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FiberGeek.....I need help with your VCCS schematic

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  • FiberGeek.....I need help with your VCCS schematic

    Hello FiberGeek,

    I am building a VCCS from your schematics. I would like to ask a few questions, if I may.

    I understand that the MOSFETS need to have their grounds isolated from the main circuit but should they also be isolated from each other?

    Reason asking is that I have a long heatsink with a fan already mounted on it that would work nicely for the three TO-220's.

    In the same line of thought, I purchased three .1 ohm 50 watt aluminum housed resistors that are connected to the "sink" leg of the TO-220's. Would there be any advantage to heatsinking these also? or could they be mounted directly to the circuit board? ( board is a "hobby" board from Radio Shack.)

    Final question.....

    On the schematic, the LM336BZ-2.5 ( labeled VR-1) does not have a connection on its 3rd leg. Does this get isolated from the circuit or is it to be connected to somewhere in the circuit?

    Since I don't understand much about how the VCCS actually works, I do appreciate your help.

    Thank you,

  • #2
    By MOSFET "ground" I think you mean the terminal marked "G" which is the gate, not ground. Connect the gates as shown; if the distance between the MOSFETs on your heatsink is less than 3 inches, no additional isolation is necessary.

    R9, R11, and R13 (0.3 Ohm 10W) are connected in parallel to provide 0.1 Ohm 30W. You can use 1 of your 0.1 Ohm 50W resistors. Heatsinking your resistor is not required. The "S" means source, not sink. The "D" means drain.

    Don't connect anything to pin 3 of VR1.

    I should write a Theory of Operation for this circuit and email it to Jnbsystems so he can post it.


    • #3
      Thanks Fibergeek,

      I appreciate the quick response.