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.

Regards

Mike Pearce - Neutronics uk



Subject: Re: How to drive big led displays


 It sounds to me like you're running out of voltage.  Each LED
 (I'm assuming RED) drops about 1.2 volts at a current of about
 20 mA.  You have 5 in series for a total of 6V (1.2x5=6).  So,
 you'll have to have a higher voltage supply, or use fewer LED's
 per segment.  You could try 4 per segment for 4.8 volts, but
 that too will be close.

                                       Regards,

                                         Jim



On Wed, 23 February 2000, Omar Martinez wrote:

>
> Gentlemen:
>
> I'm making a device that show the time and temperature with a 16F84 and it
> work fine with led display of 1" in size, so when I try to use led
displays
> of 4" size, the segments don't
> turn on...
> In the led display spec's, I saw the connection diagram and it's made with
5
> leds connected in serial mode for each segment (common cathode), except
for
> the decimal point that have only one.
> All the device is powered with standard +5 Volt regulator, the segments
are
> driving directly from PortB thru limiting resistors and each display is
turn
> on every 3 mSec. via transistor from portA.
>
> Can anyone please help me with this problem?? I will appreciate it!!!
>
> Thanks & excuse for my english....
>
> Omar

jim@jpes.com