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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 (22.214.171.124)
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.
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