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    Re: Lights,Leds and scopes etc.
    From: George Huxtable
    Date: 2003 Oct 14, 20:39 +0100

    Earlier today, I sent a mailing as follows (in part)-
    >Jared Sherman said-
    >>12 LED's, typically 40mA each. That would draw 0.48 Amps,
    >Comment from George-
    >That's not the way anyone would drive such an array, not if he was trying
    >to conserve the power taken a 12-volt power source.
    >Instead, the diodes should be strung together IN SERIES, as far as
    >possible. It depends somewhat on the details of the specified voltage drop
    >across each LED at full output, but 4 or 5 such diodes connected in series
    >should still add up to less than 10 volts, allowing a somewhat-depleted
    >battery to drive them, with a bit of "headroom" for a current-regulator to
    >work properly.
    >Each such diode string would require a separate current regulator. If a
    >12-diode array was made up of, say, 3 such strings, then the overall
    >battery drain would then be only 0.12 Amps, not 0.48 Amps as Jared
    >suggests. Three separate current regulators would be required in this
    >case, each able to cope with dissipation about 0.2 Watts to deal with the
    >enhanced voltage on the rail with a highly-charged battery and a running
    >engine: this is not very demanding.
    Some of that was certainly wrong, and I would like to backtrack, with some
    Dave Weilacher posted-
    >How about looking at this site and commenting.
    So I did; and there found some detailed specifications for superbright
    LED's, reg, green, and others.
    And discovered that for the green LEDs, the forward voltage drop at maximum
    output was significantly greater than I expected, between 3.5 and 4 volts.
    That implies that it would be quite impossible to power "4 or 5 such
    diodes" from a 12 volt supply, as I had suggested. In fact, one could
    series-up no more than two such diodes: which would still be worth doing,
    for current economy, rather than connecting them in parallel. Each  series
    pair would then need its own current driver.
    For the red diodes, the voltage drop is significantly less, and it would be
    quite practicable to connect 4 such diodes in series (but not 5), with a
    current driver.
    Sorry to get that wrong.
    In view of these higher voltage drops, the use of a current-regulated power
    supply, providing sufficient voltage step-up to drive all the diodes in a
    lamp in a single series chain (as in the "orcagreen" lamps), seems a rather
    sensible alternative.
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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