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What type of relay (or something else) for a 19" monitor???

Discussion in 'Electronic Design' started by SA Dev, Jan 8, 2004.

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  1. SA Dev

    SA Dev Guest

    Hi,

    I've got a 19" monitor that I want to power on and off automatically.
    Currently it has no power switch and is on when plugged in. Is there some
    sort of 120V AC relay I could use? I have 12V and 5V signals I could tap to
    tell the relay whether I want the monitor on or off. Any recommendations or
    part numbers?

    Thanks,

    SA Dev
     
  2. For long life and low noise, solid state relays are very good, though
    you may have to parallel the monitor with a night light bulb, to
    achieve the holding current necessary to keep them on, since the
    monitor supply will draw pulses of current only at the peaks of the
    cycle.

    Something like:
    http://dkc3.digikey.com/PDF/T041/1029.pdf
     
  3. Ian Stirling

    Ian Stirling Guest

    If modern, it may support this already.
    If it's connected to a PC, look for a software setting to turn it off.
    Interrupting the syncs should turn it off.
     
  4. SA Dev

    SA Dev Guest

    John,
    Ok. I don't understand what you mean about the night light bulb? The input
    voltage would not be coming from the monitor, but from a PC. Do I still
    need the night light? Why?

    SA Dev
     
  5. The power switch in an AC solid state relay is pulsed on at every line
    voltage zero crossing. The TRIAC switch needs a minimum load current
    to stay latched on for the rest of the half cycle. Any device powered
    by a DC supply with a capacitor energy storage filter will draw
    significant current only near the peak voltage each half cycle, so
    there is no holding current to keep the TRIAC switch on from the line
    voltage zero crossing to the peak. Adding a small resistive (or
    resistor in series with a capacitor, to draw more current as the
    voltage rises toward the peak) provides the holding current. With
    some relays, this can be as simple as a 2 watt resistor of 100 ohms or
    so in series with a microfarad (or less) of capacitance rated for
    across the AC line voltage. But a 7 watt bulb is simpler. Paint it
    black if the light bothers you.
     
  6. The power switch in an AC solid state relay is pulsed on at every line
    voltage zero crossing. The TRIAC switch needs a minimum load current
    to stay latched on for the rest of the half cycle. Any device powered
    by a DC supply with a capacitor energy storage filter will draw
    significant current only near the peak voltage each half cycle, so
    there is no holding current to keep the TRIAC switch on from the line
    voltage zero crossing to the peak. Adding a small resistive (or
    resistor in series with a capacitor, to draw more current as the
    voltage rises toward the peak) provides the holding current. With
    some relays, this can be as simple as a 2 watt resistor of 100 ohms or
    so in series with a microfarad (or less) of capacitance rated for
    across the AC line voltage. But a 7 watt bulb is simpler. Paint it
    black if the light bothers you.
     
  7. SA Dev

    SA Dev Guest

    Ian,
    It is not a standard computer monitor, but an arcade monitor. It has no
    physical power switch and also does not turn itself off even if not plugged
    into a computer. It is on when plugged in, off when unplugged.

    What I plan to do is tap a 5v or 12v signal from the PC power supply to run
    to a relay of some type to turn the monitor on or off by applying or
    removing its AC power.

    When doing this, do I need to get a relay that has 2 positions, so it can
    connect and disconnect the common and hot AC line? Does the ground line
    always remain connected?

    Thanks,

    SA Dev
     
  8. SA Dev

    SA Dev Guest

    John,
    I guess I'm still confused, I am *not* electrically brillaint. I am used to
    mechanical relays where you apply some electricity (I would use 12V DC from
    a power supply) and then it connects some terminals. Is this AC solid state
    relay different?

    Thanks,

    SA Dev
     
  9. Ian Stirling

    Ian Stirling Guest

    Ok, in that case, the simplest way is probably to use a conventional
    (not solid state) relay.
    Practically any relay will do.
    You want one with a 5V/12V coil, able to swich 110/220V (or whatever
    the monitor voltage is) at the rated current of the monitor.)
    (ideally you should double this current or so.)


    You only run the live wire through it, connecting one end to the moving
    contact, and one to either the normally open or normally closed (depending
    on whether you want the monitor on when the relay has or has power to the
    coil).
     
  10. SA Dev

    SA Dev Guest

    Ian,
    Which wire is the live wire? If I'm looking at an AC recept, I see a longer
    slot, shorter slot, and hole (ground). Is the shorter slot the live wire
    (black)? Then the longer would be the common (white)?

    Thanks,

    Alan
     
  11. N. Thornton

    N. Thornton Guest

    A standard mechanical relay wil do the job nicely - you must add a
    diode across the coil to avoid damage to the transistor driving it.

    Part numbers - find one yourself.


    Regards, NT
     
  12. Ian Stirling

    Ian Stirling Guest

    Do you have a multimeter?
    Measure the resistance from live at the plug to the monitor connections.
    (You could also use a double pole relay, and disconnect both live and
    neutral, ground is left connected.)ZZ
     
  13. Yes. it is turned on by applying a low voltage DC to an internal
    light emitting diode in series with an internal resistor or other
    current regulating mechanism. Most of the ones on the catalog page I
    sent operate with any DC voltage applied between between 3 and 32
    volts. The light beam turns on the electronic switching circuit,
    instead of connecting metal contacts.

    If you prefer an electromechanical relay, there are lots of them that
    will do the job, but they take more drive current so you will have to
    build a transistor driver to boost a logic level signal to drive them,
    and put a diode across the coil to prevent a high voltage spike when
    you turn them off.
     
  14. scada

    scada Guest

    Be sure to use a diode across the relay coil, reverse biased. If not, you
    run the risk of destroying the PC power supply, or mother board! The relay
    coil can produce hundreds of volts when turned off, the diode suppresses
    most of that voltage. The + side of the coil wire goes to the "Cathode"
    the - to the Anode. Use something with a high "PIV" rating, the 1N4007
    should do nicely (PIV 1000V). I would use the 12V line (used mostly for
    motor drive: CD, HD, etc.), not the 5V. I'm not saying you won't still fry
    something in the PC! Do it at your own risk! If I were to do it, I would use
    the 12V as a signal to an external "Box" that was independently powered to
    do the deed!
     
  15. SA Dev

    SA Dev Guest

    Hi,
    How/Why does the coil produce so much voltage? I was planning on using the
    PC power supply just to send the 12V, which should only be about 75ma. The
    AC will not be coming from the power supply at all. Does this still pose an
    issue?

    Thanks for the help!

    Alan
     
  16. No. The diode is needed only if the relay coil is disenergized by a
    switching device. If it is disenergized by a decaying power supply
    that it stays connected to, it cannot produce any voltage. Its
    magnetic energy will just hold up the supply for a few extra
    microseconds. A nonevent.
     
  17. If you are talking about the standard US 15 Amp 120 Volt AC outlet,
    yes, the long pin "SHOULD" be ground. That is, if it is wired properly,
    and not fed from a three phase source where neither side is grounded. In
    other words, don't depend on it, unless you check every outlet you plan
    on using before plugging it in.


    --
    We now return you to our normally scheduled programming.

    Take a look at this little cutie! ;-)
    http://home.earthlink.net/~mike.terrell/photos.html

    Michael A. Terrell
    Central Florida
     
  18. SA Dev

    SA Dev Guest

    John,
    So I'm safe in doing this:

    Get a 12V DC relay that is rated at 120VAC 10A.

    Connect a power connector from my PC power supply to the 12V DC coil part of
    the relay.
    Connect the monitor AC hot line to the normally open poles of the relay.

    When I turn on my PC, the relay will connect, connecting the monitor's
    power.
    When I turn off the PC, the relay will release, disconnecting the monitor's
    power.

    Is this right? Any other gotcha's?

    Thanks so much for the help,

    Alan
     
  19. SA Dev

    SA Dev Guest

    Hi Michael,
    The long round pin = ground, right?

    Then the wider slot is common (if looking at a recept with the ground
    underneath the two slots, it is the left wider slot).

    Then the narrower slow is the hot (if looking at a recept with the ground
    underneath the two slots, it is the left wider slot).

    Is this right?

    Thanks,

    Alan
     
  20. That should work, fine.
     
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