> My experience of driving larger displays shows that the
> drive voltage required per led is nearer 1.8 - 2V per LED and
> may be higher in multiplexed displays where larger drive
> currents are used for shorter periods.
> This would put your drive requirements in the 10V region plus
> additional voltage across a current control resistor.
>
> In this case an aditional driver chip would be reccomended such
> as a ULN2003A for common anode displays or the UDN2981A type for
> common cathode, acombination of these can be used to provide a
> higher voltage multiplexing scheme under PIC control.
>
> Other considerations are whether to regulate this higher voltage
> to provide a display that does not fluctuate in brightness when
> different amounts of segments are illuminated.
>
>
> Mike Pearce - Neutronics uk
>
>

Just to be difficult...
An LED should really, ideally be driven by a current source since a
leds light output is directly proportional to the current flowing
through it. In precision led control work (such as we do here from
time to time) the only way to get around voltage drop variations
between leds is to run them with a constant current source (we
have been using 13 bit accurate resolution ).
A filthy big LED display could be efficiently and accurately (no
brightness variations) run with a switch mode constant current
source.


_____________________________

Lance Allen
Technical Officer
Uni of Auckland
Psych Dept
New Zealand

www.psych.auckland.ac.nz

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