FET not supplying constant Current

Discussion in 'The Projects Forum' started by SamPhil, Apr 23, 2016.

  1. SamPhil

    Thread Starter New Member

    Apr 23, 2016
    Hi All,

    New to the forum, used the search utility prior to posting this and didn't find anything similar on the first couple of pages so here is my question.

    I have a couple of FETs lying around and I wanted to get a rough idea for what their properties are (Vtr, K). Therefore I'm supplying 12V to both the gate and drain with a LED and resistor coming out of the source. What I've found is that id is not a constant current. The led coming connected to the source blinks continuously and the current is impossible to measure as it is constant jumping. I thought this could be a problem with the power source however when wiring up an LED and resistor the LED remains on constantly. Tried both the FETs I have with identical results and also tried with different resistors. Has anyone experienced this before? What could be causing this?

    Pictures are attached:
  2. crutschow


    Mar 14, 2008
    What you found is perfectly consistent with your circuit.
    Why would you think applying 12V to the gate and drain with the load on the source would generate a constant current?

    You pictures don't tell us much. Post a schematic.
    Are they MOSFETs or junction FETs?
  3. #12


    Nov 30, 2010
  4. shortbus

    AAC Fanatic!

    Sep 30, 2009
    Do your search again for, "high side switch", this is what you have built, the search should show why it doesn't work like you thought. Took me some time to figure it out too.
  5. dannyf

    Well-Known Member

    Sep 13, 2015
    The way you connected it, it is not supposed to be a constant current source, for either mosfet or jfet.
  6. Picbuster

    Well-Known Member

    Dec 2, 2013
    You connected a load between Source and ground ( correct me when wrong)
    When the fet goes into conduction current will flow and a voltage over the load will go up in relation to the gate.
    Resulting in less G-S voltage hence the conduction reduced resulting in a higher G-S voltage and this start over again.
    Frequency is depending on resistance G-S capacity, impedance of load and connected voltage.