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MOSFET mystery

Discussion in 'General Electronics Discussion' started by NiGHTS, Nov 19, 2014.

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  1. NiGHTS


    Nov 19, 2014
    Hello there. I have a question about a simple circuit I am constructing.

    A principal part of this circuit involves two N-channel MOSFETs configured to act as high-amp switches to turn on and off a pair of LED strips. These MOSFETs are designed to control each LED strip independently and therefore should be logically isolated.

    For a reason currently unclear to me when one MOSFET is turned on and the other off, the MOSFET which is supposed to be off has current flowing through the drain.


    Attached is a schematic of my most recent and simplified experiment. As you can see from the graphic, MOSFET (B) appears to be configured so that its load is never driven, yet when the push button is closed and MOSFET (A) is turned on this seems to cause "LED Strip B" to also turn on.

    I tested the LED strips to make sure there was no short and each one works fine when powered directly without the MOSFETs. I also tested each transistor independently with its corresponding LED strip and they work as expected. Its only when both MOSFETs are involved that this problem manifests itself.

    After performing the test shown in the graphic, I removed "LED Strip B" and put a multimeter probe on the drain of MOSFET (B). When the switch is open the voltage reads a little over 6 Volts. This is curious to me because I figured it would be high impedance (floating), a capacitance effect maybe?

    When I close the switch the voltage is a solid 0 indicating that the transistor is turned on.

    Any ideas what could be wrong here and how I can independently control each LED strip?
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Hi there and welcome to Electronics Point :)

    Thanks for the clear problem description!

    Are the mounting tabs of the two MOSFETs electrically isolated from each other? The tabs are connected to the drains and if heatsinking is required, the two MOSFETs must either have separate heatsinks, or be insulated from a common heatsink using insulating pads.

    If that's not the problem, can you upload some pictures of your construction please.
    BobK likes this.
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Welcome to our forum.

    Do you use the TO220 case? The metal tab of this case in many (if not most) cases is connected to drain. If you put two transistors on a common heatsink, the heatsink can create a short circuit between the drains. In this case you need insulating washers and screws or separate heatsinks to insulate the transistors from each other.

    Otherwise your setup according to the schematic looks correct - unless you made a mistake in the real-world configuration.
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Two fools, same idea :)
    KrisBlueNZ likes this.
  5. NiGHTS


    Nov 19, 2014
    Thank you for your quick responses. You are both correct, the two transistors are joined by a common heat sink without electrical isolation. I'm sure this is what is causing the problem, it makes perfect sense!

    I will make certain to post more questions as they arise in my future projects. This has been a refreshing experience so far.

    Again, thank you very much!
  6. BobK


    Jan 5, 2010
    This thread is a testimony to the kind of results you can get when a problem is stated clearly and presents all the relevant information.

    Good work all.

    (*steve*) and KrisBlueNZ like this.
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