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Old 23rd February 2014, 04:36 PM   #11
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Originally Posted by AndrewT View Post
I changed the timing cap from 100nF to 10nF and then to 1nF to allow my analogue scope to see and measure the fast edges.
That's why Quasimodo V4 (thru hole) PCBs have a dip switch. To brighten the traces on analog scopes.
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Old 23rd February 2014, 04:47 PM   #12
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Originally Posted by AndrewT View Post
How fast should this circuit be able to go?
How fast does it need to go?
We've seen physical measurements of transformers whose optimum (Cx, Cs, Rs) snubber was (0.01u, 0.15u, 183R) ... page 10 of the Quasimodo design note. Call that "Case 1".

We've also seen physical measurements of transformers whose optimum (Cx, Cs, Rs) snubber was (0.01u, 0.15u, 4.0R) ... (post #233 in the Quasimodo thread). Call that "Case 2".

We've also learned that Quasimodo+CRC snubber is, at heart, merely a machine that measures inductance (QM post #257). So you could back-calculate the secondary leakage inductances of Case #1 and Case #2, and then set yourself a design goal such as this one:
  • My CheapoModo will work flawlessly in simulation, and give the correct answer, when I drive a Case#1 leakage inductance, AND also when I drive a Case#2 leakage inductance.
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Old 25th February 2014, 04:30 PM   #13
AndrewT is offline AndrewT  Scotland
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Originally Posted by Mark Johnson View Post
That's why Quasimodo V4 (thru hole) PCBs have a dip switch. To brighten the traces on analog scopes.
Not what I meant.

With the standard spike repetion rate of around 100Hz, the analogue scope sees an almost zero rise time and almost zero fall time.
I can use the 10times horizontal expansion switch and still it looks like near zero rise and fall times.

If I change the repetition rate to around 1400Hz using a 10nF timing cap and then use the 10times expansion of the horizontal I can discriminate an actual rise time and fall time, but it is still too short to measure.

Now changing the repetition rate to 14kHz and with 10times expansion I can see and read the rise and fall times.

Now that I have the ~ 1us rise and fall times on the spikes with the very slow decay I have rest the timing capacitor to 150nF for ~ 100Hz repetition of the spikes.

I asked:
Is this fast enough?
Or does it need to be faster?
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regards Andrew T.
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Old 25th February 2014, 05:38 PM   #14
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Post #51 in the original Quasimodo thread makes one suggestion; post #12 in this thread suggests a different simulation-based approach to obtaining insight and an answer.

Page 12 of the Quasimodo design note tells how to use a dual trace oscilloscope to trigger upon the important event, allowing you to dial up a very fast horizontal sweep rate and view the exact waveform segment you desire.

However if your scope's sweep rate is 20 nanosec/cm and your Quasimodo oscillator's period is 8.3 milliseconds (120 Hz), then your scope's electron beam paints the CRT for 200 nanosec (20ns/cm x 10cm) every 8.3 millisec; a duty cycle of 0.0024 percent. So it will be a VERY dim trace, best viewed in a darkened room. Quasimodo V4 PCB offers frequency boost of 5X and/or 25X, via dip switches, making the trace 5X less dim or 25X less dim. Digital scopes like my 190 Rigol DS1102, don't have this problem of course. They have other problems instead.
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Old 3rd March 2014, 02:25 PM   #15
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I've just gone ahead and build the thing. Used BC547/557 instead of the 2n3904/06 and the BS170 instead of the 2n7002. Specs of the components are roughly the same.

And it works like a charm! Very easy to dial in the optimum resistor value. So thanks for opting the idea to build one with junkbin parts. It has been a while since I used vero board . I've added some headers to quickly swap the caps and pot, so I can experiment with different values, dielectrics, voltages, etc. Now for a nice box...

P.S. Thanks Mark for the education in snubbing the transformer ringing in the original Quasimodo thread. I've learned A LOT.

without.jpgWith.jpgfoto (2).jpg

Last edited by funk1980; 3rd March 2014 at 02:30 PM.
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Old 3rd March 2014, 03:12 PM   #16
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Funk1980, congratulations! Your before-and-after waveforms are textbook perfect.

I'm glad that you used different part-numbers for the PNP, NPN, and Nch MOSFET. CheapoModo is not sensitive to part types*, just about anything from the junkbin will work -- which your successful board proves, quite convincingly.

* But of course you still have to get the pinouts correct! For those who may not know, BC547 and 2N3904 have different pinouts. Let the builder beware.

Last edited by Mark Johnson; 3rd March 2014 at 03:23 PM. Reason: pinout
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Old 3rd March 2014, 03:17 PM   #17
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Originally Posted by funk1980 View Post
I've just gone ahead and build the thing. Used BC547/557 instead of the 2n3904/06 and the BS170 instead of the 2n7002. Specs of the components are roughly the same.

And it works like a charm! Very easy to dial in the optimum resistor value. So thanks for opting the idea to build one with junkbin parts. It has been a while since I used vero board . I've added some headers to quickly swap the caps and pot, so I can experiment with different values, dielectrics, voltages, etc. Now for a nice box...

P.S. Thanks Mark for the education in snubbing the transformer ringing in the original Quasimodo thread. I've learned A LOT.

Attachment 403327Attachment 403328Attachment 403329
Nice build! I think that everyone would benefit if you are able to supply a layout for that, too.
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Old 3rd March 2014, 03:24 PM   #18
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Nice build! I think that everyone would benefit if you are able to supply a layout for that, too.
Thanks guys!

Well, to be honest, I've quite literally followed the LTSpice schematic for layout of the components, including the Vcc and GND rail. I'm more than willing to make a picture of the solder-side, but I doubt it'll help much, since it's a veroboard. Let me know.

One question regarding transformers with multiple secondary windings (toroidal with 250V for audio and 12.6V for heater in my case). Should every winding have it's own network, or does it suffice to only snub the winding used for the actual audio path?
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Old 3rd March 2014, 04:15 PM   #19
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I don't see any need to snub transformer windings that supply pure AC to non-rectifier loads { such as valve heaters }. No diode, means nobody to ring the bell, means no ringing.

For transformer windings that drive rectifier diodes, I recommend following the left hand column of Figure 13 (p.11) of the QM design note.
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Old 3rd March 2014, 05:24 PM   #20
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I don't see any need to snub transformer windings that supply pure AC to non-rectifier loads { such as valve heaters }. No diode, means nobody to ring the bell, means no ringing.

For transformer windings that drive rectifier diodes, I recommend following the left hand column of Figure 13 (p.11) of the QM design note.
Yes I should've been more descriptive. It's for the winding providing the voltage for a DC heater (rectifier -> cap -> LM317). I'll use the given testing method and provide independant snubbing.

Update: The heater winding came out on 9.3 ohm. So 10 ohm will do nicely.
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