Hello All,
I'm getting my butt kicked and could really use some help. I'm building a sports scoring device using very bright 3" seven segment displays. Because I can do nothing simple, these are also dual colored seven segments with a common anode for each color. I'm going to use a PIC and multiplex a total of four digits so only one is on at any given moment which I'm figuring will reduce the overall current consumption. The caveat is this is for a battery powered device that I'm trying to make as small as possible. The red color uses a typical forward voltage of 12.0V while the green calls for a 12.6V, therefore I'm planning on using eight AA batteries to get my 12V.
The biggest problem I've run into so far is driving the segments. I originally planned on using ULN2803's to drive the cathodes of the segments and an Allegro 2982 source driver for the anodes. However, I failed to realize these transistors have a 1-2V drop across the collector and emitter and the segments come out dim at best. I then tried playing with an IRL520 MOSFET, just driving the cathode of one segment to see how it looked, but the current on the power supply goes well beyond 400mA and doesn't turn on the segment at all. I've included a PDF of the test circuit - I'm not 100% sure I did it right. I've also tried opto-isolators, specifically the LTV-847. Those seem to work - the segment under test appears reasonably lit, but still not as bright as when I hook a segment directly up to the 12V source (with a 10Ω resistor for safety).
Anyone have any suggestions on how to drive an LED segment, with rapid on/off, with a 12V battery without dropping the voltage or consuming a great deal of current? Or would you recommend sticking with the ULN2803 and 2981 and increasing the batteries to 9 or 10 to get 13.5-15V?
I'm getting my butt kicked and could really use some help. I'm building a sports scoring device using very bright 3" seven segment displays. Because I can do nothing simple, these are also dual colored seven segments with a common anode for each color. I'm going to use a PIC and multiplex a total of four digits so only one is on at any given moment which I'm figuring will reduce the overall current consumption. The caveat is this is for a battery powered device that I'm trying to make as small as possible. The red color uses a typical forward voltage of 12.0V while the green calls for a 12.6V, therefore I'm planning on using eight AA batteries to get my 12V.
The biggest problem I've run into so far is driving the segments. I originally planned on using ULN2803's to drive the cathodes of the segments and an Allegro 2982 source driver for the anodes. However, I failed to realize these transistors have a 1-2V drop across the collector and emitter and the segments come out dim at best. I then tried playing with an IRL520 MOSFET, just driving the cathode of one segment to see how it looked, but the current on the power supply goes well beyond 400mA and doesn't turn on the segment at all. I've included a PDF of the test circuit - I'm not 100% sure I did it right. I've also tried opto-isolators, specifically the LTV-847. Those seem to work - the segment under test appears reasonably lit, but still not as bright as when I hook a segment directly up to the 12V source (with a 10Ω resistor for safety).
Anyone have any suggestions on how to drive an LED segment, with rapid on/off, with a 12V battery without dropping the voltage or consuming a great deal of current? Or would you recommend sticking with the ULN2803 and 2981 and increasing the batteries to 9 or 10 to get 13.5-15V?
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