7-Segment LED Displays: Uneven Brightness – How Come? - Page 1
Forum section: electronics / beginners

author: Darkwing
author_rank: Contributor
timestamp: April 23, 2014, 11:23:23 am
content: Hi folks,while examining some 7-segment LED displays in my all day environment (=clocks), I noticed that some of them have odd-looking unevenly lit digits, some of them have nice-looking evenly lit digits. I made a photo of good examples I found:Exhibit Ashows clearly, that '1' is brighter than '8'; the others are somewhere in between (the effect is more noticable in praxis than on this photo).Exhibit Bshows nothing of that effect: every number, no matter how much segments are lit up, has pretty much the same brightness.Non-exhibit C:On my little project I’m currently working on, I also use a 7-segment LED display and I’m noticing effect A, the uneven brightness. I know that in my case I’m using a common cathode display – can this have anything to do with the 'distribution of brightness' inside of a digit? Or is it a weird multiplexing-effect I’m not aware of?Just a guess, because I have absolutely no explanation for this effect. Can someone help?I’m including a generic common cathode schematic for the discussion:Uneven brightness makes no sense to me, because every single LED segment has its own pre-resistor on ABCDEFG – so every segment will get the same specific amount of current, no matter if any other segment is lit up. Uneven brightnesswouldmake sense to me, if just one 'post'-resistor per digit is used – but that’s not the case in my project, and I also suppose no one in the industry building clocks for TV receivers and ovens will do itthatwrong?…So can someone please explain the 'uneven brightness effect' and if it has something to do with common cathode / common anode displays in general?Thanks a lot!

author: spudboy488
author_rank: Regular Contributor
timestamp: April 23, 2014, 11:41:42 am
content: What happens in figure A when the display is 11.11? The dimmer digit just happens to be the one with the most segments lit. Maybe a power supply or driver issue issue? Is the effect always in the same digit location or is it random? You said you are multiplexing the display. Have you tried (for debug purposes) to slow the multiplexer clock rate down to say 1 Hz? This would light just a single digit full on so you can see if any one digit shows the effect. Most designs multiplex on one direction, for example, left to right. Have you tried reversing the digit order to see if that sheds any light (no pun intended) on the issue?

author: amyk
author_rank: Super Contributor
timestamp: April 23, 2014, 11:46:26 am
content: If you are multiplexing, you will need to increase the instantaneous current to maintain the same brightness.

author: digsys
author_rank: Unknown
timestamp: April 23, 2014, 11:48:26 am
content: Above plus also - is the mux rate / average power ABOVE the minimum LED requirement ? How accurate is your time-slice inthe MUX ? Is it a poor / cheap LED display ? I'd do a test run - pick 1 display, run individual resistors to each segment (say 560R),connect to a variable P/Supply and check consistency as you dial up the voltage.

author: kripton2035
author_rank: Super Contributor
timestamp: April 23, 2014, 12:08:31 pm
content: is the mux generated by an interrupt clock, or by program loops and delays ?that could be the answer.

author: ecat
author_rank: Frequent Contributor
timestamp: April 23, 2014, 12:35:50 pm
content: Calling on my years of software experience I'm declaring a hardware problemYou could be pulling down the supply rail?8 segments at 10 to 20mA each, I'm guessing you are switching the cathodes with some sort of transistor? Is it actually biased to pass 160mA, or if a fet is it fully on? If less than the full required current is shared between segments you would see this sort of dimming once N or more segments are active, or if the resistance of the cathode switch is sufficiently high then more segments = more current = greater voltage drop across the switch = dimmer segments.Or it could be a sw problem

author: Darkwing
author_rank: Contributor
timestamp: April 23, 2014, 07:12:44 pm
content: Ok, your replies tell me, that even if my question was understood correctly, there’s no definitive answer for now.So I did some further investigations we should discuss:Let’s put aside exhibit A and B for a moment and let me present exhibit C: the 4-digit 7-segment LED display I’m currently working with.I stopped multiplexing and now lighting only some random segments of digit 1. (Btw: I was unprecise when mentioning multiplexing ago: I don’t multiplex individual segments, I only multiplex between digits.)In the picture you can see the amount of current drawnpersegment whenxsegments are lit (I only tested up to x=3 for now).What you see: the values differ! One and the same segment draws a different amount of current, depending on how many segments are lit up in the digit. How is that possible?You can even see on the photo that the brightness decreases when starting to lighten up more segments. Is this some odd wiring inside of the display? It’s a variation of the ROHM LC-204VL and there seems to be no real datasheet for that type, only some maybe-valuesin here(amongst a lot of Japanese).Question:Is it in some way normal, that segment brightness / current consumption starts to diminish when more segments are lit up? Could it have something to do with this common cathode thing?Finally here is another picture of a fully lit '8' on digit 1. Shown values are the same for all other digits.So I hope this answers some questions from some posters in one way or the other.What’s to say about these findings? I don’t understand them.

author: sacherjj
author_rank: Frequent Contributor
timestamp: April 23, 2014, 08:31:38 pm
content: Your common cathode or common anode displays have a common source or return. Even though you have current limiting resistors on each segment, the increased current in the common path is dropping more voltage. This is causing the per segment current to drop and brightness to drop.

author: spudboy488
author_rank: Regular Contributor
timestamp: April 24, 2014, 11:36:18 am
content: If the current per segment drops by 50% from 1 lit segment to 7 lit segments, then it appears that you have a driver issue. In theory, the current through a segment should not change based on how many segments are lit. What are using to drive the segments? Schematic?

author: mmotorny
author_rank: Newbie
timestamp: May 14, 2016, 07:58:13 am
content: For all those who are reading this post in the future: I had exactly this problem and the root cause was the battery. I used CR2032 to drive both MCU and 7-segment LED. The entire circuit consumed 16 mA which was too much for CR2032 (it can only provide 4 mA continuously according to the info I found on the internet). I replaced CR2032 with 2 AA batteries and the problem was gone.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 14, 2016, 08:43:06 am
content: The higher current when more segments light up can case voltage drop. That can be the power supply or the driver for the digit (low side for the common cathode example). The effect is stronger with relatively low supply voltage and thus small resistors. So for a red display and a 5 V circuit this is usually not a big deal, but for a 3 V powered circuit this can be a problem.Software could compensate for this and adjust the time-slots to make the 8 lit longer than the 1.

author: mav_iqdirect
author_rank: Contributor
timestamp: May 14, 2016, 10:44:03 am
content: Some cheap 7-segment displays have difference in characteristics of LEDs installed inside. So you may never achieve equal brightness with these displays. Try to use higher quality displays plus observe recommended operating current per segment.

author: danadak
author_rank: Super Contributor
timestamp: May 14, 2016, 11:17:23 am
content: A basic 7 segment display is a series of die, one or more/segment, toppedby a light pipe. If there is any mismatch between die then you will seevarying brightness between segments, all other parameters equal.If you look carefully at the pics you posted you can see what looks like a"hotspot" in the segment. That's because the lightpipe does not perfectlyand uniformly diffuse light nor is the led a perfect point or area source oflight either.Additionally if die are different colors, one display to another, the die havedifferent threshold from one color to another, and will emit different lightlevel output. Usually there is a spec in 7 seg displays calling out max lightoutput deviation segment to segment.Regards, Dana.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 14, 2016, 11:25:21 am
content: without looking at the circuit of exhibit A, my guess is, the limiting resistor is put at common cathode instead of in every anode.

author: Raj
author_rank: Frequent Contributor
timestamp: May 14, 2016, 11:47:59 am
content: attempt to fix-1-try reducing the screen refresh and multiplexing speedattempt to fix-2-try changing the screen...I have 2 screens from same seller, both of same size and same pinout, but different model,the good one is aok (cpd-0315), but the bad one(nhsd 312h) has light leaking off of the lit segment into switched off segmentsthe funny thing is , the package they came in had 11 good ones and one bad one

author: newbrain
author_rank: Super Contributor
timestamp: May 14, 2016, 05:51:46 pm
content: Quote from: Darkwing on April 23, 2014, 07:12:44 pmAre you driving the segments directly from the MCU -btw, you did not tell us which one you are using- also in these pictures?Consider that all MCUs are limited in the amount of current that you can drawin totalfrom the I/Os.Each pin can be able to source, say, 20mA, but you cannot source that current from more than a small number of pins.There often are restrictions about sourcing or sinking current from adjacent pins, and this might add to the effect you're seeing; especially if you are using a 3V supply, a small variation of the output voltage would cause a larger change in the current, as the resistors used are lower in value wrt to 5V.All of this is usually "clearly" stated in the datasheets, let's take e.g. TI MSP432P401, page 43 of specification doc:QuoteOr, for an STM32F042, see the attached picture, where note 2 says:Quote