switch delay

Discussion in 'General Electronics Chat' started by samjesse, Oct 1, 2010.

  1. samjesse

    Thread Starter Active Member

    Sep 14, 2008
    202
    0
    Hi

    I have a switch which when flipped it opens a circuit A and closes a circuit B. 5v, 200mA.
    I want when the switching happens from circuit A to circuit B, that it delays the opening of circuit A while it closes circuit B immediately. i.e. both circuits will be on for a second before circuit A opens.
    How can I go about this?

    thx
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    What kind of circuit is it?

    You can use a 555 with a relay or a relay with timer functions.
     
  3. samjesse

    Thread Starter Active Member

    Sep 14, 2008
    202
    0
    do you have a link to a 555 circuit for this objective? thx it is for a device on my car.
     
  4. DonQ

    Active Member

    May 6, 2009
    320
    11
    If you want to try a search, look for "make before break". That is the term for the type of switch you are describing.

    If you want anything more than a few milliseconds, you will have to go with an electronic solution. Two relays (with relatively high coil resistances) with big capacitors across them may work. The caps would charge almost instantaneously when switched on, then each capacitor would hold its relay on for a short interval after it was released. This is a "TD" (Time to De-energize) in the PLC world.
     
  5. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    With the right caps in a well designed relay coil snubber circuit you could do this pretty easily with two separate relays.

    Is there any specific delay that you need or does it need to be just enough such that the power source isn't interrupted?
     
  6. samjesse

    Thread Starter Active Member

    Sep 14, 2008
    202
    0
    After reading the Time-delay relays link on this site. I need a Normally Closed Timed Open relay which I can control from 1/2 a second to 3 seconds range.
    The circuit to be controlled caries 12 DCV and 5A. the circuit has 4 to 8 loads "coils" in parallel, which produce spikes when each load is turned on/off.

    Since there is no off the shelf NCTO relay, I may have to design a RC circuit with a variable capacitor. Will take me some time to do the homework.

    I would appreciate any help.

    thx
     
  7. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    For what you're wanting the cap would have to be big, and they don't make large variable caps.

    As far as searching for relays, I know this may be overkill but if it's ever been used in industrial control chances are you might find one at http://www.graingers.com
     
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