Re: [NTLK] Adjustable backlight?

From: Marco Mailand (Newton2k1_at_mac.com)
Date: Tue Jul 02 2002 - 05:30:43 EDT


On 30.6.2002 7:10 Uhr, "Jim Witte" <jswitte_at_bloomington.in.us> wrote:
> I asked PCBMan a while back and he said not to put a resistor in
> series with the EL foil to drop the voltage because it could break the
> inverter (he didn't say why, but I'm guessing it might have something to
> do with the fact that the inverter puts out AC and the resistor doesn't
> have a capacitive load component.)
Since the inverter is basically a transformer it is not resistive and
neither capacitive but inductive. Whatever it is its drive electronics are
designed to handle the impedance an accept changes. An additional resistor
in series with the BL foil which appears like capacitance does at least not
overload the drive but waste energy as heat. The better approach would be t=
o
reduce the drive *input* voltage and thus have a large resulting change in
the output voltage (That=B4s what I wanted to say in my 1st post).

> Instead of trying to hook it into the inverter that's built in to the
> Newt, would it be possible to fit another inverter into the case, hooked
> in parallel to the battery/AC, and then wire a timer into the power
> switch to detect the "hold" condition (and not use Apple's BL circuit at
> all?)
Of course and I do have a spare drive electronics board, thanks to Frank
Gruendel. There is still the question of space. The transformer and the
additional circuitry will take up some space but one could sacrifice e.g.
The pen slot (that=B4s where the PCB I got is designed for.)

Another idea would be to add a chain of capacitors in series with the BL
foil and short them to get full brightness. The result will be brightness
adjustment in at least two steps but very simple and cheap.

--
Marco

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