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    Ripple Current ?

    Hi Folks,

    This is my first post here. My question is about ripple current. If a capacitor is on a circuit powered by filtered DC, is ripple current still getting to the cap? I am using a 3300 mfd 50 volt electrolytic cap as an "electronic flywheel" in parallel with a DC can motor powered by AC (the locomotive has a full wave bridge rectifier inside) from an ordinary toy train transformer with a max of 18 VAC. If I were to power the locomotive with filtered DC (filtered by a 2200 mfd 50 volt cap) instead of AC, could I use a cap of a lower voltage rating? Speaking just in theory, does a filter cap reduce or eliminate ripple current in other caps down the line?

    My basic rule of thumb is to use a cap rated at about 2-3 times the voltage it may receive. My electronics knowledge is from hobby reference materials and some basic practical experience like sizing-up the cap if it gets warm.

    I will be happy to elaborate if that would help!

    Thanks!

    Take care, Joe.

    #2
    Re: Ripple Current ?

    not sure on train and motors

    Sounds like you build a DC psu for it on board the loco

    now if I read the rest right you want to run the whole she bang On DC

    Well I would just find a decent site that deals with psu design with out the hifalutin maths.
    (there is an FAQ section here that has posts on power supplies)

    me off the top of my head 10 V~15V above the transformer AC "unloaded" voltage.

    caps will charge to peak of AC less Vdrops across diodes so say for example

    10 VAC rrms x 1.414 = 14.14V AC peak

    your case with 17 ~ 18 VAC RMS (read as = DC over time) Id say 50 V is the best for you..and I think only voltage you could get 25V would be too low
    ========================================================
    Ripple is really more to do with how fast current is pulled off the cap before it get its recharge pulse (of course it a little more complex then that crude statement.)

    so back tracking the flow of this
    the rectifier Bridge turn ac in to pulsating DC the cap acts a a storage bucket
    (smooths out the peaks, so its at one voltage level all the time just like DC
    (well it is DC)

    so what you get is a full bucket all the time if your not taking current from it.

    once you start taking current (add a load like a motor) from the cap depending on how much you take before it get it next fill determines the amount of ripple.

    Now if you already have full bucket but you add another bucket
    (here's were we will fall apart a bit, so lets just say when you add a capacitor in parallel with the filtered DC (which already has a cap) you just have a BIGGER bucket)

    The "bigger" bucket will reduce the ripple for the same amount of current.
    so yes there will be ripple just less of it due to the fact you got a bigger bucket

    I should add that depending on how far apart the caps are and the gauge of wire use to hook between them this may also effect operation.

    when ever you take DC from a psu like this it must be "from the caps" not the bridge

    Hope that makes sense

    here a few links on linear psu design and how they work

    iin the FAQ's tom41post I have links of this through out the post


    Basic PSU unregulated

    fairly in depth on psu design really more so for Audio amps but the theory is all there



    Like I said no idea with trains thought....I didn't have a "real" train set as a kid

    HTH and Welcome Joe

    Cheers
    Last edited by starfury1; 02-25-2008, 04:39 AM.
    You step into the Road, and if you don't keep your feet, there is no knowing where you may be swept off to." Bilbo Baggins ...

    Comment


      #3
      Re: Ripple Current ?

      Hi StarFury,

      Many thanks for the reply! Your reply and the links really helped me!

      Much of my electronics experience is just the practical application of simple model train electronics. I will attach 2 little diagrams that will give everyone a better idea than just my words. I would like for the filter capacitor to allow for the use of a lower rated (in volts) flywheel cap since physical size is limited by the room within a locomotive.

      So when filtered DC is fed to circuit in the locomotive (diagram 1) there is less ripple current to the flywheel cap, as opposed to just AC fed to the locomotive (diagram 2).





      I would like to achieve an effect where little if any ripple current gets to the flywheel cap. Perhaps just additional filter caps in parallel would result in less ripple to the track? Am I correct in assuming that the smoother the DC, the less ripple current? A battery has no ripple current? I guess the big question is: Can I build a supply with little or no ripple current just by adding more filter caps?

      Thanks!

      Take care, Joe.
      Attached Files

      Comment


        #4
        Re: Ripple Current ?

        should have mention too that the supply is unregulated.

        I am not really up on the best way to feed a DC motor.

        you first diagram shows 2 Bridge rectifiers, the second one(in loco) apart from a slight vDrop (2V say) across it would be pretty much the same voltage as the first DC voltage.

        it would have a "steering effect" on the DC
        (which possibly isn't such a bad Idea, Old trick for idiots with CB's)

        So your problem is really physical size of cap?

        As the circuits stands you will need close to the other caps voltage I would say
        the only way to reduce the size is to reduce the capacitance of the second cap

        you may possibly counter balance by having more on the first cap say make it 3300uf
        (yeah now you remind me there is a 35 Volt range with caps)

        Now I know nothing of how they work,

        so is that AC transformer a variable type Between 8Vac and 18Vac?

        So dc Aprox is 12V~26V DC (the 1.414 thing for peak AC)

        yeah more caps less ripple but you do this with in reason as other factors come into play

        So next guess is the varying DC is being used to control speed of loco?
        more capacitance will probably result in a longer latency in control.

        What you could do I think is actually build a "regulated variable DC supply" this would reduce ripple way down and you would get away with less capacitance in the loco I think.

        I'll have to go learn how train sets work and the motors in regard to them I suppose.

        what might be another factor is you are basically building a DC transmission system which is variable
        So you have long lengths of wires acting as ant plus DC resistance of wire and contacts.
        (AC is much more effective for power transmission and thats why all of mains is AC)

        BTW the Bridge and cap in loco is that a standard thing?
        (or did you do that, think from original post you did)

        anyway I'll look into it.

        Cheers
        You step into the Road, and if you don't keep your feet, there is no knowing where you may be swept off to." Bilbo Baggins ...

        Comment


          #5
          Re: Ripple Current ?

          Thanks for the diagrams it helps understanding.

          Diagram 2 does not supply AC to the engine because the rectifier has converted AC to DC with ripple controlled by the cap.

          Diagram 1 is waste of time in that the first rectifier creates DC and the second simply passes that same DC and adds a circuit loss in the second rectifier. Connecting both caps together will lower the ripple, leave out the second rectifier. I can see that you are trying to run the track on DC . Why? Running on AC reduces contact arking.

          I would like to achieve an effect where little if any ripple current gets to the flywheel cap. Perhaps just additional filter caps in parallel would result in less ripple to the track? Am I correct in assuming that the smoother the DC, the less ripple current? A battery has no ripple current? I guess the big question is: Can I build a supply with little or no ripple current just by adding more filter caps?
          You cannot avoid ripple. It will always be there, except when the train motor is not a load. As soon as a load is applied ripple will be present. The amount is a product of the load current and the total capacitance. So a large cap = less ripple. The only way to completely get rid of ripple is to build a regulated power supply. Having said that, a motor could not careless about ripple, it is not a problem, most DC motors will run with a large ripple and in fact with no capacitor. It is only electronic circuits that are bothered by ripple.
          Last edited by davmax; 02-25-2008, 11:26 PM.
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            #6
            Re: Ripple Current ?

            A/C is only more efficient for power transmission in the middle distances. Under 3 miles or so, DC works okay. Over 1,000 miles, high voltage DC works better than AC.

            From 3-600 some-odd miles, A/C is better.

            Comment


              #7
              Re: Ripple Current ?

              Diagram 1 is waste of time in that the first rectifier creates DC and the second simply passes that same DC and adds a circuit loss in the second rectifier. Connecting both caps together will lower the ripple, leave out the second rectifier. I can see that you are trying to run the track on DC . Why? Running on AC reduces contact arking.
              On bridge 2 True its pointless as far as psu goes DC wise
              Its a removable thing the loco and the line was AC which is why I asked
              it does allow the option of running on AC track
              Also it would ensure DC is feed in right polarity

              I knew there was a reason I liked the AC "contact arking" thanks

              OK on the motor..which is something I am unsure about, from what Davmax has said
              I suppose the question is "why do you want to reduce ripple" to it?

              I know weird and wonderful things are dome with electric train sets these days
              but I know nothing on them really.

              Cheers

              True bookworm and in a train set it probably wont matter that much
              but like davmax said there must be other reasons why its been used
              Last edited by starfury1; 02-26-2008, 12:41 AM.
              You step into the Road, and if you don't keep your feet, there is no knowing where you may be swept off to." Bilbo Baggins ...

              Comment


                #8
                Re: Ripple Current ?

                AC

                I should have clarified the mains remark above your quite correct bookworm
                I was talking really in regards to power grid
                but with train set I think there possibly a small advantage as one point made by davmax

                like I said in another post Tesla should have made Bill Gates look like a down and out, wealth wise
                but he died with nothing blew what money he did get on other things)
                He not Edison is the man you bow down to and say "were not worthy"

                Every power gird in the world is of a direct consequence of his work.

                Cheers
                Last edited by starfury1; 02-26-2008, 12:58 AM.
                You step into the Road, and if you don't keep your feet, there is no knowing where you may be swept off to." Bilbo Baggins ...

                Comment


                  #9
                  Re: Ripple Current ?

                  Every computer, every power grid, every low power light fixture, every radio transmission.....

                  (He developed the concept behind the 'AND' logic gate, required by modern computers).

                  He also invented radio, not Marconi. Marconi, it turns out, was a student at one of the universities where Tesla spent time lecturing on the transmission of information through electromagnetic radiation

                  Comment


                    #10
                    Re: Ripple Current ?

                    A better solution would be a PI filter.

                    Although, as davmax said, a DC motor isn't particularly sensitive to ripple.

                    .
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                      #11
                      Re: Ripple Current ?

                      PI filter is not worth it for a motor.

                      but with train set I think there possibly a small advantage as one point made by davmax
                      It is more than a small advantage, AC on the rail is put there for a good reason, DC arcing burns contacts.
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                        #12
                        Re: Ripple Current ?

                        ok big advantage (i just knew there was a reason or more why I liked the AC idea)

                        so on that note,
                        if you don't mind blowing holes in your contacts
                        (and it seems from this site some don't, although there is on another page how to add contacts...maybe the above is why)

                        I found a site for trains that had this psu

                        3Amp psu using a LM350K.. here
                        (note the reg will need to go on a fair sized heatsink)


                        main page Here

                        lots of stuff there should keep you amused for hours Joe

                        Cheers
                        Last edited by starfury1; 02-26-2008, 09:09 AM.
                        You step into the Road, and if you don't keep your feet, there is no knowing where you may be swept off to." Bilbo Baggins ...

                        Comment


                          #13
                          Re: Ripple Current ?

                          Hi Folks,

                          Many thanks for all the replies and great information!

                          The reason there is a bridge rectifier in the loco in the example is so it can be run on both AC or DC, and to keep the polarity of the cap correct. And without the bridge in the loco, someone could easily place the loco on the DC rails in the wrong direction and the cap would get reversed polarity, or place the loco in AC rails. I do eliminate the bridge in the loco when I use a bipolar cap on DC rails, but I didn't want to get into that to confuse the issue. (I have 3 rail O gauge track powered by either lightly filtered DC from a wall transformer at a fixed voltage or variable AC from old, but safe, train transformers, some quite powerful. Also 2 rail HO and S gauge.)

                          The size of the cap for my purposes in the loco needs to be 3300 mfd at least. What I want to do is use a cap rated at just about 16 volts because of the physical size limitations. The transformer would deliver approx. 8-16 VAC, but when my house is supplied with about 120 VAC, those toy transformers and wall transformers that aren't regulated will go high. Once I measured 122 VAC from the wall receptacle. However, I will never give the loco more than about 14 VAC.

                          As far the power supply is concerned, the caps can be any size there. I have a few 6800 mfd 50 volt caps that I could used as filter caps on the supply.

                          Would this help to eliminate a good bit of the ripple?



                          Many thanks!

                          Take care, Joe.
                          Attached Files

                          Comment


                            #14
                            Re: Ripple Current ?

                            Hi Folks,

                            I forgot to add that the reason you run on DC is for direction control of a locomotive by reversing polarity. Some of the AC equipment has a reversing circuit for a DC can motor, but that takes up space I don't have in small locos, handcars, railcars, etc.

                            Thanks!

                            Take care, Joe.

                            Comment


                              #15
                              Re: Ripple Current ?

                              Originally posted by Joe Rampolla
                              Hi Folks,

                              I forgot to add that the reason you run on DC is for direction control of a locomotive by reversing polarity. Some of the AC equipment has a reversing circuit for a DC can motor, but that takes up space I don't have in small locos, handcars, railcars, etc.

                              Thanks!

                              Take care, Joe.
                              Sorry Joe I am trying to make sense of what you are doing.
                              1 Why do you require low ripple? Motors do not.
                              2. You say DC on track to reverse motion of loco. If polarity is reversed with a rectifier in loco the direction will not reverse. If polarity is reversed without a loco rectifier there must be no polarised cap in loco.

                              I assume you are going for option 2, keeping cap outside loco.
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                                #16
                                Re: Ripple Current ?

                                I don't understand the point of what you are doing either but I'll play along.

                                You don't need low ESR caps for these filters. Standard caps should do fine.

                                You say you only need 3300uF yet your drawing shows 20400. ??

                                How much room do you actually have for mounting caps?

                                Where is the polarity (direction) reversing switch in relation to the capacitors, rectifier, and track?

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                                  #17
                                  Re: Ripple Current ?

                                  well I think your limitation is voltage, the cap as you are aware MUST be higher then the DC Voltage.

                                  The only way you can use lower voltage cap is to lower the voltage from supply
                                  regulate or possibly use a power resistor which maybe a drama getting the drop right
                                  (not to mention heat)

                                  So if you need 3300 uf your going to be stuck some what with that size
                                  and yeah I still trying to figure out exactly why you need reduced ripple, since the comment above don't think you need to worry about it

                                  But no HO loco engineer here

                                  Humm yeah well if by reverse you mean plus+ becomes minus- and minus- becomes plus+ on the rails
                                  then Iam with the others too
                                  it cant happen in the loco with the bridge and cap
                                  simply the bridge will "Steer" it
                                  (old trick to save CB rigs from those that liked to reverse the DC PWR leads called idiot proofing...and I hear all the ARRL members saying WHY????)

                                  So how is the reversing done in your case with that setup? as above

                                  Now without the bridge and a Bipolar cap yeah that will work I think

                                  You can make a Bipolar cap by putting 2 caps in series with either the + tied together or the -

                                  ----[]---+---+----[]-----

                                  You can force equal voltage sharing by using resistors of same value
                                  (1 in parallel with each cap)
                                  Other wise its at the whim of the actual capacitors were the mid voltage rests

                                  But the BIG draw back here is you going to have to double the capacitors uf to get the same capacitance you want
                                  like for 3300uf you need 2 X 6600uf
                                  get the idea...now that isnt going to fit

                                  Also how well they work in a power type application I am not sure...maybe bonze or davmax can answer that better.
                                  (I am thinking it might be a case to use lower ESR types to reduce internal resistance, I could be wrong)
                                  Mostly you see this done with Audio.

                                  Thats if I remember it all correctly.

                                  Id say that site has something on this "reversing thing" so guess i'll go have a read.

                                  HTH

                                  Cheers
                                  You step into the Road, and if you don't keep your feet, there is no knowing where you may be swept off to." Bilbo Baggins ...

                                  Comment


                                    #18
                                    Re: Ripple Current ?

                                    The reason there is a bridge rectifier in the loco in the example is so it can be run on both AC or DC, and to keep the polarity of the cap correct.
                                    Are ok you know that bout the bridge
                                    You step into the Road, and if you don't keep your feet, there is no knowing where you may be swept off to." Bilbo Baggins ...

                                    Comment


                                      #19
                                      Re: Ripple Current ?

                                      humm ok the more caps you wack on sure the less ripple but you are also pulling a lot more peak current through the bridge so your going to need a nice beefy bridge on the psu.

                                      As I said its an within reason thing,
                                      go to far from the specs of bridge or transformer and you could create problems.

                                      Also the more you have the larger Dump you will get from the cap current wise if you accidentally short something.

                                      20,000 uf has good potential to blow chunks out of things.

                                      cheers
                                      You step into the Road, and if you don't keep your feet, there is no knowing where you may be swept off to." Bilbo Baggins ...

                                      Comment


                                        #20
                                        Re: Ripple Current ?

                                        Starfury you are right about power release from caps. It is bad enough using DC that increases arcing damage, adding large cap values will create even more contact damage.


                                        ----[]---+---+----[]-----

                                        You can force equal voltage sharing by using resistors of same value
                                        (1 in parallel with each cap)
                                        Other wise its at the whim of the actual capacitors were the mid voltage rests

                                        This is not correct. Do not even try it. The caps will not voltage share on one polarity or the other. The one with reverse voltage/current will have a very low resistance. Reverse voltage on these caps should never exceed 1 volt max. Voltage sharing resistors are only used when two caps are connected in series and both in the same polarity.

                                        We still do not know why low ripple is required!!!!!!!!!!!!! If we know why then another solution may be applied.
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