Using a Mosfet

Discussion in 'General Electronics Chat' started by tazntex, Jul 13, 2009.

  1. tazntex

    Thread Starter Member

    Sep 29, 2008
    Hello to all,
    I've have a few NTE2389 N-CH Enhancement Mode High speed switch. I would like to drive the gate with an output pin from my 16f628A.

    Ok, now the real question,

    I want to use this with the DRAIN connected to 12VDC and the SOURCE connected one terminal on the load, LAMP or Solenoid, and the other terminal to Ground. I've heard this can be done but have not seen it myself, But is this really possible?

    Another question,After searching this forum I seen suggestion for CHARGE PUMPS, or mosfet drivers and I am trying to find some examples of a charge pump. It seems that I must have more voltage at the gate than what I am trying to transfer to the load but how do I calculate the required voltage needed?. I would like to be able to use what I have in my junk box, etc instead of buying a component to drive these with.

    Also, I know how to calculate the base resistor for transistors, but is there a formula for the resistor need for the Gate? Several examples show a 1M or 10M resistor between gate and source, is there formula for that?

    I know I am asking a lot of questions but I've been searching the Forum and googled before posting. Just trying to learn.

    Thanks to all.
  2. John5788

    Active Member

    Apr 2, 2009
    depending on the size of the motor or load, you should look into something called a high-side driver.
  3. jpanhalt


    Jan 18, 2008
    Yes, it is called a high-side configuration.

    It is not a calculation. It is in the datasheet. For most N-channel mosfets, the gate needs to be about 10V higher than the source. Logic-level mosfets turn on at lower voltages.

    Then I recommend that you connect the load to your 12 VDC, the mosfet drain to the other terminal of the load, and the mosfet source to ground. That is so-called low-side.

    No. The gate resistor is for a different purpose than the base resistor. Fairchild has an app. note on choosing the gate resistor. In general, it is used to control the turn on (and off) time. If there is no resistor, you may get gate oscillation, so then one is added to slow down the turn on and prevent the oscillation. Another reason is to limit the maximum current from the driver, which comes to play when the driver is a PIC , which is limited to 25 mA. If you have a high-current source, then gate resistors may be from a few ohms to 50 ohms; if it is to protect the PIC, then I have used up to 200 ohm. The high-value resistor between gate and source is to ensure the gate is not floating, as when first powering up the circuit. It insures that the gate is off in that case.

    NOTE: It is important to be sure the mosfet you choose is logic level, if you intend to drive it directly from the PIC.

  4. tazntex

    Thread Starter Member

    Sep 29, 2008
    Thanks to all for answering my questions and for the quick reply.

    I will do some more research on high side drivers and visit the Fairchild site for the app note. I don't intend on driving the FET directly from the Pic that was why I asked about the charge pump.

    Thanks again.
  5. millwood


    that's called high side switching: where the switcher (your mosfet) sides at a potential higher than the load.

    you can do it with a level shifter, or a high side driver (discrete or ic).

    unless that somehow provides a distinct advantage to your particular application, I would suggest that you do NOT do it.