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Circuit to power on devices when one other device is turned on?

Discussion in 'Beginner Electronics' started by Noozer, May 18, 2004.

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  1. Noozer

    Noozer Guest

    Does anyone have a simple circuit that would power on a few outlets when the
    power draw on another outlet is more than 15 watts or so?

    Basically, I want to turn on a powerbar whenever a specific outlet is
    drawing more than 15 watts and to turn off whenever that specific outlet
    draws under 15 or so watts.

  2. Tam/WB2TT

    Tam/WB2TT Guest

    Are you trying to turn on a printer and monitor when your PC is off standby?
    I would use a current transformer, and operate a relay when the current
    exceeds a certain value,

  3. These are around but tend to be more expensive than the usual power
    strip with a master switch. I'm not sure what the correct acronym for
    this is.

    If you want to build one, then you should look for a good, reasonably
    high current relay like the ones that are used on a remote controlled
    consumer device like a TV or VCR. I recently built a proximity switch
    and used a Matsushita relay labeled JR1a-DC12V-TV, whch pretty much sums
    up what its ratings are. These relays are reasonably sensitive and
    should handle 10 to 15 amps or more.

    You also have to remember that the master device may have quite a bit of
    variance in the amount of current it draws. So if you set your power
    strip improperly, it could shut off during periods of low current draw,
    such as when your monitor, for example, shows a screen that is
    completely black.

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  4. Noozer

    Noozer Guest

    Thanks for all the help folks, but I was hoping for something a little more

    Engineer I am not, but I can solder and etch a circuit board if necessary. I
    know that a 1/8 watt resistor won't do where a 1/4 watt is required, etc.

    With the number of external devices used on computers I would have thought
    that this would have been a pretty common request.

    Basically I want to plug my PC into the controlling outlet, so when the PC
    shuts off it will turn off my monitor and USB/Firewire drives.

    What would really be nice is a "Consumer Connection Protocol" where anything
    could be connected to anything - even just to power on/off the linked
    devices. It would be great for PC's, not to mention multimedia centers where
    you have a separate tuner, monitor and audio devices.
  5. Noozer

    Noozer Guest

    But I've never seen one and trying to find one here in Calgary has been

    What are these things officially named?
  6. Tweetldee

    Tweetldee Guest

    The proper term for these power strips is "Smart Strip". You can buy one at for $27.95 plus
    shipping. If you're really interested in building your own, a small current
    transformer feeding an opamp with a detector and relay driver should be
    fairly easy to design. However, for ~ $30, I wouldn't bother with a
    homebrew unit.. the electronic parts would likely cost $10 to $15, adding
    the cost of the electrical sockets and a suitable housing, plus the
    construction time could easily drive the total cost above the cost of the
    Smart Strip.

  7. Soeren

    Soeren Guest

    Use a 5V relay (with contacts rated for mains voltage at the current you
    need) or a Solid State Relay.

    Drive it from a USB-port or the game port (which both contains 5V).
  8. Noozer

    Noozer Guest

    That's what I was thinking at this point... but (and I haven't checked) I
    don't think USB will work if "wake on USB" is enabled as the 5V will always
    be present.

    Still... The gameport is a good idea. PS/2 port won't provide enough current
    to fire a relay, but maybe enough to trigger something else...

    I'll do some digging.
  9. The gameport is ok, but the PS/2 port (either mouse or keyboard) should
    also be good for 100mA. It shouldn't be too difficult to find a suitable
    relay. I would consider it preferable to use a 2-pole relay so that both
    poles get cut off. Also, don't forget the flywheel diode.

    You could of course build a device that doesn't need the auxiliary power
    from the PC. Other have offered their suggestions here, but have
    neglected to mention that the extra electronics need to have their own
    power supply. The overall design effort for doing this properly isn't
    trivial, and probably only justified when you want to manufacture in volume.

    Still, it would be interesting to know how the commercial devices are
    solving this problem. I've got a few ideas, bouncing around in my head
    but haven't tried any yet.

    A very simple solution would be to have a relay with a special
    high-current low-voltage AC coil that can be operated in series with the
    master device. If such a thing existed (and I haven't a clue whether it
    does - does anyone know?) it would be the only component needed except
    maybe for a fuse.
  10. Ms_Squiggles

    Ms_Squiggles Guest

    Noozer wrote:

    Silicon Chip magazine did a project for exactly this type of application in
    July 2003.

    You can order back issues of the magazine, or just a copy of the article if
    the magazine is out of stock.

    I just looked through my pile of magazines and the July edition is not
    there!!! The following little blurb is from the current Jacar Electronics

    Auto Power up Kit for Mains Applications.
    Refer Silicon Chip July 2003.
    Turn on all your equipment at the flick of a switch. This could include your
    PC and all its peripherals, TV and the rest of your home theatre, or even
    the workshop appliances.
    The kit connects to the main unit and switched on the power to the remaining
    units, saving you from powering up the other units seperately.

    Power level to switch slave:1 to 25 watts adjustable.
    Maximum load (master and slave outputs combined) 6 A @ 240V AC or 1440 W
    Standby current 18.5 mA

    Catalogue number KC-5363 $69.95 Australian.

    Jaycar do mail orders.

    I hope this helps

  11. Tam/WB2TT

    Tam/WB2TT Guest

    You could use a 5V DIP relay to operate the power relay, which would have a
    125 VAC coil. Radio Shack sells a double pole 10 A with a 125 VAC 16 ma

  12. Soeren

    Soeren Guest

    I guess you are right.

    The gameport can deliver almost whatever you need, as it is connected
    directly to the 5V line (the limiting factor is the circuit board traces
    current handling ability).
  13. (in danish)

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