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Old 15th March 2014, 03:36 PM   #31
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mcandmar, I recommend you connect a fixed inductor whose inductance is known within 10%, to Cheapomodo. Then calculate the ideally perfect Rs value which gives zeta=1.000 (at this L and this Cx) using the equation in Appendix A. Set your potentiometer to this Rs value. Look at the CheapoModo waveform on your scope; at the ideal perfect zeta=1.000, your scope trace is not a flat line. The goal is not to achieve a flat line, the goal is to achieve zeta=1.000. The ideal perfect scope waveform for zeta=1.000 is shown in post #143 of the Quasimodo thread, and in several Figures of the design note.
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Old 14th September 2014, 08:39 PM   #32
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Mark any news on the osh park pcb ? Are they safe for ordering?

Thanks a lot for a very nice idea and implementation
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Old 14th September 2014, 09:16 PM   #33
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Quote:
Originally Posted by dimkasta View Post
Mark any news on the osh park pcb ? Are they safe for ordering?
I built it and found that I hated it. The board functioned correctly, and I successfully used it to find zeta=1 snubbers for a few transformers. Despite this I hated it.

Why did I hate it: because it has too few external I/O pins. In particular, it doesn't have pins for the transformer secondary AND for the oscilloscope probe & groundclip. To use the board you have to wire up a little shiite-rig of crocodile clip jumpers, which connect the board to the transformer AND to the scope probe. Bleah.

So I tweaked the board design and made a rev.2. It worked and I didn't hate it. There were plenty of I/Os for the transformer and for the scope probe. Maybe I'll upload the PCB fab "Gerber" files in a few weeks, if I can eventually find them. Stuff is kinda jumbled up around here.

If anyone on DIYaudio wants to lay out a CheapoModo PCB, and share it with others (or sell it), please go right ahead. You have my blessings and good wishes.
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Old 15th September 2014, 12:05 AM   #34
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Fair enough.

Do you guys mind having a look at my schematic to verify that I have not done any booboo?

Click the image to open in full size.

And here is the board

Click the image to open in full size.

It's 51x16mm and cost is going to be 6 dollars for 3 pieces from OSH park, including shipment...

How is that for a cheapo?

Click the image to open in full size.

Any comments more than welcome. I plan to order the samples at the end of the week.
Attached Images
File Type: png sch.PNG (32.5 KB, 458 views)
File Type: png brd.PNG (121.3 KB, 459 views)
File Type: png osh.png (64.2 KB, 294 views)
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Old 15th September 2014, 01:57 PM   #35
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I forgot to add pins for the second channel of the scope. I will add them later today
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Old 15th September 2014, 04:52 PM   #36
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Here we go

Pins for two channel scopes, holes to attach clips to grab the secondaries, fixed width 10nF and 150nF caps (it is a cheapo anyway), mosfet drains marked with D to help orient the device of your choice and a 3pin socket for a trimmer.

Click the image to open in full size.

Oh and it is a tiny bit smaller, so it now costs 5,85 for three pieces

Have I forgotten anything?
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Old 15th September 2014, 07:38 PM   #37
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Will you include a GND flood on both blue and maroon layers? The naked via next to CS1 suggests that you will. It would be a good idea.

I don't see any electrolytic bypass caps; check your deltaV = (deltaT * I / C) calculations to be sure the supply ripple will be okay when operating from a 9V battery with its relatively high internal resistance.

If you're going to discard the 2N7000 junkbox MOSFET anyway, why not replace it with a low-RDSon TO-92 MOSFET ? The one in the attachment has the same pinout as your BS170, but 15X lower Rds. You could build one of your 3 boards with good FETs and the other 2 boards with cheap FETs and see if there's a difference. {On high current 6.3VAC heater windings/transformers, there may indeed be}
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Old 15th September 2014, 08:03 PM   #38
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Yes ground planes on both sides.

I used the BS170 because I saw it as an alternative on a previous post and I found it in the eagle libraries. I m keeping a note for the Zetex It is available on mouser for about a euro.

About the electrolytic bypass, I got a rush as soon as I saw that function
Any rough guestimates on what should be used? Something like low esr 100uF?
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Last edited by dimkasta; 15th September 2014 at 08:07 PM.
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Old 21st September 2014, 10:01 AM   #39
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Default One more successful Cheapomodo

Similar to the previous poster, I also used BC556/546, some BS170 mosfets and a prototype board to produce a cheapomodo.

It works like a charm, thanks a bunch Mark.
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File Type: jpg 300VA no snubber.jpg (789.6 KB, 68 views)
File Type: jpg 300VA 72ohm.jpg (796.4 KB, 57 views)
File Type: jpg IMG_20140921_203038.jpg (625.0 KB, 78 views)
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Old 21st September 2014, 07:18 PM   #40
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Congratulations Oliphant! Nice power supply (homebrew?) in photo 3.

I assume the unconnected transformer wire in photo 3 is an electrostatic shield, rather than a primary winding connection?
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