time delay/relay emulation

Discussion in 'The Projects Forum' started by bgraham, Jun 23, 2012.

  1. bgraham

    Thread Starter New Member

    Jun 23, 2012
    2
    0
    I'm building a circuit using both depletion and enhancement mode Mosfets.
    When power is off, the depletion mode Mosfet is conducting (of course and the enhancement mode one is off.
    The state of each toggles when power is applied.
    Problem is, I need about 30-50 mSec time delay between changeover states. i.e. there must be a period of time during each transition where both types are "off"
    1. Power Off, Depl Mosfet On, Enh Mosfet Off
    2. Power On, Depl Mosfet Off, (time delay), Enh Mosfet On
    3. Power Off, Enh Mosfet Off, (time delay), Depl Mosfet On.

    I'm trying to emulate an electromechanical relay where it is specified that NC and NO contacts can never be closed at the same time.
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    The Completed Projects Forum is for Completed Projects only. It is meant to allow members to show plans for projects they built so other members can duplicate them if desired. New threads are also automatically moderated per Moderator review for this reason. Your thread does not belong in this forum, and was moved here.
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    It's hard to speculate without a schematic. You could put a RC tank on the gate to slow down the switching, but this would be bad thing if you're switching a large current. (The MOSFET would not be fully saturated and would have to dissipate some power.) You could put an RC tank and a comparator on the gate, to accomplish the delay while still maintaining on/off control.
     
  4. #12

    Expert

    Nov 30, 2010
    16,321
    6,818
    Nothing I can add. wayneh covered everything I can think of.
     
  5. bgraham

    Thread Starter New Member

    Jun 23, 2012
    2
    0
    Apologies for original mispost to incorrect forum.
    I think I'll go back to my original plan using a micro + storage cap to delay turning on the eMosfet on power up. When power is removed, the micro shold run on the stored energy long enough to delay enabling the dMosfet.
    I was hoping someone had a clever solution.
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    Clever costs extra. ;)
    But seriously, without more information, there's no way to define clever. You were given two low-parts, easy-to-implement solutions. Not clever?
     
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