14 Replies Latest reply on Sep 27, 2020 9:02 AM by opalko

    Understanding 555 timer in astable mode but stuck on capacitors

    opalko

      So I am still plowing my way through Platt's Make: Electronics and into chapter 4 working with the 555 timer IC.  The astable mode of the timer is giving me trouble as I don't understand how

       

      When C1 acquires 2/3 of the positive voltage powering the circuit, the 555 reverses its output on Pin 3 from positive to negative and forces C1 to discharge itself through R2.

      (This last sentence is actually from Make Magazine issue 10 where Platt is explaining the same thing as in chapter 4 of his book - see image below)

       

      555 timer in astable mode

       

      What I am confused about is the discharging through R2.  I am obviously stuck on what is going on with C1. As I understand it, it first gains a positive charge on what I will call the top plate in the picture above, and a negative charge on the bottom plate (next to where the label C1 is).  Maybe I have read too many analogies of water with electricity but I think of the capacitor as a balloon (in fact there are many analogies in Platt's book like this).  So how is the balloon releasing the pressure in what seems like a reverse direction back through R2?  Here is a similar circuit I found on circuitlab albeit the pins arranged differently and it shows that reversing of "flow":

       

      circuitlab 555 astable mode image

       

      Is there not +5V into R2 from R1 at the same time C1 is discharging??

      Finally here is Platt's "internal" view of the 555 timer in astable mode.  I assume pin 7 gets grounded to pin 3 but I am still confused about the seemingly reversal of flow through R2.

       

      555 internals in astable mode image

      I've read through this many times and still stuck. Any additional help is appreciated!

       

      Cheers

       

      Message was edited by: Robert Opalko same day

        • Re: Understanding 555 timer in astable mode but stuck on capacitors
          jw0752

          Hi Robert,

           

          During the discharge part of the cycle the Pin 7 is attached to ground. (see the blue - in the last picture). Once pin 7 is connected to Ground the electricity in C1 starts to move through R2 and into the Ground. The grounding of Pin 7 does also connect R-1 to Ground but this has nothing to do with R2 and C1 at this time since there is a ground between them. The last picture also shows that Pin 3 is connected to ground at this time but here again it has nothing to do with R2 and C1 since there is a ground between them.

           

           

          See if this helps? Keep in mind that the ground that I drew outside of Pin 7 is actually inside the chip and it is temporary. As soon as the charge on C1 hits 1/3 of Vcc it once again goes open and allows the current coming from R1 to recharge C1 through R2.

           

          John

          6 of 6 people found this helpful
            • Re: Understanding 555 timer in astable mode but stuck on capacitors
              opalko

              Thank you that makes it more obvious now with your diagram.  I should have realized R1 would flow the same way into ground.  See my question below to Jon about polarized vs non-polarized caps..would that matter? 

               

              I wish I could mark 2 answers as correct here, both are such good explanations.


              Cheers

                • Re: Understanding 555 timer in astable mode but stuck on capacitors
                  jw0752

                  Hi Robert,

                   

                  The polarized capacitors must be used in the correct polarity as they will fail if more than a token voltage is applied in the reverse direction. The polarity on the capacitor  is not like that of a diode. In the case of the polarized capacitor if you hook it up in the proper polarity it will work and if you hook it up in the incorrect polarity it will conduct  current, get hot and potentially explode provided there is enough energy available in the circuit. Generally speaking the electrolytic capacitors have higher values of capacitance for their size than do the non-polarized types. There are other differences too but probably not important to this application. The number and variety of components that can be used to make any particular circuit application is what makes electronics an art. The 555 that you are working with is a great example. There are literally hundreds of different applications that it can be applied to. Check out:

                   

                  http://www.talkingelectronics.com/projects/50%20-%20555%20Circuits/50%20-%20555%20Circuits.html

                   

                  If there is ever any conflict between what I am telling you and what jc2048  is telling you your best bet is to listen to him. He is a real expert while i am just an enthusiast.

                   

                  John

                  4 of 4 people found this helpful
                  • Re: Understanding 555 timer in astable mode but stuck on capacitors
                    jc2048

                    I wish I could mark 2 answers as correct here, both are such good explanations.

                    Give it to John: he got to the finishing post first. I was just adding a little more detail to the answer he'd already given.

                • Re: Understanding 555 timer in astable mode but stuck on capacitors
                  jw0752

                  PS you are doing an excellent job of presenting your questions. I would bet that several times you have actually answered them yourself in the process of preparing the question.

                   

                  John

                    • Re: Understanding 555 timer in astable mode but stuck on capacitors
                      colporteur

                      Great response JW.

                       

                      I wanted to acknowledge your contributions. I have indicated in my past posts, the responses from members to question posted have value. Someone took the time to respond. This post is a perfect example demonstrating that. To individuals that have an understanding the response may seem trivial. To the learner, stuck in spot in theory that seems incongruent, small clarification such as what you have provided is immense in furthering their progress.

                       

                      I doff my chapeau to you, in acknowledgement!

                      1 of 1 people found this helpful
                    • Re: Understanding 555 timer in astable mode but stuck on capacitors
                      fmilburn

                      This is one of those times when animated graphics might help. The 555 timer was not intuitive to me at first either from just the schematics and block diagrams. Karen on the Learning Circuit might be helpful.  Also this video 

                      5 of 5 people found this helpful
                      • Re: Understanding 555 timer in astable mode but stuck on capacitors
                        jc2048

                        When you charge a capacitor, the increasing imbalance of charge causes the voltage between the plates to increase and

                        the resulting electric field between the plates then stores energy. When you discharge the capacitor, that

                        energy is then powering the discharge circuit.

                         

                        Personally, I don't much like bucket [water] and balloon [air] analogies. A capacitor isn't simply a

                        container that you to pour something in to. When you add charge to one plate, you always remove the same

                        charge from the other. It's the resulting imbalance between the two that stores energy and it can be either

                        way round (leading to a voltage in either direction across the capacitor). Negative amounts of water and

                        negative pressures are a bit weird, when you think about it. [Yes, the maths works with 'negative water'

                        too, but what's the point? You're then struggling to explain an analogy that's supposed to make things

                        easier to understand.]

                         

                        When we charge or discharge a capacitor, that flow of charge constitutes a current. Current paths are loops

                        'driven' by voltage sources.

                         

                        Here is the current path [red] when the capacitor charges

                         

                         

                        and here is what happens when the 555 connects pin 7 to ground [black] and the capacitor discharges

                         

                         

                        There are now two paths. The current through R1 [red] now flows back to the supply through the 555. At the

                        same time, the capacitor voltage drives a current through R2 [green] that returns to the other side of the

                        capacitor through the 555. That current discharges the capacitor. [As it discharges, the voltage across the

                        capacitor reduces until it reaches the lower threshold, at one third the supply voltage, at which point the

                        555 removes the connection from pin 7 to gound and the capacitor starts charging again.]

                        5 of 5 people found this helpful