Simple, no-math transformer snubber using Quasimodo test-jig - Page 8 - diyAudio
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Old 5th November 2013, 10:48 PM   #71
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Ctrlx, do you have the power transformer in your possession, the one that you'll use in your final, end-use, project build? Have you built and debugged a Quasimodo test-jig yet? I don't see your userID on the list of people who purchased one of my leftover PCBoards, so I guess you'll construct it on a Vero board or even a solderless breadboard as shown in post #18 of this thread.

The first thing to do is to complete the longest lead-time activities: obtain the transformer, and construct the bellringer jig. Then you can start planning which R&C component values to use "at least as a starting point for further experimentation." (QM design note p.7).
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Old 6th November 2013, 10:01 AM   #72
ctrlx is offline ctrlx  United Kingdom
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Quote:
Originally Posted by transistormarkj View Post
If you want to replicate the Quasimodo idea, the testbench audio amp will be the piece which provides an ultra low output impedance. The signal generator produces an 0.5V peak-to-peak square wave @ 120Hz, that you feed into the testbench audio amp. The audio amp has a voltage gain of ~ 20X, so the amp's output drives the injection capacitor Cx with a 10V peak-to-peak square wave. Cx drives (C3 + Rs) and the transformer secondary, replicating the Quasimodo topology. Just make sure the power amp has sufficient bandwidth to give < 30 nanosecond rise time (in this case: fall time!) on the square wave's edges.
Thats what i did, using the last setup of Fig 13 labeled centre tap secondaries.
As shown there the transformer is driving one rectifier, but i am driving two recitifiers (see post 64), hence my question regarding the cap values.
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Old 8th November 2013, 12:54 PM   #73
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Ctrlx, one way to analyze the situation when you double the number of rectifiers (thereby doubling the rectifier capacitance contribution to Ctotal), is to consider this a variation in rectifier capacitance. A +100% variation in Crectifier.

If, before doubling, you had chosen Cx to be much much greater than (Ctransformer + Crectifier), so that your Ctotal was insensitive to variations in Crectifier and/or Ctransformer (reason #2 to use a CRC snubber), then a doubling of Crectifier does not change Ctotal = (Cx + Ctransformer + Crectifier) significantly. Thus the resonant frequency omega_n does not change significantly, and the optimum snubber resistance does not change significantly.

Another design approach might be, to estimate the capacitance of the second set of rectifiers (conservatively over-estimate), and increase Cx by this amount.

A third approach might be, to arbitrarily increase Cx by 46.2%. Or to arbitrarily increase Cx by P% for some value of P that you happen to like best.

Whichever snubber component values you select, you will still (A) build a snubber with those component values; (B) apply a bellringer test to your transformer with that exact snubber, and (C) dial in the optimum Rsnub (zeta = 1.0) for your particular transformer and for that particular choice of snubber capacitances. As long as your snubber gives critical damping with your transformer, it is optimum. Whether you have increased Cx, left Cx unchanged, or decreased Cx.
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Old 9th November 2013, 09:28 AM   #74
FdW is offline FdW  Netherlands
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A few quick pictures of my 'final' implementation of the Quasimodo device
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File Type: jpg Proof, IMG_0738.jpg (212.2 KB, 402 views)

Last edited by FdW; 9th November 2013 at 09:43 AM.
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Old 9th November 2013, 11:50 AM   #75
ctrlx is offline ctrlx  United Kingdom
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Quote:
Originally Posted by transistormarkj View Post
chosen Cx to be much much greater than (Ctransformer + Crectifier), so that your Ctotal was insensitive to variations in Crectifier and/or Ctransformer (reason #2 to use a CRC snubber), then a doubling of Crectifier does not change Ctotal = (Cx + Ctransformer + Crectifier) significantly. Thus the resonant frequency omega_n does not change significantly, and the optimum snubber resistance does not change significantly.
ok, i will go with 15nF (since i have stock) and 220nF.
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Old 9th November 2013, 01:21 PM   #76
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Quote:
Originally Posted by FdW View Post
A few quick pictures of my 'final' implementation of the Quasimodo device
FdW: Lovely build, congratulations!

Your arrangement of binding posts is very clever; it lets you swap in whatever values of Cx and Cs you wish. It also lets you measure the resistance of potentiometer RV1. Finally it gives you the option to install a fixed resistor in parallel with RV1 if you so choose, to reduce the ohms-per-degree-of-shaft-rotation sensitivity. You know, to get those 3rd and 4th digits of precision

Would you please say what the BNC connector is for? Is it sync-out to the oscilloscope? Is it DC-power-in? Or is there a battery inside the enclosure?

Excellent job, thanks for showing us!

MarkJ
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Old 9th November 2013, 03:20 PM   #77
FdW is offline FdW  Netherlands
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Quote:
Originally Posted by transistormarkj View Post
Would you please say what the BNC connector is for? Is it sync-out to the oscilloscope? Is it DC-power-in? Or is there a battery inside the enclosure?
It's the scope, 2x 9V inside
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Old 10th November 2013, 04:00 PM   #78
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Default Quasimodo the Bell Ringer

Excellent build FdW. Very good inspiration for me. I am still using the Quasimodo completely naked . Maybe it will be a good idea to cover it like you did. Thanks for showing us the nice pictures.

Karsten
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Old 22nd November 2013, 03:49 PM   #79
SGK is offline SGK
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Anyone interested in a group buy of the V4 through-hole version? I have no experience in ordering PCBs and I'm a newbie to electronics and so I'm unlikely to be the right person to coordinate a buy but I would surely like to participate in one (even if the organiser earnt a return for their hassle).
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Old 22nd November 2013, 09:56 PM   #80
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I would be interested, but I also have no means of organising and placing an order right now.
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