A power transformer snubber is a wonderful thing for reducing or eliminating RFI from rectifier-induced LCR ringing. Unfortunately it's a huge pain to design and optimize a snubber. First you have to measure the transformer's leakage inductance and secondary capacitance, at about 100 kHz, which is not especially easy. Then you have to estimate the capacitance of your rectifier(s), which does not always appear on datasheets. Finally you plug these numbers into a formula that spits out snubber values -- and then you hope it's all correct.
Shown here is a little test jig called "Quasimodo the bell-ringer" which makes this process a great deal simpler. Quasimodo connects an actual snubber across the transformer, smacks the transformer to make it ring, and you observe the ringing on an oscilloscope. Then you adjust a (25 turn) potentiometer on the jig, watching the scope to find the setting which completely damps out all ringing. And you're done! Just use the same snubber values in the end product, as you used on the test-jig, and success is yours. (The 25 turn trimpot is socketed for ease of measuring the final resistance that gave perfect damping).
Attached are some scope waveforms showing a Quasimodo jig driving an Avel Lindberg toroidal transformer (from the Akitika GT-101 power amp). I set the trimpot to 5 different resistance values, and got the 5 traces shown. When the trimpot was removed from its socket (R = infinity), I got the black trace. A setting of Rtrimpot = 109 ohms (red trace) gave "critical damping" with no ringing: see yellow arrowhead.
Quasimodo gave an optimum snubber having critical damping (Greek letter zeta = 1.0), without requiring any calculations and without measuring the transformer's inductance or capacitance. It is a quick procedure, too: set Rtrim to max, observe waveshape on scope while reducing Rtrim, stop when all ringing is completely damped out, remove Rtrim from socket and measure it with an ohmmeter. About 3 minutes from start to finish.
This Avel Lindberg transformer has dual primaries for 115V/230V operation. So I configured the primaries the other way and ran Quasimodo again: figure attached. Not surprisingly, the optimum snubber for 230V operation is (slightly) different than the optimum for 115V operation: 120 ohms versus 109 ohms. Plus or minus the error tolerance of my ohmmeter, of course!
I've also attached a .pdf note that contains more details, schematics, construction tips, user guide, more "Quasimodo in action" scope photos, plus a bit of theory and a list of references.
EDIT 1: I had a small number of extra PCBoards + kits of all parts, which I sold at my cost, in October - December, 2013. They are all gone now, and I have no plans to sell any more. I encourage any energetic and enthusiastic diyAudio member to organize a Group Buy, using the PCB Gerber files and Bills Of Materials I provide in this thread. "CheapoModo" (a low cost version of Quasimodo) kits and PCBs are available in the Vendor's Bazaar, here.
EDIT 2: I have attached the PCB Gerber files and the Bill Of Materials, for both boards (V3_SMD, and V4_thru_hole) right here, to post #1. So now they are very easy to find!
EDIT 3: (April 2022) I just stumbled across this "Yes Math!" application note from Texas Instruments, which calculates RC snubbers to eliminate ringing from switch mode power supplies: (link)
EDIT 4: Answers to frequently asked questions:
Shown here is a little test jig called "Quasimodo the bell-ringer" which makes this process a great deal simpler. Quasimodo connects an actual snubber across the transformer, smacks the transformer to make it ring, and you observe the ringing on an oscilloscope. Then you adjust a (25 turn) potentiometer on the jig, watching the scope to find the setting which completely damps out all ringing. And you're done! Just use the same snubber values in the end product, as you used on the test-jig, and success is yours. (The 25 turn trimpot is socketed for ease of measuring the final resistance that gave perfect damping).
Attached are some scope waveforms showing a Quasimodo jig driving an Avel Lindberg toroidal transformer (from the Akitika GT-101 power amp). I set the trimpot to 5 different resistance values, and got the 5 traces shown. When the trimpot was removed from its socket (R = infinity), I got the black trace. A setting of Rtrimpot = 109 ohms (red trace) gave "critical damping" with no ringing: see yellow arrowhead.
Quasimodo gave an optimum snubber having critical damping (Greek letter zeta = 1.0), without requiring any calculations and without measuring the transformer's inductance or capacitance. It is a quick procedure, too: set Rtrim to max, observe waveshape on scope while reducing Rtrim, stop when all ringing is completely damped out, remove Rtrim from socket and measure it with an ohmmeter. About 3 minutes from start to finish.
This Avel Lindberg transformer has dual primaries for 115V/230V operation. So I configured the primaries the other way and ran Quasimodo again: figure attached. Not surprisingly, the optimum snubber for 230V operation is (slightly) different than the optimum for 115V operation: 120 ohms versus 109 ohms. Plus or minus the error tolerance of my ohmmeter, of course!
I've also attached a .pdf note that contains more details, schematics, construction tips, user guide, more "Quasimodo in action" scope photos, plus a bit of theory and a list of references.
EDIT 1: I had a small number of extra PCBoards + kits of all parts, which I sold at my cost, in October - December, 2013. They are all gone now, and I have no plans to sell any more. I encourage any energetic and enthusiastic diyAudio member to organize a Group Buy, using the PCB Gerber files and Bills Of Materials I provide in this thread. "CheapoModo" (a low cost version of Quasimodo) kits and PCBs are available in the Vendor's Bazaar, here.
EDIT 2: I have attached the PCB Gerber files and the Bill Of Materials, for both boards (V3_SMD, and V4_thru_hole) right here, to post #1. So now they are very easy to find!
EDIT 3: (April 2022) I just stumbled across this "Yes Math!" application note from Texas Instruments, which calculates RC snubbers to eliminate ringing from switch mode power supplies: (link)
EDIT 4: Answers to frequently asked questions:
- Build guide for V3 (SMD) is found in post #27
- Build guide for V4 (thru hole) is found in post #103
- 2 hour build time, quick-and-dirty Quasimodo on solderless protoboard (no PCB!) is found in post #18 and in (the CheapoModo thread)
- Substitute parts recommendations are found in the Bill Of Materials
- How to choose a MOSFET besides the ones in the BOM, is found in post #175
- How to check your own BOM before purchasing components, is found in post #203
- diyAudio members who have ordered their own sets of PCBoards from a PCB fab, using the Gerbers provided here, include: gazzagazza, luvdunhill, Borges, stormsonic, cwtim01, normundss, dsolodov, EUVL, kissmurphy, stephengrenfell, Piersma, SyncTronX, yoaudio, andrensairr. You can PM them to find out how easy or difficult it was.
Attachments
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Avel_USA.png78.5 KB · Views: 25,140
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Avel_Europe.png70.1 KB · Views: 24,779
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Quasimodo_V3_SMD_sch.png48.6 KB · Views: 25,181
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Quasimodo_V4_TH_sch.png47.7 KB · Views: 25,076
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Quasimodo_jig_revA.pdf817.4 KB · Views: 8,163
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QuasimodoSMD_Gerbers_and_everything_else.zip104.7 KB · Views: 1,660
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Quasimodo_TH_Gerbers_and_everything_else.zip347.1 KB · Views: 2,539
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