enter image description hereenter image description here

I'm making a magnet which gets turned of and on by a pwm signal and a irfz44n mosfet. The problem is; //The magnet gets current even though I don't apply current at the gate.// I tried a lot of things like switching the connection and such but it won't bother.

Might the MOSFETs be counterfeit? I bought them from BangGood (Chinese site).enter image description here

  • \$\begingroup\$ Please provide a schematic instead of the first photo. \$\endgroup\$ – try-catch-finally Feb 13 '17 at 20:44
  • \$\begingroup\$ Use the tool, edit your question and post a schematic thanks \$\endgroup\$ – Voltage Spike Feb 13 '17 at 20:48
  • 1
    \$\begingroup\$ Does it also get hot? If so you've probably blown the gate oxide, possibly even if it is not getting hot. Breadboards are a very poor choice for power circuits, especially those made with components that can easily suffer electrical damage like MOSFETs. \$\endgroup\$ – Chris Stratton Feb 13 '17 at 21:00
  • \$\begingroup\$ In your schematic, the 1k resistor is always conducting. The connection should be between the gate and ground. \$\endgroup\$ – Donald Gibson Feb 14 '17 at 1:16

A MOSFET doesn't have gate current, except when the gate voltage is changing. When your switch is closed, the gate charges, and when the switch is open, the gate stays charged and the device stays on. You need to provide a way to discharge the gate to turn it off. You also need to provide a path (such as a diode in antiparallel with your magnet coil) for the coil current when the MOSFET switches off, otherwise the coil inductance will cause a voltage spike that will avalanche and eventually destroy the MOSFET.

  • \$\begingroup\$ Seems like a reasonable solution/prevention, I will try it thank you :) \$\endgroup\$ – Electronics Knoopje Feb 13 '17 at 21:11

Your gate is floating and therefore your mosfet is partially turned on. Move the resistor to across the gate and source. You should also use a flyback diode across your solenoid to protect your mosfet from back emfs when it turns off. Of course it may already be damaged and the normal failure mode is for the mosfet to remain conducting.

  • \$\begingroup\$ Although the drawing seems to show that, the breadboard photo seems to show the resistor between the gate and a breadboard rail to which the source is also connected, ie presumably ground for some meaning thereof. \$\endgroup\$ – Chris Stratton Feb 13 '17 at 20:59
  • \$\begingroup\$ I will try it with you tips \$\endgroup\$ – Electronics Knoopje Feb 13 '17 at 21:10

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.