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???
01/27/05 14:15
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 		 #85935 - LEDs are in danger!
Responding to: ???'s previous message
Payam said:
Also there is no resistor in series with the LEDs, It may look crazy, but due to the fast nature of the display (I guess) nothing goes wrong, only a small drop in the 12v line.

As Erik already mentioned, there MUST be some sort of reliable current limitation otherwise you will totally blow your LEDs!!!

Anyway, even if there's a current limit, your LEDs are in danger! Why?
If you mulitplex your display, then you have to run the LEDs by a very high peak current. A peak current which the LEDs couldn't withstand when flowing permantly. And there's the danger: If for any reason the multiplexing fails, then this high multiplex current can flow permantly, damaging your LEDs.
So, your design should provide some protection scheme. We have already discussed this issue here:

http://www.8052.com/forum/read.phtml?id=76303

In this link a CD4538 monoflop was periodically strobed to enable the LED current drivers. If a failure occurs, all the LEDs will be turned-off after 10msec.

Kai

List of 242 messages in thread
TopicAuthorDate
BIG I/O expansion            01/01/70 00:00      
   Like I said            01/01/70 00:00      
      Carry on            01/01/70 00:00      
   As i said before            01/01/70 00:00      
   Fanout            01/01/70 00:00      
      Great idea, but 2 things            01/01/70 00:00      
         Thanx Sasha            01/01/70 00:00      
   Why?            01/01/70 00:00      
      Yep, nice guess            01/01/70 00:00      
   outputs            01/01/70 00:00      
      I really need'em and can't multiplex'em            01/01/70 00:00      
         But why??            01/01/70 00:00      
         Can't you go serial?            01/01/70 00:00      
            Serial??            01/01/70 00:00      
            output of a CPLD            01/01/70 00:00      
               using CPLD outputs ..            01/01/70 00:00      
                  To buffer or not to buffer            01/01/70 00:00      
                     to buffer or not            01/01/70 00:00      
   timing problems            01/01/70 00:00      
   what on earth are you doing            01/01/70 00:00      
      Exactly!            01/01/70 00:00      
   complaints and more info.            01/01/70 00:00      
      Don't worry            01/01/70 00:00      
      Consensus - not conspiracy!            01/01/70 00:00      
      Head Bashing            01/01/70 00:00      
         Head banging            01/01/70 00:00      
      more questions            01/01/70 00:00      
      Summary of data and a suggestion            01/01/70 00:00      
         A typo correction.            01/01/70 00:00      
   well            01/01/70 00:00      
   More info. on the project            01/01/70 00:00      
      More info?            01/01/70 00:00      
      thevenin terminations on LE signals            01/01/70 00:00      
         Cable            01/01/70 00:00      
            Termination must exactly match!            01/01/70 00:00      
               Explanation            01/01/70 00:00      
   Stranger and stranger            01/01/70 00:00      
      Humor            01/01/70 00:00      
         Russel            01/01/70 00:00      
            tolerance goes both ways            01/01/70 00:00      
   Bingo !            01/01/70 00:00      
   Bingo !            01/01/70 00:00      
      repeating myself            01/01/70 00:00      
         bare CPLD            01/01/70 00:00      
            Not professional?            01/01/70 00:00      
   Forget about the second Bingo ! :)            01/01/70 00:00      
      Payam            01/01/70 00:00      
         The new decoder            01/01/70 00:00      
   Cos i am just too good to you :)            01/01/70 00:00      
      Wouldn't this be a bit expensive?            01/01/70 00:00      
   Trifle expensive            01/01/70 00:00      
      synchronous design            01/01/70 00:00      
         synchronicity            01/01/70 00:00      
   Not really            01/01/70 00:00      
      ...but there are clocks            01/01/70 00:00      
   a total mess            01/01/70 00:00      
      Dear Eric            01/01/70 00:00      
         Why didn't you just say....            01/01/70 00:00      
            Sub-micros and RS485            01/01/70 00:00      
         wrong approach            01/01/70 00:00      
            here are the links (I forgot)            01/01/70 00:00      
            Eric            01/01/70 00:00      
               serial/parallel            01/01/70 00:00      
                  Cables, Multiplexing            01/01/70 00:00      
                     more on MPX            01/01/70 00:00      
                        sunlight            01/01/70 00:00      
                           more on sunlight            01/01/70 00:00      
                              Could you please explain more?            01/01/70 00:00      
                                 Where is the current limiting from            01/01/70 00:00      
                                 more explaining            01/01/70 00:00      
                           LEDs are in danger!            01/01/70 00:00      
                              4538ing is nice            01/01/70 00:00      
                              What about this            01/01/70 00:00      
                              instead of 4538            01/01/70 00:00      
                                 all over the globe            01/01/70 00:00      
                                    RE: to do what            01/01/70 00:00      
                                       no need if non-mpx            01/01/70 00:00      
                  Question to Erik            01/01/70 00:00      
                     ST2221            01/01/70 00:00      
               EMI!            01/01/70 00:00      
                  not out, but in            01/01/70 00:00      
                  Transmitting parallel            01/01/70 00:00      
                     Series termination            01/01/70 00:00      
                        correct impedance of series termination            01/01/70 00:00      
                           termination            01/01/70 00:00      
                              Output current            01/01/70 00:00      
                                 Kai, is that right?            01/01/70 00:00      
                                    Series termination methode            01/01/70 00:00      
                                       serial termination, got it            01/01/70 00:00      
                                    series termination            01/01/70 00:00      
                                 cable of 100Ohm            01/01/70 00:00      
                                    Of course!            01/01/70 00:00      
                              used for parallel of thevenin terminatio            01/01/70 00:00      
                           Impedance of flat ribbon cable            01/01/70 00:00      
                              anyhow, termination may not be needed            01/01/70 00:00      
                                 Is needed for 2m long cables!            01/01/70 00:00      
                                    that long not needed            01/01/70 00:00      
                                       Agreed            01/01/70 00:00      
                                 re: termination may not be needed            01/01/70 00:00      
                              Thanx Kai            01/01/70 00:00      
                              Flat cable characteristic impedance            01/01/70 00:00      
                                 Look closely...            01/01/70 00:00      
                                    the the            01/01/70 00:00      
                                    Doh!            01/01/70 00:00      
                                       Initial Misconception            01/01/70 00:00      
                                          I can't even spell that...            01/01/70 00:00      
                        ferrite            01/01/70 00:00      
                           ground is part of signal path            01/01/70 00:00      
                              Vcc IS part of the signal path            01/01/70 00:00      
                                 Vcc PIN is part of signal path            01/01/70 00:00      
                           Depends on what you want to filter            01/01/70 00:00      
                              PC power supply            01/01/70 00:00      
                                 4700micro is too high            01/01/70 00:00      
                                    paralell            01/01/70 00:00      
                                       This cannot work!            01/01/70 00:00      
                                          we have a problem            01/01/70 00:00      
                                             Kai, Eric            01/01/70 00:00      
                                                you need to think            01/01/70 00:00      
                                                paralell resistor current limiter !            01/01/70 00:00      
                                                   Thanks Farshid, Reinforcing PC power sup            01/01/70 00:00      
                                                      Reinforcing PC power            01/01/70 00:00      
   Jan            01/01/70 00:00      
      We don't understand each other            01/01/70 00:00      
         What if only BIG I expansion            01/01/70 00:00      
            Your Own Thread            01/01/70 00:00      
      Metastability            01/01/70 00:00      
   how about            01/01/70 00:00      
   Maybe            01/01/70 00:00      
      Divide the problem            01/01/70 00:00      
      Own ideas            01/01/70 00:00      
      sidesteps            01/01/70 00:00      
   Thanks            01/01/70 00:00      
   I dunno Kai..            01/01/70 00:00      
      I dunnoJez            01/01/70 00:00      
      Agreed            01/01/70 00:00      
         CE radiation            01/01/70 00:00      
   Thanx for criticisims from Eric & Kai            01/01/70 00:00      
      the original poster            01/01/70 00:00      
         Eric, Kai            01/01/70 00:00      
            if you can't explain            01/01/70 00:00      
               paralell resistor current limiter !            01/01/70 00:00      
      Then, why don't you discuss it?            01/01/70 00:00      
         taking our warnings seriously and discus            01/01/70 00:00      
      world record            01/01/70 00:00      
         world record +1            01/01/70 00:00      
   Eric            01/01/70 00:00      
      Cost Is Final Issue            01/01/70 00:00      
         lowest cost            01/01/70 00:00      
            Yeah.......            01/01/70 00:00      
            now drop the multiplex            01/01/70 00:00      
      Let's compare parallel vs. serial            01/01/70 00:00      
         Time to confess            01/01/70 00:00      
            simple bitbang            01/01/70 00:00      
               something ain't mising here?            01/01/70 00:00      
                  more            01/01/70 00:00      
               not even that...            01/01/70 00:00      
                  you can, but            01/01/70 00:00      
                  Cool            01/01/70 00:00      
      urband legends            01/01/70 00:00      
      No influence            01/01/70 00:00      
   This heart of mine on fire            01/01/70 00:00      
      HC595's and ULN2803's            01/01/70 00:00      
         typpo            01/01/70 00:00      
         column xistors            01/01/70 00:00      
            Instantaneous!            01/01/70 00:00      
               saturation and off state not on and off            01/01/70 00:00      
                  What is a 'xistor'??            01/01/70 00:00      
                     Okay, transistor            01/01/70 00:00      
                     xistor            01/01/70 00:00      
                  Saturation and Gain            01/01/70 00:00      
                     FETs and gate capacitance            01/01/70 00:00      
                        Fets Vs Transistors            01/01/70 00:00      
                           Which MOSFET            01/01/70 00:00      
                           More Qs on P-MOSFETs            01/01/70 00:00      
                              Best text ...            01/01/70 00:00      
                              Mosfets            01/01/70 00:00      
                              Low 'Ugs' and low 'Ron' is crucial!            01/01/70 00:00      
                                 Bingo!            01/01/70 00:00      
                                 Soft-start circuit            01/01/70 00:00      
      answers and comments            01/01/70 00:00      
         ghosting            01/01/70 00:00      
            what            01/01/70 00:00      
               ghosting            01/01/70 00:00      
                  ok            01/01/70 00:00      
                     this is probably not the case with Payam            01/01/70 00:00      
                        no, worse            01/01/70 00:00      
            P-MOSFETs            01/01/70 00:00      
         typicals            01/01/70 00:00      
            These probably might go            01/01/70 00:00      
            yes, you are and so what            01/01/70 00:00      
               "them" explained            01/01/70 00:00      
   another solution            01/01/70 00:00      
   Serial is a must!            01/01/70 00:00      
      Serial is not a must!            01/01/70 00:00      
   Additional facts            01/01/70 00:00      
      Multiprocessor?            01/01/70 00:00      
      To ULN or not to ULN            01/01/70 00:00      
         ULN2803, human eye response            01/01/70 00:00      
            typical again            01/01/70 00:00      
         consider everything            01/01/70 00:00      
         Equivalent brightness with pulsing            01/01/70 00:00      
      Re: Additional facts            01/01/70 00:00      
         You put many of them...            01/01/70 00:00      
            Re: You put many of them            01/01/70 00:00      
               the matrix revisited            01/01/70 00:00      
   here are some more additional facts            01/01/70 00:00      
      NFI            01/01/70 00:00      
         Me, not subtle ???            01/01/70 00:00      
            Subtle as a brick!            01/01/70 00:00      
   one little detail            01/01/70 00:00      
      30° is it enough?            01/01/70 00:00      
   Calculating ... please wait!            01/01/70 00:00      
      answers/comments            01/01/70 00:00      
         six-layer boards?            01/01/70 00:00      
            no 2 layers possible            01/01/70 00:00      
               6 layer for 20$???            01/01/70 00:00      
                  here            01/01/70 00:00      
                     PCB discussion extracted            01/01/70 00:00      
               a trace about 1 inch            01/01/70 00:00      
                  20A on a 2mm track!            01/01/70 00:00      
                  holy smoke            01/01/70 00:00      
                     Okay.            01/01/70 00:00      
                        again            01/01/70 00:00      
                           colloquialism            01/01/70 00:00      
               Calculating a bit...            01/01/70 00:00      
                  'Three' layer design            01/01/70 00:00      
                     layers - LED - EMI            01/01/70 00:00      
                        LEDs don't understand ground bounce            01/01/70 00:00      
                           But the four 74HC595 and ULN2804 do!            01/01/70 00:00      
                              595s don't, ULN ... not important            01/01/70 00:00      
                                 not so            01/01/70 00:00      
                           Ground bounce            01/01/70 00:00      
                              Sure, shall take care of it.            01/01/70 00:00      
      Don't underestimate heat dissipation!            01/01/70 00:00      
   Kai            01/01/70 00:00      
      Payam            01/01/70 00:00      
         less complex/less expensive            01/01/70 00:00      
         Did you try the multiplex?            01/01/70 00:00      
            Yes            01/01/70 00:00      
            go ahead and try            01/01/70 00:00      
            Square root increase            01/01/70 00:00      
               Distributed power supply            01/01/70 00:00      

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