RE: How to drive big led displays

Mike:

I have approx. 13.6 V from the unregulated power source and the device is a Common Cathode (red) led display. Can you explain me how to use 2 transistor to manage the segments? And what type of transistor should be use??

Thanks again for your time & response !!!

-----Mensaje original-----
De: Michael Rigby-Jones [mailto:mrjones@NORTELNETWORKS.COM]
Enviado el: MiŽrcoles 23 de Febrero de 2000 14:55
Para: PICLIST@MITVMA.MIT.EDU
Asunto: Re: How to drive big led displays

-----Original Message-----
From:   Omar Martinez [SMTP:OMARTINEZ@HARTFORD.COM.AR]
Sent:   Wednesday, February 23, 2000 4:04 PM
To:     PICLIST@MITVMA.MIT.EDU
Subject:        How to drive big led displays
Importance:     High

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

If the segments are made from 5 series connected LED's the total forward voltage drop will be of the order of 5*2 = 10volts. There is no way to drive this display from 5 volts. If your unregulated voltage is greater than 10 volts and the display is a common anode device then you could drive the cathodes via NPN transistors driven from PORTB. If it's a common cathode then you'll need two transistors per segment.

Regards

Mike Rigby-Jones