The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
From: Eric Bohlman
Subject: Re: Current Source For Large LED Displays
Date: 18 Sep 2002 06:59:02 GMT
Organization: OMS Development
NNTP-Posting-Host: 1cust107.tnt1.franklin.il.da.uu.net (126.96.36.199)
firstname.lastname@example.org (Ed Edmondson) wrote in
> I am building a timer which counts down from 59 minutes and 59 seconds
> to zero. The displays are 2.3 inch green common anode or cathode. Each
> of the four digits are multiplexed. The forward voltage for the
> segments is 12 volts minimum @ 30 mA (static) and 200 mA (1 mS pulse
> width @ 1/10 duty cycle). The DP has a forward voltage of 6 volts
> minimum @ 30 mA (static) and 50 mA (1 mS pulse width @1/10 duty
> cycle). The counter chip is the old Maxim 4 digit presettable Up/Down
> Counter ICM7217.
> The question is would you use a constant current source instead of
> current limiting resistors to keep the light intensity at the same
> level regardless of which segments are turned on.
> Would it be better to have a constant current source for each set of 4
> segments (total of 8 segments) or for each digit?
> Where could I find a simple constant current source for powering these
> The counter chip will provide a digit output current of 500 mA and a
> segment output of 100 mA. What would be a simple way to interface the
> large displays with the ICM7217?
You're going to need either a constant current source or a limiting
resistor for each segment; if you try to feed a group of segments (or an
entire digit) off of either, you'll get exactly the effect you're trying to
avoid, because you'll have a fixed amount of current that has to be divided
by the number of segments lit, a number which varies according to what's
I'd just go with one current-limiting resistor per segment.
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup