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???
11/26/09 15:46
Modified:
  11/26/09 15:52
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 		 #171202 - Vgs > Vgs(th)!
Responding to: ???'s previous message
Chico said:
This means, I need a higher voltage than Vgs to "switch" the MOSFET?

MOSFETs have the drain source path differently controlled, namely by an electric field, than a NPN/PNP-transistor his collector emitter path, where a current needs to flow across the base emitter junction.

MOSFETs are controlled by a voltage between gate and source, called Vgs. The gate source threshold voltage Vgs(th) is the minimum voltage needed at gate and source to make a current flowing along the drain source path. So, to turn on a N-channel enhancement MOSFET a gate source voltage Vgs > Vgs(th) is needed. And, the more current shall flow the more voltage is needed.

Kai Klaas


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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