floating switch

Discussion in 'The Projects Forum' started by ees2, Mar 29, 2012.

  1. ees2

    Thread Starter New Member

    Mar 29, 2012
    1
    0
    I have a circuit with a constant DC input, the output is three resistors in series with same value.

    If you short the middle resistor R2 by turning an analog switch crossing the R2 on and off, the voltage cross R2 will be either 1/3 V or 0V.

    Now if I set up a control circuit to control the p type mosfet (pmos) to switch the R2 on and off, I notice that I cannot short the R2 completely.

    The control circuit consists two npn transistors, one pnp, one diode, and a few resistors to pull the pmos gate high or low. the control circuit reference point is the ground.

    Any suggestion?

    Thanks.
     
  2. strantor

    AAC Fanatic!

    Oct 3, 2010
    4,302
    1,988
    You're probably not turning the MOSFET completely on due to low gate drive voltage or your MOSFET has a high Rds(on)
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,155
    3,061
    +1

    You don't say what R2 is, but assuming it's much larger than Rds(on), the problem is that the MOSFET is not fully "on", and that's usually because the gate voltage is not enough. An N-channel MOSFET might be a better choice, btw, since it will have a lower Rds(on) all else equal.

    A diagram with labeled components would be nice, so we don't have to guess.
     
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