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Extension block switches neutral not live

Discussion in 'Electrical Engineering' started by Chris Glen-Smith, Jan 5, 2005.

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  1. Hi

    I have a master slave extension block that automatically switches on/off
    6 slave sockets when whatever is plugged into a master socket is
    switched on/off, in this case my PC. It is labelled TCM, I bought it
    almost a year ago.

    The other day it stopped working and me being me I took it apart to see
    what was wrong (I was trained as an electronics engineer).

    I found the problem (a blown thermal fuse) but I was gob smacked to
    discover that it switches the neutral and not the live, i.e. the slave
    sockets are turned off by isolating the neutral and the live is still
    live. *

    Is that legal? (I'm in the UK).

    * That explained an odd behaviour I saw a few months ago when a PC
    speaker system I bought remained powered when plugged into the slave
    sockets even when they were off, it guess it earthed some part of it's
    PSU for some reason.

  2. Rusty

    Rusty Guest

    I wouldn't think it is legal in any part of the world. It certainly
    isn't in North America. The potential for shock and death is very
    high, especially if a child or infirm adult touched something hot
    while grounded. Throw it away and check any replacement you buy.
  3. Roy Q.T.

    Roy Q.T. Guest

    Only if it is a Low Voltage Control Master/Slave System

    Where the AC [household] Outlet is shut on & off by the Masters Command
    to Trigger the Slaves.

    No Killer AC actually runs through it, Only Conventional Receptacle
    Dangers Apply, check your Polarities.
  4. Roy Q.T.

    Roy Q.T. Guest

    Is it an X-10 system ?

    That's very cool, how much did it cost you?
  5. Roy Q.T.

    Roy Q.T. Guest

    If it keeps giving you trouble, do llike Rusty says, throw it away
    immediatley ! }:-o teeheehee

    Toss It All in Rustys' Garbage Can}-)

    or givem here, i'll dispose ovem for ya };)
  6. Get a power strip with a switch on,like mine.When you switch off your
    computer, switch off that too, so that everything connected on it will be
    off.They are cheap, here you can get one for less than 5 euro.
  7. Is this product CE marked?

    I think that you will find that the device is not legal and you should
    speak to the Trading Standards Office in your area.

    Paul E. Bennett ....................<email://[email protected]>
    Forth based HIDECS Consultancy .....<>
    Mob: +44 (0)7811-639972 .........NOW AVAILABLE:- HIDECS COURSE......
    Tel: +44 (0)1235-811095 .... see for details.
    Going Forth Safely ..... EBA.
  8. Hi again

    I don't know what an "X-10 system" is, sounds flash, this is just bit of
    flex with a mains plug one end and a box of sockets etc the other, it
    cost 12 UK pounds of so I think, from a gadget shot in the mall.

    It worked well, it was convenient being able to select shutdown and walk
    away know everything would switch off after the PC shutdown. I can
    easily repair it but I'm not sure I want to use it now.

    Killer AC definately runs through it! Except for the sockets it's all on
    a PCB. The feed to the Master 230VAC live does a couple of turns round a
    toroid core to sense the load which drives some electronics which drives
    a relay. The relay contacts are in series with the slave neutral, the
    slave live is wired directly to incoming live via a PCD mounted thermal
    fuse (temperature sensing solid state thing I think, not a "fuse" fuse.

    The master socket is always on (i.e. it's unswitched) while it is
    plugged into the wall socket, perhaps that makes it OK to leave the
    slave socket lives live even when it switches them off.

    Mostly I'm just amazed it was certified (or whatever the approval is)
    for sale.

  9. Hi Paul

    Yes it has a CE marking, I just checked.
  10. It is likely that your wireless device has an input circuit with a dc voltage
    across the inputs when the input is open (that is how the open contacts
    are sensed). One side is likely common with one terminal of the

    You should try putting a power FET device across the input (source
    and drain) and using a capacitor from source to gate to hold the
    FET on for a period after your relay opens. A resistor across the
    capacitor sets the discharge period. I would start with 470 MF and
    1meg resistor for 4-5 minutes delay (depends on gate voltage on
    turn-off, device specific).

    If the battery negative is common to one of the input terminals,
    use a N-channel FET (I did something similar with an IRF520,
    way over kill on current, but you are not likely to damage it).
    If the wireless device voltage is only 3 volts, you will need a
    FET designed for logic level gate voltages.

    Connect as follows:
    Source of FET to - input terminal
    Drain of FET to the other input terminal
    Capacitor negative (use 16V cap) to Source
    Capacitor positive to Gate
    1 meg resistor across capacitor
    100K resistor to battery +, other side of 100K to one of your relay contacts
    Other relay contact to Gate.

    Use a grounded soldering iron as the FET gate can be
    damaged by static electricity.

    If the battery positive is common to one of the input terminals,
    use a P-channel FET and reverse all polarities listed above.

    When the relay contacts are closed, the 100K resistor uses the
    battery to charge the capacitor. Voltage on the capacitor keeps
    the FET ON.

    When the relay contacts open (on power fail) the capacitor holds
    a charge while the 1 meg resistor discharges the capacitor. At
    some low voltage on the Gate, the wireless sensor will detect an
    open circuit and send the alarm. Experiment with resistance and
    capacitance to get an acceptable time delay.

    If you shop the parts, likely cost per circuit is
    USD $2.00-4 .00.
    Bill Kaszeta
    Photovoltaic Resources Int'l
    Tempe Arizona USA
  11. OOPS! Sorry about previous post, I replied to the wrong question.

    Bill Kaszeta
    Photovoltaic Resources Int'l
    Tempe Arizona USA
  12. Roy Q.T.

    Roy Q.T. Guest

    X 10 is registered mark and has a totally wireless control for
    household ac receptacles.

    Your's is just a Hardwired Extension with receptacles attached to it,
    Not much remotes controll as we assumed, furher more, you have 220vac
    and you will always have a Live Wire present on your Receptacles Line
    In., the system you have employed is obviously for use on a 110 system
    where there is 1Hot and 1Neutral] { In your 220vac household system
    there's 1Hot and 1Return also Hot for control purposes there is a little
    more considertion that one wire isolation for the elctronics to perform.

    Tidy it up and send to one of your US Buddies we would have no problem
    with it.

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