Mosfet RE current issue?

Discussion in 'General Electronics Chat' started by rougie, Mar 9, 2013.

  1. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    Hello,

    Now I understand that a mosfet is not exactly like a resistor as it just limits the current by restricting the current through the n channel. But what happens when we put a resistor in series with the n- channel as shown
    in attachment below. Here the amp meter reads 4.07ma.

    If I change R2 to 20 ohms instead of 51 ohms, the amp meter reads 2.94ma?

    Why??

    Wouldn't lowering the resistance allow more current through the circuit?? :confused:

    All help appreciated!
     
  2. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    Try connecting the source directly to Gnd and put the R2 resistor(s) in series on the drain side instead.
    Then you should get the results you expected.
     
    Last edited: Mar 9, 2013
  3. Uber_Goober

    Member

    Jan 19, 2013
    45
    4
    I just posted about my experience learning about MOSFETs as well. It's probably is due to the Vgs (the voltage drop from gate to source). Adding that resistor (with the voltage drop on the resistor) raises the voltage seen by the source. The corresponding drop in Vgs will result in a slightly higher internal on resistance Rds-On. On the device I'm working on the change is slight- it will go from about 0.3 ohms to 0.5 ohms. But that is probably enough to cause what you are seeing.
     
  4. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    Hi tubeguy, I did what you recommended and its a little better in the sense that now as my resistance lowers the current doesn't get lower !!!!!

    In other words, the current is always 4.78ma weather 51 or 20 ohms is connected! So wouldn't even know why I would need any R2 resistor there..... except for the odd situation where the mosfet would allow its channel to fully open hence R2 would be there for security....!!!!

    Its funny though, I was expecting the current to rise as R2 would get lower and by the same token, the current to lower as R2 would get higher! Bof!!!!

    Thanks all for your replies

    r
     
    Last edited: Mar 9, 2013
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