Mosfet 'normal' behaviour.

Discussion in 'General Electronics Chat' started by cjdelphi, Jun 17, 2009.

  1. cjdelphi

    Thread Starter New Member

    Mar 26, 2009
    I noticed 2 different things happening depending on the power transistor I use.

    If i put >5v through at gate, let's say we're powering an LED (it could be anything) the source is from 12v the drain goes through a resistor into an LED simple as 123.

    Now which is normal, connect 5v to Gate the LED lights up, remove 5v from the gate the LED goes off that happens about half the time, the rest of them put 5v through the gate the LED lights up but if you simply pull the wire out (not supplying 0v just pull the plug) the LED stays lit up and continues to do so until 0v is passed where as the rest as soon as you pull the wire out to the gate the LED stops being fed.

    I can give you an example of these two power transistors the TIP31C, 5v fed to gate LED comes on, pulling the wire or supplying 0v makes LED go off.

    now the IRFZ44N supply 5v the LED comes on, simply pull the wire out from the gate the LED stays on, it's not until you feed 0v to gate will the transistor switch off the LED.

    is this normal or are the ones that fail to switch off the LED when i pull the cable out faulty and a sign that a transistor is bad?
  2. SgtWookie


    Jul 17, 2007
    The TIP31 is a bjt, or bipolar junction transistor. It does not have a "gate", it has a "base". It requires current on the base to control the current on the collector. When the source of current through the base is removed, the transistor turns off. Thus, it is a current controlled device.

    The IRFZ44N is an enhanced power MOSFET. It has a gate, a source, and a drain. It does not have a "base". The gate of a power MOSFET acts more or less like a capacitor. You charge the gate to a voltage relative to the source to turn it on or off. If Vgs=0v, the MOSFET is fully turned off. If Vgs=10v, the MOSFET is fully turned ON. If Vgs is somewhere between 2v and 8v, it is only partially conducting, and may burn up if the current through it is high enough. There are logic-level MOSFETs available which will turn on fully when Vgs is greater than around 4.5v. The IRLZ44 is an example (notice the "L" in the part number).

    There is not a real current path from the gate to the source or drain terminals; the impedance is very high. Once the gate is charged to a certain voltage level, it may stay there for quite a while.

    If you want the MOSFET to automatically turn off when you remove the voltage, connect a 10k Ohm resistor from the gate terminal to the source terminal. This is frequently done to keep MOSFETs turned off if there is a failure in the MOSFETs' gate driver circuit.
  3. eblc1388

    AAC Fanatic!

    Nov 28, 2008
    The gate is a capacitor and it stores the charges, keeping the MOSFET turning ON, until the charge is leak away.

    Try placing a 1K resistor from gate to source and now every MOSFET you own will behave "normal" in your case.
  4. Audioguru


    Dec 20, 2007
    If you try to feed +5V to the base of a TIP31 and its emitter is connected to 0V then the transistor or the battery will blow up without having a series resistor to limit the current.
  5. cjdelphi

    Thread Starter New Member

    Mar 26, 2009

    Luckily for me I run all my little projects through an LM317T Regulator (practically indestructible :D )

    And thanks for explaining, makes perfect sense now, one runs on Current the other on Voltage (basically)