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Such thing as a momentary relay?

Discussion in 'Beginner Electronics' started by nefarious, Sep 4, 2003.

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  1. nefarious

    nefarious Guest

    .... or a circuit to accomplish the same thing.

    Situation: Switching on 110VAC, need to turn on a 5VDC source for NO MORE
    than 1 sec about 50ms after the AC power is flowing.

    Any inexpensive solutions?

  2. Put the relay in series with a capacitor and resistor, driven through
    a diode? Also a small cap across the relay to give 50mS?
  3. nefarious

    nefarious Guest

    Forgive my lack of knowledge.. I know what each of the components functions
    are, but sizing etc I'll be lost on. Any chance of an ASCII schematic? ;-)

  4. I don't mean to be harsh, but if you are not able to put that together
    then you should not. It could be very dangerous for you. We are
    talking about lethal DC voltages from half-wave rectified AC.
    Why not start with some safer projects using batteries.
    Please accept my apologies.
  5. OK, here is the ASCII schematic, but only to look at:

    | | | |
    +-100K-+ +---Csmall_--+

    Note: relay may close again upon power-off

    It is a crappy circuit but may be good enough.
    The cap and resistor values depend on your relay.
    I'd use a 5 watt resistor for R and a photoflash cap for Cbig
    Relay is 220VAC or 120VAC.

    The diode is 1n4007. The circuit needs time to bleed off capacitor
    charges between invocations.

    MUCH safer to do this in low voltage DC. DC is easier to design but
    much more complicated circuit.
  6. No, it won't close again at power-off.What am i thinking?
  7. Oh yes, and the caps may EXPLODE if you put them in backwards.
  8. nefarious

    nefarious Guest

    No need to apologize, but I think putting in three phase circuits and
    discharging CRTs is more dangerous than anything we're talking about here
    ... and I do those things all the time ;-)


    ps... thanks for the circuit diagram
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