Hi
EL panels can be blown quite easily.  Continued use reduces the life which
I think is in the region of 2000 hours at reasonable voltage levels.  At
120V, 400Hz the panel gives it's best output to life ratio and raising the
voltage higher only gives a very small output increase.  A 200V spike will
destroy the panel instantly as sparks pass through the panel causing black
spots to appear.  Usually the inverter goes before the panel.  BTW does
anyone know where I can by large and I mean large sheets of this stuff
(about square metres if possible)


Cheers


Tim


At 18:58 31/08/97 -0400, you wrote:
>On Sun, 31 Aug 1997 15:11:16 -0700 Tim <lagushka@BEST.COM> writes:
>The made-in-China watch I have has a backlit LCD similar to Indiglo,
>maybe not quite as bright but adequate.  The circuit is very
>conventional.  It uses two SOT-23 transistors, a coil, and a diode.  The
>transistors are driven by two outputs from the CMOS watch chip.  When the
>light button is pressed, two outputs from the chip produce sequences
>similar to these:
>
>A: 1010101010101010101010101010101000000000000000000000000000000000
>
>B: 0000000000000000000000000000000011111111111111111111111111111111
>
>The frequency of A is I believe 32 KHz direct from the crystal.  When
>output A is one the first transistor turns on.  It pulls 3V across the
>coil.  When A goes to 0, the transistor turns off, the coil volatge flies
>up, turns the diode on and is applied to the EL panel.  After several
>cycles the voltage across the EL panel reaches about 60V.  The only thing
>limiting the rise of voltage is the capacitance of the panel.
>
>Just energizing an EL panel with DC will not make it light up.  It is
>necessary to use AC, preferably 400 to 1000 Hz.  Therefore, the other
>transistor, controlled by output B, is used to discharge the panel back
>to 0V.  I forget exactly how this is wired to prevent shorting out the
>battery through the coil and diode during the discharge phase.  This is
>the gist of the circuit though.  No exotic chips, etc. just a coil, 2
>transistors, and a diode.  The coil looked like the same type that boosts
>the voltage for the alarm beeper.  I think the transistors were bipolar
>though they could be FETs.
>
>Of course a PIC could generate sequences A and B easily.  I don't know if
>EL's can electrolyse like LCDs.  If they do, leaving an EL on constantly
>with this type of drive may damage it due to the DC component.  Adding
>another capacitor to the circuit to couple the voltage to the EL would
>solve that.
>
>


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