Center tapped transformer rectifier question

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  • Dannyx
    CertifiedAxhole
    • Aug 2016
    • 3912
    • Romania

    #1

    Center tapped transformer rectifier question

    Good day folks. I'm going to use a center tapped UPS transformer for a project and would like some advice on how to rectify its output. The two windings are 18v each and I already use those on the AC legs of a bridge rectifier to get around 40v out which is fine (the center tap is floating at this point), however a separate part of my project requires a lower voltage of just 18v, so how can I achieve that ? Obviously I could just place another bridge rectifier between the center tap and one of the legs, but that's not very space-efficient, so I was thinking of using 2 diodes on the center tap and other 2 already inside the rectifier....complicated I know, so I put together the rather horrible schematic to try and explain how I see it. Is this correct ?
    Attached Files
    Wattevah...
  • stj
    Great Sage 齊天大聖
    • Dec 2009
    • 30917
    • Albion

    #2
    Re: Center tapped transformer rectifier question

    follow your current paths - boom.
    your shorting the windings with the diodes!!

    Comment

    • budm
      Badcaps Legend
      • Feb 2010
      • 40746
      • USA

      #3
      Re: Center tapped transformer rectifier question

      Build it and find out for yourself if your theory is correct.
      Look real close at your drawing.
      Never stop learning
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      Comment

      • fester
        Senior Member
        • May 2017
        • 158
        • Slovakia

        #4
        Re: Center tapped transformer rectifier question

        you cant achieve that,only if you have separate grounds for every voltage and you dont plan to connect it together. Maybe the simplest thing will be using of some voltage regulator with higher allowed max. voltage on input

        Comment

        • Dannyx
          CertifiedAxhole
          • Aug 2016
          • 3912
          • Romania

          #5
          Re: Center tapped transformer rectifier question

          It's obviously wrong given that two people so far have sarcastically suggested trying it out, so I'm not gonna do it - can't really afford to scrap components like that Let's take it step by step then, since my brain hurts when working with center tappers like these....step one: rectify the 40v output - no problems here, I'm positive....pun intended
          Attached Files
          Wattevah...

          Comment

          • fester
            Senior Member
            • May 2017
            • 158
            • Slovakia

            #6
            Re: Center tapped transformer rectifier question

            Or if you accept more ripple on output,you can make two separeted half-wave rectifiers with lower winding as a ground

            Comment

            • Dannyx
              CertifiedAxhole
              • Aug 2016
              • 3912
              • Romania

              #7
              Re: Center tapped transformer rectifier question

              Then I guess the only option is to have 4 diodes (a full bridge rectifier) for the other winding as well (can't mix them together): AC side of said rectifier goes to the center winding and one of the remaining ones and the output will be 18v DC.
              Wattevah...

              Comment

              • fester
                Senior Member
                • May 2017
                • 158
                • Slovakia

                #8
                Re: Center tapped transformer rectifier question

                something like this
                Attached Files

                Comment

                • Dannyx
                  CertifiedAxhole
                  • Aug 2016
                  • 3912
                  • Romania

                  #9
                  Re: Center tapped transformer rectifier question

                  Originally posted by fester
                  something like this
                  I have seen that before, but I believe that's a half wave rectifier.
                  Wattevah...

                  Comment

                  • fester
                    Senior Member
                    • May 2017
                    • 158
                    • Slovakia

                    #10
                    Re: Center tapped transformer rectifier question

                    yes it is but if you dont need big current,it is also the solution..

                    Comment

                    • mariushm
                      Badcaps Legend
                      • May 2011
                      • 3799

                      #11
                      Re: Center tapped transformer rectifier question

                      Something like this would probably work :



                      Note that with two 18v windings you get 36v AC and that's rectified to DC voltage by the bridge rectifier and you get 36 x 1.414 - 2 x voltage drop so actually you'll have a DC voltage with a peak of ~ 50v
                      Also note that it's probably a 230v to 36v transformer, but here at around 3-4 am where there's small load on the network, i get up to 245-248v at the mains socket which is still legal (it's 230v +/- 10% in Romania and most of Europe, so up to 253v is legal)

                      The point is with 245v at the input, you're not gonna have 36v you may have 48-40v ac , and you may have 50-55v peak dc voltage. So for example, use capacitors rated for 63v or with higher voltage rating.

                      As for your 18v requirement ... you'll have 18v rms , if you rectify it to dc you'll have much more. what voltage you actually need? And more to the point, how much current you expect you're gonna need?

                      If it's just let's say 50-100mA for some opamps or something, you could just use a LM317 HV (which works up to 60v input voltage) adjustable linear regulator to and get it over with .. 40-50v in (since i doubt you're gonna install 100000 uF) , 18v DC out at 100mA means it would only waste (~40v-18v) x 0.1A = 2.2 watts of heat, which can be doable with a heatsink. Or, you can use a 3-5w resistor to drop some voltage before the regulator and then some power would be dissipated on the resistor and some on the regulator ... the regulator would only need around 2v above the output voltage to work.

                      For lower currents you could even use a zener diode... see App Note AN954 page 6/14 Resistive Transformerless Power Supply : https://cdn.badcaps-static.com/pdfs/...5df14516be.pdf
                      ... change the zener diode to 15v one or 18v or whatever you're happy with, change the resistor to the value matching your peak current (keeping in mind that you'll have ~ 36v AC and not 110v/230v AC) and everyone should be happy.
                      Attached Files
                      Last edited by mariushm; 10-22-2017, 04:02 PM.

                      Comment

                      • Dannyx
                        CertifiedAxhole
                        • Aug 2016
                        • 3912
                        • Romania

                        #12
                        Re: Center tapped transformer rectifier question

                        The "high" output of the transformer powers one of those ebay power supply modules and I also need a lower output voltage to power a DC-DC converter which runs a cooling fan for the thing...won't go into too many details on how I built this, but I tested it extensively with a variable laptop power brick which goes up to 24v and it works as it should, regulating the output to the fan based on temperature (nothing WOW there), so the voltage, while not too critical, I wouldn't dare let it go beyond 24v either. Current draw ? I'd say around 1A absolute maximum, if that (perhaps if the fan stalls or jams, worst case scenario).
                        Wattevah...

                        Comment

                        • stj
                          Great Sage 齊天大聖
                          • Dec 2009
                          • 30917
                          • Albion

                          #13
                          Re: Center tapped transformer rectifier question

                          big clive would run the fan from mains with a capacitive dropper!!

                          Comment

                          • Dannyx
                            CertifiedAxhole
                            • Aug 2016
                            • 3912
                            • Romania

                            #14
                            Re: Center tapped transformer rectifier question

                            Originally posted by stj
                            big clive would run the fan from mains with a capacitive dropper!!
                            That would work too, but the amount of components required to do so outweighs the benefits of using the unused winding like I want to do, plus it's isolated and "to standard" so to say
                            Wattevah...

                            Comment

                            • goontron
                              5000!
                              • Dec 2011
                              • 4108
                              • US

                              #15
                              Re: Center tapped transformer rectifier question

                              Keep it simple?
                              Attached Files
                              Things I've fixed: anything from semis to crappy Chinese $2 radios, and now an IoT Dildo....

                              "Dude, this is Wyoming, i hopped on and sent 'er. No fucking around." -- Me

                              Excuse me while i do something dangerous


                              You must have a sad, sad boring life if you hate on people harmlessly enjoying life with an animal costume.

                              Sometimes you need to break shit to fix it.... Thats why my lawnmower doesn't have a deadman switch or engine brake anymore

                              Follow the white rabbit.

                              Comment

                              • petehall347
                                Badcaps Legend
                                • Jan 2015
                                • 4422
                                • United Kingdom

                                #16
                                Re: Center tapped transformer rectifier question

                                Originally posted by goontron
                                Keep it simple?
                                is the 9v plus or minus ?

                                Comment

                                • goontron
                                  5000!
                                  • Dec 2011
                                  • 4108
                                  • US

                                  #17
                                  Re: Center tapped transformer rectifier question

                                  ^ Positive.
                                  Last edited by goontron; 10-24-2017, 06:35 AM.
                                  Things I've fixed: anything from semis to crappy Chinese $2 radios, and now an IoT Dildo....

                                  "Dude, this is Wyoming, i hopped on and sent 'er. No fucking around." -- Me

                                  Excuse me while i do something dangerous


                                  You must have a sad, sad boring life if you hate on people harmlessly enjoying life with an animal costume.

                                  Sometimes you need to break shit to fix it.... Thats why my lawnmower doesn't have a deadman switch or engine brake anymore

                                  Follow the white rabbit.

                                  Comment

                                  • goontron
                                    5000!
                                    • Dec 2011
                                    • 4108
                                    • US

                                    #18
                                    Re: Center tapped transformer rectifier question

                                    It seems i forgot C3 between the 18v rail and 0v rail.... Ooops.
                                    Things I've fixed: anything from semis to crappy Chinese $2 radios, and now an IoT Dildo....

                                    "Dude, this is Wyoming, i hopped on and sent 'er. No fucking around." -- Me

                                    Excuse me while i do something dangerous


                                    You must have a sad, sad boring life if you hate on people harmlessly enjoying life with an animal costume.

                                    Sometimes you need to break shit to fix it.... Thats why my lawnmower doesn't have a deadman switch or engine brake anymore

                                    Follow the white rabbit.

                                    Comment

                                    • Dannyx
                                      CertifiedAxhole
                                      • Aug 2016
                                      • 3912
                                      • Romania

                                      #19
                                      Re: Center tapped transformer rectifier question

                                      How does THIS looks ? It's actually the original idea I had and I then thought I could simplify by omitting two diodes. One of you guys posted something similar, but the values were off, so THIS is what I believe it should look like.
                                      Attached Files
                                      Wattevah...

                                      Comment

                                      • eccerr0r
                                        Solder Sloth
                                        • Nov 2012
                                        • 8662
                                        • USA

                                        #20
                                        Re: Center tapped transformer rectifier question

                                        mariushm has the only workable solution as far as I can tell - only 4 diodes needed and though his drawing only has 7.5V windings, you can use full 12V (24V/CT). A lot of these other "solutions" don't quite work because the two windings are not isolated from each other. If there is a need for isolation of the two GNDs (i.e., 0V potentials), then you'll need two isolating switching converters to get this done or another transformer that has isolated secondaries...

                                        Also note that the capacitance on the output legs will need to be fairly large to filter the higher voltage output...
                                        Last edited by eccerr0r; 10-24-2017, 08:03 PM.

                                        Comment

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