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From: firstname.lastname@example.org (Tim Shoppa)
Subject: Re: Current Source For Large LED Displays
Date: 18 Sep 2002 05:02:21 -0700
NNTP-Posting-Date: 18 Sep 2002 12:02:21 GMT
email@example.com (Ed Edmondson) wrote in message news:<firstname.lastname@example.org>...
> 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.
As a practical matter the brightness matching between segments in a 7-segment
display can be up to 50% variance and the viewer will hardly notice. And
typical 7-segment display specsheets only guarantee segment luminosity matching
to that level. It seems a bit silly to match the segment currents to a few
percent when the brightness will still vary by 50 percent and nobody will
Electrically, yes, you do need something there to set the segment current,
but what you do will depend on what supply you have available. If you've
got 15V or 18V semi-un-regulated then resistors will do just fine. If
you've got 24V or 30V then you probably want to step it down with a small
switcher to the 15-18V level, then use resistors.
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