10 Replies Latest reply on Oct 21, 2019 11:23 AM by johnnybraveau

    Bridge rectifier magic?

    johnnybraveau

      Good morning. First-time newbie here. 2 years ago I yanked all the fluorescent fixtures from my shop and replaced them with LED strips, powered by cheap Chinese power supplies, see below (120VAC>12VDC, 30A).

      Some have begun to fail, and so I bought some new  ones - exact replacements - and am using one as a standard against which to compare values, and help deduce faulty components. (not that I don’t have some clear “winners” already, i. e. swollen caps, blown thermistors).

      But here’s where it gets goofy: in the process of sampling voltages on the “good” unit, I discovered that DC voltages aft of the bridge rectifier (GBU6J, GBU808, GBU810GBU810, or GBU406GBU406, depending on which board) are 2.75 times that of the AC input. Specifically, I’m getting 330VDC on the back side, with 120VAC going in. I’m including a picture of a “deduced” schematic - up to a point - along with the board itself. Power flows - on the board - from bottom left, anticlockwise around to top left. On the hand-written schematic, C1 and C6 are too small to read, so they got the “teeninie” designation, and probably don’t figure into the puzzle anyway. The pin-out of the semiconductors (2SC2625) after the 680uF electrolytic caps I am not certain of, and so I did not assign a collector/emitter, but based on their description on the spec sheet (“high power, high speed”) I am assuming this is where the 330VDC is chopped up (or modulated) and processed back into AC. The transformer represented by the “?” is the one all the way to the right on the board, nestled behind the big 680uF caps. I can’t begin to guess at the pinouts on that, but there are 3 terminals on one side, and 5 on the other.

       

      so, to reiterate: 120VAC to 330VDC...?

       

      As Professor Quirrell once said “What magic is this?”

        • Re: Bridge rectifier magic?
          colporteur

          G'Day,

          I am going to speculate from the picture the unit is a switching power supply. The Difference Between a Linear Supply and a Switched Mode Power Supply Switching mode P.S. operate differently than traditional linear power supply. The link is a recent article describing the differences.

           

          Before providing further details, I give you a cautionary note. One of the important safety concerns that can be present in Switching P.S. is what is called a floating grounds. You can let the smoke out of your test equipment if you confuse the grounds, especially with an oscilloscope.

           

          I'm going to suggest your inconsistent reading may have something to do with the ground reference used to take measurements. I am blowing the dust of my electronics career when switching P.S. were used in televisions. The price of switching P.S. is so cheap, I just pitch them when they fail.

           

          I am reluctant to suggest "try this and try that" approach based on your knowledge level. I'm not saying you can't troubleshoot the unit. Switching P.S. are unique birds and require some knowledge to deal with. If you take a look at your schematic, note you only show one ground reference. What about the reference for the output side of the circuit?

           

          I just not confident posted instructions you result in you or your equipment getting damaged in the process.

           

          Sean

            • Re: Bridge rectifier magic?
              johnnybraveau

              Sean, thanks for your reply, mate.

               

              Yes, it is a switched power supply.

              The measurements were taken directly across the terminals, with the board removed from the enclosure, and without respect to ground. Meter used was a Fluke 115.

              Gordon

                • Re: Bridge rectifier magic?
                  johnnybraveau

                  Sean,

                  one more thing: the ground reference (trace) does not extend beyond those two tiny capacitors on the output side of the isolation transformer. So - if I am reading your question correctly, the answer would be no, there is no ground reference on the high-voltage side of this board, nor on the lower voltage (12VDC) circuit after the high voltage portion.

                    • Re: Bridge rectifier magic?
                      colporteur

                      I'm confused! First,that is normal. I seem to always be confused so, it may have nothing to do with our discussion.

                       

                      Far from mate. Yes I grew up on an island but it was off the coast of Canada.

                       

                      I'm not sure of where the terminal you took the measurement are located. If I look at your schematic there is one point where you indicate a high DC voltage. If I look around that area I don't see a ground reference. How did you do a DV measurement without the reference or what did you use as a reference?

                       

                      I may cause you more confusion as a result of my own confusion. I'm recalling my electronic college lab instructors berating some 40 years ago. What did you expect to see for a measurement before you did the measurement? Not what did you find and then try to figure it out.

                       

                      My electronics knowledge is so old, electrons may not even flow in the same direction anymore. What that conventional or electron current flow we were discussing?

                       

                      Sean

                • Re: Bridge rectifier magic?
                  dougw

                  For a sine wave

                  120 Vac RMS = 339.4 V peak-to-peak

                  It sounds like you are measuring the peak-to-peak voltage.

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                    • Re: Bridge rectifier magic?
                      johnnybraveau

                      I would hope the answer to be as simple (as my ignorance). I was raised to believe that what was coming out of the outlet (here in the US Colonies) is 60hz, 120VAC, peak to peak.
                      So you’re telling me that is not the case? (That it’s really 120V rms?)

                      Or am I still missing the point?

                      • Re: Bridge rectifier magic?
                        colporteur

                        Wasn't the high voltage approx 300 DC and not AC?

                         

                        Sean

                          • Re: Bridge rectifier magic?
                            dougw

                            Yes, it would seam the measurement was DC.

                            The peak-to-peak voltage may be 330 V but it is never present between the AC wires.

                            To recover the peak-to-peak voltage as a DC measurement, I think you would need something resembling the circuit below:

                            The circuit above has these components, but not shown as connected like this.

                              • Re: Bridge rectifier magic?
                                johnnybraveau

                                Douglas,

                                I appreciate your answer(s). This has been quite a larnin’ experience.
                                Sean, I’m sorry for being less than clear on my notation of voltage sampling. I’ve included an updated sketch, which shows the points at which the probes made contact. I’m sure there is some industry-wide standard for this type of notation, but this dog grew up in the 70’s, and took only one high school class in electronics, so I apologize to all for my backwoods skills. I promise to do better!

                                 

                                and last... you may notice in this newer

                                sketch that what I thought was an isolating transformer was actually a choke (or chock, as Dear Leader might say). I simply assumed its function, instead of yanking the thing and putting a meter to it. Again, take me to the flogging post

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