LED display multiplexing in uSupply to reduce power consumption - Page 1
Forum section: electronics / projects

author: vikasvjois
author_rank: Newbie
timestamp: January 09, 2013, 05:16:18 am
content: Hi Dave,This is a suggestion on a multiplexing method for LED displays to reduce power consumption in your uSupply. The idea is to sequentially turn on the segments in the display so that only one LED is on at a time. Attached is the schematic to implement this. It simply uses a ring counter and AND gates to gate the LED display input to turn on one segment at a time. This method should be combined with the usual method of turning on one digit at a time.Also attached is a logisim (http://ozark.hendrix.edu/~burch/logisim/) circuit file (mux_display_logisim.zip) showing the simulation and a GIF animation showing it in action.Thanks,Vikas

author: EEVblog
author_rank: Unknown
timestamp: January 09, 2013, 05:55:19 am
content: Unfortunately you don't get a free lunch here.For a 6 digit display + DP, worst case with all digits on, each led is only on 1/48th of the time.So you need a related increase of current over driving say 8 segments at a time at lower current for 1/6th the time.Yes, there is the theory that shorter high current pulsed LED's appear "brighter", but you'd have to experiment to determine the trade-off and any actual current saved.I recall this was this covered in another thread maybe?Dave.

author: vikasvjois
author_rank: Newbie
timestamp: January 09, 2013, 06:57:00 am
content: Quote from: EEVblog on January 09, 2013, 05:55:19 amCan't we just increase the frequency at the same current so that even though each LED is on 1/48th of the time, it exceeds the POV time.Vikas.

author: vikasvjois
author_rank: Newbie
timestamp: January 09, 2013, 11:59:50 am
content: Oh. I understand it now. I found this in a previous discussion (Topic: EEVblog #298 - Dave's Decade Digit Display - USB Supply Part 3):QuoteOkay. So, we cannot make the LEDs light up bright enough to make it look lit even though it is off for such a long time (1/48 ON, rest OFF). Even if we could, it might need more current that the advantage is nullified. Also, such a high current switching might lead to some problems.

author: JoannaK
author_rank: Frequent Contributor
timestamp: January 10, 2013, 05:30:07 pm
content: Muxing is possible but fast high current switching tends to lead to power supply noise and EMI. Also the increased parts and shielding may rise price/weight of the unit.For example this circuit at the first picture.. there seems to be 4 new chips and 4 discrete transistors. And after adding the necessary filtering caps and current limiting resistors, it will become quite messy.

author: ballanux
author_rank: Contributor
timestamp: January 10, 2013, 08:35:13 pm
content: You don't need those gates to do the muxing. You only need an output+resistor for each segment and an output+transistor for each digit. To light a segment in a digit you just put VCC at the cathode and GND the anode you want.

author: JoannaK
author_rank: Frequent Contributor
timestamp: January 10, 2013, 09:36:25 pm
content: As long as the microcontroller is up to it, using it's IO pins for direct led driving can save a lot components. That's one of the neat features I like on (for example) low end Pic chips, many other chips ain't that sturdy.