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current sensing switches

Discussion in 'Electronic Basics' started by Nick, Jan 27, 2004.

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  1. Nick

    Nick Guest


    I'm wiring up my shop. I have a total of 8 separate wall circuits.
    One circuit is solely for the dust collector. Two of them are 220's.
    The rest are simple 20 amp circuits. The lights are on completely
    unrelated circuits. They will all come into the breaker box via the
    bottom. I am interested in wiring them up so that whenever current is
    being drawn by any of the seven, the dust collector circuit will
    "sense" that a tool is on and then kick in the dust collector.
    Finally, it would be nice to make it so that the dust collector
    remains running for an additional 10 or 15 seconds after the tool is
    shut down. I wouldn't need to have all seven circuits hooked up this
    way maybe just four of the 20 amps and one of the 220's. With ample
    instructions I think I can handle the job myself. I'm not that
    familiar with relays but if you can point me to the parts I would need
    and give me some wiring tips I should be able to do this. At first I
    was just going to get one of those remote control gadgets from Penn
    State or wire in a half dozen 4-way switches scattered around the room
    but this solution would be neater and fully automatic.


  2. John Fields

    John Fields Guest

    That sounds like a perfect application for a current transformer (CT),
    like one of the ones at

    What you could do would be to run all the returns from the machines
    through the hole in the CT before you connect them to neutral in the
    box, and then use the output from the CT to turn on the relay/contactor
    for the dust collector.

    If you rectified and filtered the CT's output you could use that to turn
    on a solid-state relay which in turn could turn on the dust collector's
    contactor, or turn on the dust collector itself if the SSR could handle
    the current. Also, if you arranged for the filter cap on the input of
    the SSR to be part of an RC circuit you could use its decay to give you
    the turn-off delay you want. The main thing you'd have to make sure of,
    though, is that you can get enough out of the CT with the lowest-current
    tool running to do what you want to do, yet with all of the tools
    running that its output is made incapable of hurting the SSR's inout.

  3. John Fields

    John Fields Guest

  4. One method would be to run one conductor from each of those circuits
    through a single current transformer. The current transformer output
    can be rectified with a bridge and its voltage clamped with a big
    zener diode so that a small DC relay (coil voltage same as zener
    voltage) can be powered by this regulated output The contact from that
    relay can pass power to the coil of a time delay after disenergize
    relay that operates a bigger contactor that operates the dust
    collector motor. Manual switches could also be tied into this relay
    system to force the collector on or off. The design trick it sizing
    the current transformer, zener and relay to respond to the minimum
    load and not be damaged by the maximum possible load. If the range of
    loads is too large, you will need several current transformers,
    bridges, zeners and small relays, with all the small relay contacts in
    parallel. Ideally there would be one of these current sensing
    circuits for each power circuit. But you may be able to get by with
    fewer, especially if their currents pass through the current
    transformer in a way that half of them cancel the other half. The
    chance of having two loads that are so nearly equal that they cancel
    is not likely.

    At least, this may give you some ideas.
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