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???
10/07/09 01:40
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 		 #169482 - No surprise...
Responding to: ???'s previous message
Chico said:
I discovered that putting a pullup in the output the ULN starts to work normally...


That's no surprise at all, because the AT89S52's outputs cannot source much current. No more than 300µA at an output voltage of 2V, at "worst case".

If you connect an input of ULN2803 to an output of AT89S52, then you must forward bias two base emitter junctions. Makes 1,4V. In parallel to these pn-junctions about 10kOhm is present, which results in a current of 1,4V / 10kOhm = 140µA each. This current must also flow through a 27kOhm resistor, which gives a further voltage drop of about 3.8V. So, each output of AT89S52 would have to deliver 280µA at an output voltage of about 5V, not even mentioning the needed base currents. This is way too much load for a standard 8051's output.

Your added pull-up then turned the situation and the ULN2803 started to work properly.

Chico said:
Well... OK... BUT... why was working with the other microcontroller (the now burnt one) ???


Manufacturing tolerances! The old one could, the new one cannot.

Kai

List of 168 messages in thread
TopicAuthorDate
Trouble with a simple project with ULN2803            01/01/70 00:00      
   Start to search step by step            01/01/70 00:00      
      back to breadboard            01/01/70 00:00      
         No surprise...            01/01/70 00:00      
            Thanks Kai            01/01/70 00:00      
               In short            01/01/70 00:00      
               Then have a look at the datasheet!            01/01/70 00:00      
                  Don't think so            01/01/70 00:00      
                     No, Per, the actual reason is...            01/01/70 00:00      
                        'working' again            01/01/70 00:00      
                           But why?            01/01/70 00:00      
                              Lack of knowledge of the unknown            01/01/70 00:00      
                                 Ah yes            01/01/70 00:00      
                                 I think people don't just realize how much they do not know.            01/01/70 00:00      
                              There is a reason ...            01/01/70 00:00      
         why was working with the other microcontroller            01/01/70 00:00      
            Keep in mind it's on a "breadboard"            01/01/70 00:00      
               Infinite Improbability Drive?            01/01/70 00:00      
                  re: Infinite Improbability Drive            01/01/70 00:00      
                     Pah            01/01/70 00:00      
                        Vote: Offensive/Flame!            01/01/70 00:00      
                           ;-)            01/01/70 00:00      
                              Reference for the uninitiated            01/01/70 00:00      
         Without pull-up???            01/01/70 00:00      
         because it was not working, but 'working'            01/01/70 00:00      
   What else is on your board...            01/01/70 00:00      
   It's a transistor array ... not a magic box            01/01/70 00:00      
   AGAIN!!! =(            01/01/70 00:00      
      What are you connecting to outputs?            01/01/70 00:00      
         Yes...!            01/01/70 00:00      
            new measures with load            01/01/70 00:00      
               How?            01/01/70 00:00      
               Can use an oscilloscope?            01/01/70 00:00      
                  Laser and oscilloscope            01/01/70 00:00      
                     Need current control, not voltage control            01/01/70 00:00      
                        re Per            01/01/70 00:00      
                           Definitely a good way go let out the magic smoke            01/01/70 00:00      
                              not even then            01/01/70 00:00      
                                 The problem is not the laser.            01/01/70 00:00      
                                    Drive the ULN harder.            01/01/70 00:00      
                                    Factor 10 off?            01/01/70 00:00      
                                    I guess, you forgot the Ucesat of ULN2803...            01/01/70 00:00      
                                       74HC04            01/01/70 00:00      
                                          Better to have them..            01/01/70 00:00      
                                             up the garden path            01/01/70 00:00      
                                                Definitely consider min/max values from datasheet            01/01/70 00:00      
                                                   Nothing more instructive than...            01/01/70 00:00      
                                                      but he won't            01/01/70 00:00      
                                                         Yes, that's true...            01/01/70 00:00      
                                                            a better way            01/01/70 00:00      
                                                               Datasheet            01/01/70 00:00      
                                                                  OH BOY            01/01/70 00:00      
                                                                  Logic level MOSFET...            01/01/70 00:00      
                                                                  A new test I made, bizarre...            01/01/70 00:00      
                                                                     It seems the power supply...            01/01/70 00:00      
                                                                     Lack of information            01/01/70 00:00      
                                                                     yep. It IS very much!            01/01/70 00:00      
                                                                        Laser and peltier            01/01/70 00:00      
                                                                           Dont konw wich regulator you use.            01/01/70 00:00      
                                                                              well... no regulator at all...            01/01/70 00:00      
                                                                                 PC supplies are not very well regulated            01/01/70 00:00      
                                                                           For 3V you should have a 3V regulator            01/01/70 00:00      
                                                                     That's the explanation!            01/01/70 00:00      
                                                                        3v            01/01/70 00:00      
                                                                           Minimum load            01/01/70 00:00      
                                                                              Supply and 7805            01/01/70 00:00      
                                                                                 you want help??            01/01/70 00:00      
                                                                                    No laser Datasheet            01/01/70 00:00      
                                                                                       then return it and DEMAND one with a datasheet            01/01/70 00:00      
                                                                                          Already mentioned lack of datasheet            01/01/70 00:00      
                                                                                             suggested as first step            01/01/70 00:00      
                                                                                             laser            01/01/70 00:00      
                                                                                                Hard to tell but I think you should mind the voltage            01/01/70 00:00      
                                                                                 Perhaps you should use an adjustable regulator            01/01/70 00:00      
                                                                                    I do not think so            01/01/70 00:00      
                                                                                       and how would YOU build a current source?            01/01/70 00:00      
                                                                                          line anyone else - a 'cookbook' design            01/01/70 00:00      
                                                                                             can you do that without a regulated voltage source?            01/01/70 00:00      
                                                                                                yes            01/01/70 00:00      
                                                                                                   Guys, Chico has a Laser modul, already...            01/01/70 00:00      
                                                                                                      two possibilities            01/01/70 00:00      
                                                                                                         assumptions are dangerous            01/01/70 00:00      
                                                                                                            But I didn't assume, that it has a good current control!            01/01/70 00:00      
                                                                                                               Stable is one thing - what voltage is another            01/01/70 00:00      
                                                                                                                  If only Chico had sent us a link to his Laser modul...            01/01/70 00:00      
                                                                                                                     probably highly unstable            01/01/70 00:00      
                                                                                                         Yes, and a good linear regulator can supply that!            01/01/70 00:00      
                                                                                                      That was my point!            01/01/70 00:00      
                                                                                                      we can't know for certain, but ...            01/01/70 00:00      
                                                                                                   What is an ST2221            01/01/70 00:00      
                                                                                                      ST2221a != ST2221            01/01/70 00:00      
                                                                                                         didn't you notice the reference to an earlier post?            01/01/70 00:00      
                                                                                                            you will need one of these            01/01/70 00:00      
                                                                                                               Yes, if you have a good, stabile supply without much noise            01/01/70 00:00      
                                                                                                                  A current source isn't a magic beast            01/01/70 00:00      
                                                                                                                     ... but only if ...            01/01/70 00:00      
                                                                                                                        and I thought this was turkey day            01/01/70 00:00      
                                                                                                                        Are you assuming CC has to be passive?            01/01/70 00:00      
                                                                                                                           that would be ridiculous            01/01/70 00:00      
                                                                                                                              Ask Richard.            01/01/70 00:00      
                                                                                                                                 other than Richard?            01/01/70 00:00      
                                                                                                                                    Now define "constant"            01/01/70 00:00      
                                                                                                                                    Consider this ...            01/01/70 00:00      
                                                                                                                                       Of course, it is...            01/01/70 00:00      
                                                                                                                                       Current control != Constant Current            01/01/70 00:00      
                                                                                                                                       irrelevant            01/01/70 00:00      
                                                                                                                                          Why do you continue in this way, Erik?            01/01/70 00:00      
                                                                                                                                             Minimalistic const current regulator            01/01/70 00:00      
                                                                                                                                                It's a matter of definition            01/01/70 00:00      
                                                                                                                                                   Ideal constant current source...            01/01/70 00:00      
                                                                                                                                                      That's where this whole thing began ...            01/01/70 00:00      
                                                                                                                                                         Don't jump around wildly when cornered            01/01/70 00:00      
                                                                                                                                                            That's not what was said            01/01/70 00:00      
                                                                                                                                                               That's why we should avoid to offend each other...            01/01/70 00:00      
                                                                                                                                                               Still trying to smash in open doors            01/01/70 00:00      
                                                                                                                                                                  The world is flat!            01/01/70 00:00      
                                                                                                                                                               VERY "poor choice of wording"            01/01/70 00:00      
                                                                                                                                                                  It wasn't MY wording to which I referred, Erik            01/01/70 00:00      
                                                                                                                                                                     Memory lapse?            01/01/70 00:00      
                                                                                                                                                                        You missed the point, Per            01/01/70 00:00      
                                                                                                                                                                     I didn't even bother ....            01/01/70 00:00      
                                                                                                                                                                        Why, then did you mention it?            01/01/70 00:00      
                                                                                                                                                                           Lots of ridiculous claims later...            01/01/70 00:00      
                                                                                                                                                                              ABSOLUTELY NOT            01/01/70 00:00      
                                                                                                                                                                              Cunning ?            01/01/70 00:00      
                                                                                                                                                                                 Perhaps, but ...            01/01/70 00:00      
                                                                                                                                                   What definition? CC/CV both have line regulation tolerances            01/01/70 00:00      
                                                                                                                                                   same old, same old, same old            01/01/70 00:00      
                                                                                                                                                      There you go again ... waving your hands ...            01/01/70 00:00      
                                                                                                                                             already answered            01/01/70 00:00      
                                                                                                                                                is that what you think?            01/01/70 00:00      
                                                                                                                                                   language, language, language            01/01/70 00:00      
                                                                                                                                                      a rose by any other name ...            01/01/70 00:00      
                                                                                                                                 nearly any linear regulator can do that            01/01/70 00:00      
                                                                                                                              Yes, indeed            01/01/70 00:00      
                                                                                                                                 confound it!!!            01/01/70 00:00      
                                                                                                                                    Will an AC supply be adequate for you?            01/01/70 00:00      
                                                                                                                                       that does not deserve an answer            01/01/70 00:00      
                                                                                                                                          If not, then show us what is adequate!            01/01/70 00:00      
                                                                                                                                    Now see here ...            01/01/70 00:00      
                                                                                                   Not according to the datasheet I have ...            01/01/70 00:00      
                                                                                                      of course it does            01/01/70 00:00      
                                                                                                         So what does that schematic mean?            01/01/70 00:00      
                                                                                                            re your babble about a constant supply voltage            01/01/70 00:00      
                                                                                                               If that were the case ...            01/01/70 00:00      
                                                                                                                  Time to return to earth.            01/01/70 00:00      
                                                                                                                     What are you trying to prove?            01/01/70 00:00      
                                                                                             Perhaps, but one with no supply?            01/01/70 00:00      
                                                                           NOBODY, NOBODY can make this work ....            01/01/70 00:00      
                                                                           How about LM317? Look it up!            01/01/70 00:00      
                                                                           As I said here...            01/01/70 00:00      
                                    Just a minute, Chico ...            01/01/70 00:00      
                     Double check            01/01/70 00:00      
      I see no pullups            01/01/70 00:00      
         "The commom of the resistor array is connected to +5v"            01/01/70 00:00      
            Not what I mean!            01/01/70 00:00      
               He probably needs pullups on both inputs and outputs.            01/01/70 00:00      
                  be specific!!!            01/01/70 00:00      
                     Now I understand ...            01/01/70 00:00      
                        au contraire            01/01/70 00:00      
   Gate Threshold Voltage, what is?            01/01/70 00:00      
      Did you really try Google?            01/01/70 00:00      
         I tried google...            01/01/70 00:00      
            Vgs > Vgs(th)!            01/01/70 00:00      
   I GAVE UP!            01/01/70 00:00      
      Don't give up!            01/01/70 00:00      
      Yes, don't give up!            01/01/70 00:00      
      DONT GIVE UP, did you try ...            01/01/70 00:00      

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