discussion in 'General Electronics Chat' started by Crispin, Jul 14, 2014.

  1. Crispin

    Thread Starter Member

    Jul 4, 2011
    88
    2
    Hi folks,

    Sorry, could not think of a better title...

    My circuit: [​IMG]

    MOSFET is IRL540N

    I have a Pb battery with a smart charger connected to it.
    When there is a load on the battery I want to disconnect the charger. Both switching of load and charger are arduino-controlled.

    Charger is a 25A charger, the battery is 3 390Ah batteries in parallel.

    In testing I substitute the smart charger for a PSU and the battery-to-be-charged with a dummy load. Everything works as it should. The MOSFET turns off, the load (and load LED) turns off and vice versa.

    All happy so far.

    Now I test with the battery charger(s) (I tried more than one) and the battery and this is where I get confused. The charger sees the battery even though the MOSFET is turned off. The voltage drop across the fet is 200mV so it is conducting even though the gate is held low. This happens with the fet disconnected from the arduino (but still with the pull-down resistor in place)

    I'm confused. Why does this work when it is a normal power source and load but swap the power source for a charger and the load for a discharged battery and it misbehaves?

    Grounds are all common, Arduino is powered from USB or separate battery.


    Any help on where I have gone wrong / misunderstood?


    TIA

    Crispin
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,014
    3,234
    Don't understand why the FET is not turning off, but I trust the LED has a resistor in series with it, otherwise you will zap the LED.
     
  3. Crispin

    Thread Starter Member

    Jul 4, 2011
    88
    2
    Yup, ommission in the diagram :(

    Forgot to mention, when the charger is connected and the fet appears on, I confirmed the gate was at 0V.

    Regards,
    Crispin
     
  4. RichardO

    Well-Known Member

    May 4, 2013
    1,235
    384
    The 200 mv on voltage is hard to explain, but... I wonder if you have the source and drain of the FET swapped. This would forward bias the body diode inside the FET.
     
  5. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,290
    1,255
    Maybe post the battery and charger hook up as well.
     
  6. Crispin

    Thread Starter Member

    Jul 4, 2011
    88
    2
    I have checked and rechecked the FET and am sure it's correct. I have also replaced it thinking the previous one was wrong.



    The charger is a CTEK charger which is what the 9V source (V1) is in the diagram. The 9V battery (B1) + switch is mimicking the Arduino (sorry, should have made it 5V).

    The battery to be charged is then connected at CN1.

    The charger seems to wait for a voltage before it starts. While disconnected from everything I bridged a 470ohm across it and it did not start. Open circuit is the same - it sits and waits.

    I then thought that maybe there is some sort of leakage somewhere (common ground everywhere) so and put a 1k resistor inline with it and the battery but again it failed to start up. Does not really prove anything though.


    The confusing part for me though is that this works just fine with the LED + resistive load (50W car bulb) but when I use the charger as a source and the battery-to-be-charged as a load, the FET turns on.

    My previous version used a relay and that was fine...
     
  7. pwdixon

    Member

    Oct 11, 2012
    488
    56
    I would be tempted to add a resistor in line with the LED of course.

    Not sure I completely understand the question though as your schematic doesn't really match your explanation. Perhaps a better schematic might help.
     
  8. Crispin

    Thread Starter Member

    Jul 4, 2011
    88
    2
    Sorry, in the real world there is a resistor inline with the LED. I forgot it on the diagram. (build first, draw diagram later ;) )

    Not sure how else to explain it - the diagram is what is is happening (except the LED's resistor).

    Maybe a different tack,
    I need to turn a battery charger on and off with an Arduino - why would the circuit above not work? The FET stays on all the time when the charger (V1) is connected even though the gate is pulled low (and confirmed low).

    What is wrong in the above diagram from what I need?
     
  9. pwdixon

    Member

    Oct 11, 2012
    488
    56
    So where's the Arduino?
     
Loading...