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How to switch an EL inverter with my HDD LED pins?

Discussion in 'Electronic Basics' started by h, May 19, 2004.

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  1. h

    h Guest

    I have an EL wire (incorrectly known as 'neon string' I believe) that I'd
    like to use as a replacement for the HDD LED on my PC. The HDD LED pins are
    usually at around 5v each but one of them (that I'll call the switching
    line) drops to about 2v when the HDD is active. The EL wire's inverter
    requires 5v at about 200mA.

    I think I need to wire an NPN transistor up so that the collector is
    connected to 5v, the emitter feeds the +ve side of the load (with the
    load's -ve connected to 0v) and the base is connected via a resistor to the
    HDD LED header's switching line, so to speak.

    On paper it looks super (and simple) but I'm getting stuck on the maths:
    IIRC an NPN transistor will allow current to flow from collector to emitter
    as long as there's a [proportionally smaller] current drawn from the base. I
    can work out that the HDD LED was probably drawing around 30mA and so I
    shouldn't hurt my motherboard if I draw around the same current through the
    base of my transistor (LED usually has 3V across it when on, so a 100 ohm
    resistor between the HDD LED pin and the transistor base will still look
    roughly like a LED to the m/bd, right?).

    I'm drawing a current through the collector and base, so the transistor
    should turn on and allow current to flow between the collector and emitter,
    powering the EL inverter.

    This is where I start to need help ( / reassurance / slap upside head etc ):

    - What transistor should I be using? I have a selection pack in front of me,
    witha very useful reference sheet, and I can certainly choose one that is
    NPN and will handle 200mA, but what other considerations are there? They
    only seem to come in black or silver, which doesn't match the rest of my
    colour scheme [joke]
    - If the EL inverter has anything fancy going on inside it like transformers
    and capacitors and suchlike, should I be providing my one-resistor,
    one-transistor circuit with ... ummm.... backlash protection?
    - Every circuit diagram I ever look at has caps all over it, some of which
    are AIUI to absorb RF and stop it interfering, getting amplified etc - will
    my circuit need any? Sort of feel it's lacking without. Not sure what
    flavour / value to use.

    Any suggestions will be much appreciated before I let the magic smoke out of
    my motherboard :)

    Thanks for reading,

  2. cpemma

    cpemma Guest

    A *PNP* transistor would work with the active-low system, but for *minimum*
    risk to the motherboard use a cheap opto-isolator with its diode connected
    in place of the case LED, then boost the current-handling with a NPN such as
    a 2N2222A, which will switch 600mA or more.

    Example circuit at , shown for
    12V, for 5V you can drop the base resistor to 1k.

    You can add a 1N4148 diode in series with the optoisolator's IR led to bring
    the combined forward voltage more in line with a 2V green or amber case led,
    but it's not essential, the motherboard's current-limiting will cope with
    the lower Vf.

    The inverter has an oscillator as first stage, so AFAICS you won't need a
    diode across the output.
  3. h

    h Guest

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