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    #21
    Re: Current flowing through transistor even when base is shunted to ground.

    Originally posted by EasyGoing1 View Post
    Which is technically what I'm doing here. However, hypothetically, if your source voltage for the base of the transistor was ... let's say 5V but it was capable of supplying 5 amps of current, without that base resistor, wouldn't it fry the BE junction?
    The V-I characteristics of the base-emitter junction are those of a diode. I.e., at low voltages (very close to 0), there is no current flowing. Once you get to about 0.6-0.7 volts, current flows. Pushing "extra" current through the diode will increase the voltage drop... but not very much! If you look at the curve, it heads ALMOST straight up beyond 0.7V.

    So, imagine how high it will be (height = current) by the time the voltage has increased to equal that 5V that you are IMPOSING.

    Ans: it will be so high that the diode will fry! :>

    Interesting that simply adding the transistor can complicate the LED circuit that much.
    You're using the transistor to give you current gain (and increased voltage compliance). You can't source (or sink) the amount of current that the LEDs want/need so use the transistor for "muscle".

    It makes perfect sense of course. As it happens to be, when these LEDs turn on in this application, they are plenty bright enough to where normally, I would not look for any reason to change any of the parameters of the circuit. Based on what you're saying, I'm most likely not giving the LEDs their full 150ma, which is fine because that only means that I'm driving them at a safer current and hence they will last longer and the risk of any of them being damaged from overcurrent is a non-issue.
    You can see what the ACTUAL current is by measuring the voltage drop across the 6R8 resistor and dividing by 6.8.

    Based on what you're saying, since the output of pin 4 is roughly 5 volts (minus the .7 for the B-E junction), then when I divide that by 1.7k, I get a current of 2.5ma ... which shouldn't be enough to saturate the transistor.
    At the minimum hfe of ~30, that would give you ~75mA of collector current. If 75mA flowing through the LEDs and 6R8 leaves you with ~0.2V to spare, then you're saturating the transistor.

    If, OTOH, the LEDs only dropped 2.2V each, you might see a different result; the "excess/residual voltage" appearing across the transistor, increasing the power dissipated in it.

    Note that there are other consequences to using a reduced drive... it also reduces the amount of conducted and radiated "noise" from the circuit (assuming you are flashing them).

    [In your case, you wouldn't likely be flashing them fast enough to make a noticeable difference. But, imagine you had to drive 100 sets of LEDs. You'd likely drive some subset of them with circuitry SHARED among tham all -- multiplexing. If you drove 10 sets at a time, then moved on to the next set of ten, etc. until every set had been driven... lather, rinse, repeat... and did this fast enough that a person wouldn't see the fact that the LEDs are actually "flashing very very quickly", you start to generate noise that can upset other bits of nearby electronics.

    Similarly, if you tried to pulse-width-modulate the drive to give you an "intensity" control, you'd be switching the *single* set of LEDs on and off very quickly to give the illusion of being lit continuously -- albeit at a varying intensity.

    However, the LEDs are plenty bright enough (so much so, that you can't look at them directly without causing some discomfort - but they will be diffused in the final application of the circuit) ... so since I'm only driving the base at 2.5ma (the source is capable of 40ma), does that mean that the LEDs are not getting the full current that I originally intended?
    Likely NOT getting 150mA -- simply because of the 6R8 (redo the math).

    Comment


      #22
      Re: Current flowing through transistor even when base is shunted to ground.

      Originally posted by EasyGoing1 View Post
      I followed everything you said here, thank you. I get it now... my final circuit has a BC337 and its datasheet lists Hfe as a function of collector current. The range of Hfe being from 100 to 630. Since I'm using a resistor at the base of 1.7k and assuming a 5v output from the Arduino pin, my Ibe would be 2.5ma. The minimum gain is 100 so worst case, I can have 250ma at the collector, which would be limited by that 6.8Ω resistor in series with the LEDs, so since my actual source voltage for the LEDs is 9.8, and considering the voltage drop Vce, I am most likely giving those LEDs less than 150ma, which means they are being driven well within tolerance.
      Correct.
      Keyword here being "less than".

      And the best part is, you can test that by placing an Ammeter in series with the LED's path to measure the current and see if the circuit you built actually agrees with your theory (it probably will, or would be very close.) You can also move the Ammeter between the Emitter of the BJT and ground and measure that current. Then, if you compare Ie and Ic, you will see that they are almost identical, with Ie being only slightly higher due to the added Ib current.

      Originally posted by EasyGoing1 View Post
      What I also realized reading your post, is that it would theoretically be possible to drive LEDs without a series resistor by properly selecting your base current and knowing exactly what your H value would be at that base current and voltage... which would take some experimentation to determine, but in theory it would be possible, would it not?
      It is possible, correct.
      However, since h_FE can vary not only with current, but also with temperature and between even supposedly identical transistor parts, it's not practical to do.

      That said, there is a way to "stabilize" the linear operation of the transistor and make it practically independent of h_FE by placing a resistor between Emitter and ground. With this, if any change occurs to the transistor h_FE so that it increases, the Emitter current would also try to increase for a given Base current. However, with the resistor between Emitter and ground, an increased current through it will cause a greater voltage drop, which would in turn raise the voltage observed at the Base of the transistor. And if a series resistor is used to limit Base current, then the Base current would decrease, which in turn would cause the Emitter current to decrease back to its original value, regardless of the increase in h_FE. In simpler words, the Emitter resistor in that case would provide negative feedback - a property extensively used in BJT amplifier circuits. But here, we are not doing that, so keeping the LED series resistor makes more sense.

      Originally posted by budm View Post
      Do the experiment, set power supply to 5V with 5A limit then apply it to the E-B junction of spare Transistor you have and see what happen.
      Or just do the math.

      Let's see here... 3904 BJT in standard TO-92 case is rated at maximum of 625 mWatts dissipation. No mention of the maximum Base current (though some datasheets do include that.)

      Assuming a constant Vbe of 0.7V and dumping 5 Amps of current there would produce 0.7 x 5 = 3.5 Watts or 3500 mW - about 5.5 times more than what the whole transistor can handle.

      So short answer: YES, it would fry the transistor.
      ... possible exception may exist if the Base current is pulsed, though. And in such case, the pulse width would have to be extremely narrow to no blow the BE junction. Also, the 625 mW rating is for the whole transistor die, so any current flowing through C-E multiplied by the Saturation voltage would add up to the dissipated power as well.

      *EDIT*
      Seems that CG beat me to it with the explanation.
      But yeah, this is why you do need to limit current at the Base of a BJT. Otherwise, it can burn out the BE junction, or worse.
      Last edited by momaka; 09-09-2020, 05:20 PM.

      Comment


        #23
        Re: Current flowing through transistor even when base is shunted to ground.

        Originally posted by EasyGoing1 View Post
        Do I need a complete transistor circuit to perform the test? In other words, do I need a circuit to complete C-E before B-E will saturate? I'm thinking not???
        Nope, Just hook up the power +5V to the Base of NPN, Negative to the Emitter of a spare Transistor you have and then watch the current draw and see the effect.
        Last edited by budm; 09-09-2020, 05:55 PM.
        Never stop learning
        Basic LCD TV and Monitor troubleshooting guides.
        http://www.badcaps.net/forum/showthr...956#post305956

        Voltage Regulator (LDO) testing:
        http://www.badcaps.net/forum/showthr...999#post300999

        Inverter testing using old CFL:
        http://www.badcaps.net/forum/showthr...er+testing+cfl

        Tear down pictures : Hit the ">" Show Albums and stories" on the left side
        http://s807.photobucket.com/user/budm/library/

        TV Factory reset codes listing:
        http://www.badcaps.net/forum/showthread.php?t=24809

        Comment


          #24
          Re: Current flowing through transistor even when base is shunted to ground.

          Originally posted by momaka View Post
          Correct.
          Keyword here being "less than".

          And the best part is, you can test that by placing an Ammeter in series with the LED's path to measure the current and see if the circuit you built actually agrees with your theory (it probably will, or would be very close.) You can also move the Ammeter between the Emitter of the BJT and ground and measure that current. Then, if you compare Ie and Ic, you will see that they are almost identical, with Ie being only slightly higher due to the added Ib current.


          It is possible, correct.
          However, since h_FE can vary not only with current, but also with temperature and between even supposedly identical transistor parts, it's not practical to do.

          That said, there is a way to "stabilize" the linear operation of the transistor and make it practically independent of h_FE by placing a resistor between Emitter and ground. With this, if any change occurs to the transistor h_FE so that it increases, the Emitter current would also try to increase for a given Base current. However, with the resistor between Emitter and ground, an increased current through it will cause a greater voltage drop, which would in turn raise the voltage observed at the Base of the transistor. And if a series resistor is used to limit Base current, then the Base current would decrease, which in turn would cause the Emitter current to decrease back to its original value, regardless of the increase in h_FE. In simpler words, the Emitter resistor in that case would provide negative feedback - a property extensively used in BJT amplifier circuits. But here, we are not doing that, so keeping the LED series resistor makes more sense.


          Or just do the math.

          Let's see here... 3904 BJT in standard TO-92 case is rated at maximum of 625 mWatts dissipation. No mention of the maximum Base current (though some datasheets do include that.)

          Assuming a constant Vbe of 0.7V and dumping 5 Amps of current there would produce 0.7 x 5 = 3.5 Watts or 3500 mW - about 5.5 times more than what the whole transistor can handle.

          So short answer: YES, it would fry the transistor.
          ... possible exception may exist if the Base current is pulsed, though. And in such case, the pulse width would have to be extremely narrow to no blow the BE junction. Also, the 625 mW rating is for the whole transistor die, so any current flowing through C-E multiplied by the Saturation voltage would add up to the dissipated power as well.

          *EDIT*
          Seems that CG beat me to it with the explanation.
          But yeah, this is why you do need to limit current at the Base of a BJT. Otherwise, it can burn out the BE junction, or worse.
          It is more fun for OP to do hands on!
          Never stop learning
          Basic LCD TV and Monitor troubleshooting guides.
          http://www.badcaps.net/forum/showthr...956#post305956

          Voltage Regulator (LDO) testing:
          http://www.badcaps.net/forum/showthr...999#post300999

          Inverter testing using old CFL:
          http://www.badcaps.net/forum/showthr...er+testing+cfl

          Tear down pictures : Hit the ">" Show Albums and stories" on the left side
          http://s807.photobucket.com/user/budm/library/

          TV Factory reset codes listing:
          http://www.badcaps.net/forum/showthread.php?t=24809

          Comment


            #25
            Re: Current flowing through transistor even when base is shunted to ground.

            i was wanting to do it with this but it wont let me ..maybe its my computer i dunno ..
            http://www.falstad.com/circuit/e-npn.html

            Comment


              #26
              Re: Current flowing through transistor even when base is shunted to ground.

              Originally posted by budm View Post
              It is more fun for OP to do hands on!
              Yeah, I know.
              Sorry for being the party pooper.

              On the other hand, if O/P doesn't have a bench PSU with good or any current limiting, that test could end up like some of ElectroBOOM's videos

              ... and it would waste a good working part
              ... and the fumes smell bad
              Last edited by momaka; 09-09-2020, 06:17 PM.

              Comment


                #27
                Re: Current flowing through transistor even when base is shunted to ground.

                with a bench supply it could be a well controlled destruction . throw in a video of it including infra red temp readings should spice things up a little .

                Comment


                  #28
                  Re: Current flowing through transistor even when base is shunted to ground.

                  Originally posted by budm View Post
                  It is more fun for OP to do hands on!
                  I'm still gonna do the experiment ... it's fun letting the smoke out ... and I am curious to know how long it will take to destroy the junction... if it's as fast as an LED, I MIGHT see a number on the current meter when I light up the circuit. If it's a little more resilient than I will for sure.

                  If I could measure current with my scope, I'd set it to snapshot the turn-on event but low-end scopes don't come with leads for current.

                  What I find fascinating about electronics, in general (in this application anyway), is that all we need to do to prevent the damage is drop a 1k before the base and then the amount of current available at the source is irrelevant.

                  Comment


                    #29
                    Re: Current flowing through transistor even when base is shunted to ground.

                    Originally posted by petehall347 View Post
                    with a bench supply it could be a well controlled destruction . throw in a video of it including infra red temp readings should spice things up a little
                    I don't have a bench supply, but I'm going to purchase one in about a month-ish. What I do have are those adjustable DC to DC buck converter circuits (you can get a hand full on Amazon for about $15) that also has the ability to adjust available current. I usually drive those with a 19V laptop power supply so I can set the buck to 5V and I'm sure I can push the amperage up to 5 amps with that power supply, but I'm sure anything over a couple of amps would do the job.

                    Comment


                      #30
                      Re: Current flowing through transistor even when base is shunted to ground.

                      I just want to say that around 1990, I went through the electronics courses at my local junior college, starting from basic electronics and ending with two semesters of electrical calculus. At the end of the courses, we had the option of taking the CET exam which I did and I passed. But then I went on and built my career as a Network Engineer so I never really used my electronics education specifically ... but there were many times when that education served me well when I needed to troubleshoot servers and workstations...

                      Transistors were always something that I didn't feel like I fully understood internally, even though I could run the calculations and answer the questions.

                      This thread really helped me make more sense of transistors ... must have been something in the way you guys explained it. I know there is much more to know about them, especially when you start getting into oscillating current such as in audio applications etc., but in terms of the basics, I definitely feel like I have a much better grasp on them than I did before I started this post.

                      Thank you - all of you - who took the time to write what you did to explain things to me. I really do appreciate you making that effort.

                      Comment


                        #31
                        Re: Current flowing through transistor even when base is shunted to ground.

                        Old thread but I have an interesting comment about MMBT3906 mentioned here. NTE says NTE2407 is the equivilent but it has higher mA and Voltage ratings like 40v 200mA for 3906 and 60v 600mA for 2407. Close enough I suppose.

                        Comment


                          #32
                          Re: Current flowing through transistor even when base is shunted to ground.

                          NTE devices are designed to be replacements for other parts not the other way around. Hence why the NTE part has higher specs than the one it replaces.
                          Originally posted by PeteS in CA
                          Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
                          A working TV? How boring!

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