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Low-power transformer - and other stuff

Discussion in 'Electronic Basics' started by Pete Verdon, Jan 7, 2009.

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  1. Pete Verdon

    Pete Verdon Guest


    As part of a bathroom project, I want more sophisticated control of the
    extractor fan than just turning it on with the overhead light. At
    present, I'm thinking in terms of a flow-switch on the shower supply (so
    it extracts steam while showering) and a push-switch (for occasional
    toilet-related necessity :) ). Both of these would also involve
    over-run timer(s).

    Since in the UK I can't legally have a mains (240v) switch in the
    location I'd want (well, except for a high-IP-rating one, all of which
    are ugly) I'm thinking in terms of a 6-12-ish volt "signal" circuit
    going down to the switches in the room, and a suitable timer
    module/relay/etc assembly in the (accessible) roof-space above the
    bathroom where the fan will be. I envisage a very simple circuit on
    stripboard or similar, in a project box.

    My main question is how best to power the "signal" circuit with minimum
    wasted energy when not in use. I've had a quick look at small
    transformers (eg
    ) but don't understand all the figures. Things like "efficiency 30%" and
    "core losses 1.8W" scare me when all I want to do is energise a relay a
    couple of times a day - but perhaps these only apply under load? In
    addition, I'm a little confused by the apparent two outputs - do I get a
    different voltage across each compared to across both or something like

    Secondarily, any advice on "chunky" self-contained parts for things like
    my overrun timers? I don't really want to get into detailed electronics
    on this project, just slap together big chunks of ready-made
    functionality on a dumb board. It's electrics more than electronics.


  2. Phil Allison

    Phil Allison Guest

    "Pete Verdon"

    ** Core loss will apply with no load.

    ** There are two identical windings - so you can use them in series or

    The voltage rating will be double in series and the current will be double
    in parallel.

    ...... Phil
  3. Steve Sousa

    Steve Sousa Guest

    Why not replace the single switch on the outside with a double were one
    turns on the light, the other light+fan ?
    Cheap, simple, safe, foolproof and uses no power...
  4. Randy Day

    Randy Day Guest

    Does your house have an electric doorbell?
    Maybe you could tap off the transformer for that.

    If not, you could buy a doorbell transformer from
    a local home supply center.

  5. amdx

    amdx Guest

    I like the two switch idea, but why not make the light+fan a mechanical
    timer. Turn it to the time needed. 5 minutes for me, 30 minutes for my teen
    daughter :)
  6. Jasen Betts

    Jasen Betts Guest

    have you looked at pneumatically operated remote switches, often the
    buttons for them aren't so ugly.
    I think 'core losses' is related to magenetising current and so you
    pay that all the time. 30% is extra costs while you're using the power.
    yeah , there's a number of different ways you can hook them up.
    basically you can either have two isolated outputs, or parallel them
    or connect them in series.

    and I'm guessing you're wanting DC ad then end...

    But I'd consider putting an outlet in the ceiling crawlspace/attic and plugging
    a plugpack DC supply into it,
  7. Rich Grise

    Rich Grise Guest

    I just replaced the switch with a dupllex one and fished a new wire to
    the fixture.

    Have Fun!
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