Simple, no-math transformer snubber using Quasimodo test-jig

If you model most rectifier/capacitor power supplies, you find that the charging looks sine-like, and not pulse like. And if you measure it you might be surprised to find that capacitor charging is about 20% of the mains half cycle time. They are not short pulse like events. Bear in mind that the charging current is limited by the transformer secondary resistance plus the reflected primary resistance.
 
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I have measured 3 Toroidy 60VA transformers.

2x - 2x18V Cx=10nF Cs=150nF Rs=920 Ohm

1x - 2x18V+1x8V Cx=10nF Cs=150nF Rs=920 Ohm

I think your concerns are well founded; 920 ohms is bigger than ANY of the optimum snubbing resistors found by diyAudio members and tabulated in the Quasimodo results (ONLY) thread. Probably there is a mistake, either in your lab procedures or in your Quasimodo fixture itself.

To find out, I recommend you use Quasimodo to find the optimum C+RC snubber for a fixed inductor whose inductance and tolerance are known and provided by the inductor manufacturer. Choose an inductor whose inductance is anywhere between approx 20 microhenries and approx 1 millihenry. Post your optimum snubber values (Cx= aaaaa , Cy= bbbbb , Rs= ccccc , Inductor= ddddd) and we'll go through the math here in the thread. This will determine whether your Quasimodo fixture is working, or broken.
 
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You are right. I have also measured other transformers and they al give the same Rs value. The Quasimodo has worked properly before, so I don't get it. Have attached some photos. Perhaps I am doing something wrong.
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1. Buy an inductor with known inductance (between 20uH and 1000uH) and known tolerance. Use Quasimodo to optimize a C+RC snubber for this inductor and report its values.

2. Re-read the Quasimodo Design Note and study the material before and after Figure 4. Double check that you are measuring the resistance of the trimmer with the appropriate two pins shorted together (as they are on the PCB).

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Hi All,
has anyone done the whole procedure on you tube - explaining how to select the 2nd cap value, what to look for, and or, set up the board for a test?
I'll be (eventually) using it on a Transformer with 3 separate secondary outputs so wondered if there was anything visual out there.
Best
Joe
 
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I have some info here

Its for the surface mount version because that's the PWB I was able to buy.

I can add some pics to show how I connected it, its pretty straightforward.

Recommended cap values are:
CX = 10nf
CS=150nf

CX is C2 on the Quasi
Thank you for your reply.
My main interest is due to transformer (hum)buzz in a SS power amp and I initially wondered if the rectifier was an issue (BYW22, original fitment). Then I came across this thread and thought it might be worth looking into it from this angle as well.
So really the main purpose of the Quasimodo is to identify the resistor value required? I take it the voltage values of the caps should be a factor above the voltage the transformer puts out into the highest winding? I will re read the entire thread again.
That additional site is helpful. I’ve a V4 pcb on its way and I’ll hopefully understand more when I actually have it in front of me. If you have the time to post some photos I’d appreciate it.
Many thanks
Joe
 
Thank you for your reply.
My main interest is due to transformer (hum)buzz in a SS power amp and I initially wondered if the rectifier was an issue (BYW22, original fitment). Then I came across this thread and thought it might be worth looking into it from this angle as well.
So really the main purpose of the Quasimodo is to identify the resistor value required? I take it the voltage values of the caps should be a factor above the voltage the transformer puts out into the highest winding? I will re read the entire thread again.

Joe
Yes, the main purpose of the Quasi is to determine a resistor value to cut down ringing from the transformer.

Caps are relatively small value, so easy to get in higher voltages.

You should download and read the Quasimodo_jig_revA PDF attached to the first post of this thread. Mark explains the theory there.

Randy
 
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Yes, the main purpose of the Quasi is to determine a resistor value to cut down ringing from the transformer.

Caps are relatively small value, so easy to get in higher voltages.

You should download and read the Quasimodo_jig_revA PDF attached to the first post of this thread. Mark explains the theory there.

Randy
Hi Randy,
Yes I downloaded it and am working my way through it. Thanks for confirming the process, the idea of having two variables (C2 and R) was foxing me a bit😊
Best
Joe
 
Hi Randy,
Yes I downloaded it and am working my way through it. Thanks for confirming the process, the idea of having two variables (C2 and R) was foxing me a bit😊
Best
Joe
Added a pic with comments on where to connect things for my V3 surface mount Quasi.

I only connected one scope channel, with the scope I was using it triggered fine.
 
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Hello to All,
Hopefully following thread rules for posting.

I do not have a QModo/Oscope. Would anyone have a test done on an Antek 3230 or 3232, 300VA, 115V primaries transformer's. l have reviewed the results of all the Antek test posted in the results page, and I am thinking that Cx=10nF, Cs=150nF, Rs=12-18 ohm, will be close. I will watch for the posting on the QModo results thread. Thanks in advance for the help.
Regards
MM
 
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Resourceful diyAudio members have successfully assembled a Quasimodo test fixture without using anybody's PCB. Examples below, demonstrating "where there's a will, there's a way".

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Very nice! I worry that the 96 kHz bandwidth might prevent it from displaying typical Quasimodo waveforms. I tried the infamous $18 oscilloscope kit called DSO138, with Quasimodo (link) and its 200 kHz bandwidth, plus very rudimentary triggering, prevented it from being a useful tool. I hope VA turns out to work much better!