[GreenKeys] John Nagle's USB <-> 60 mA loop board

[Bruce Phillips]

Hi John,

Thank you so very much for your post. I am truly grateful for your hard 
work, and also your blessing to build some of these.

I have been following your updates on Github, and know how much work you 
have put into this project. (and Steve Garrison).

As mentioned in an earlier post, I didn't actually check the pcb gerber 
files until I was ready to make changes......I made the silly assumption 
(like the famous saying!) that the layout was still largely through hole 
(from Github photo). However when I discovered this was not the case I 
opted to not make any changes, so please don't think I was implying your 
design needed any modification.

Thanks for your additional details....they will be really helpful.

I do SMD work every (working) day, and have decent soldering equipment, 
including an aging Ersa IR550A system....so can handle most things.

I plan to make my unit into a custom polished wooden box, with engraved 
black Bakelite panels front and back, and a vintage style current meter, 
switches etc..

I've ordered most of the extra parts I need, just trying to find the 
best value LT3750's...waiting on a couple of quotes.

Thanks again;

Regards,

Bruce Phillips





On 23/02/2018 1:57 PM, John Nagle wrote:
>> On Wed, Feb 21, 2018 at 3:24 AM, Bruce Phillips
>>>
>>> I was planning to build a few in the very near future.......I'm in
>>> Australia, but postage should not be too much for a small item like 
>>> that if
>>> there is any interest. I was very conscious not to tread on anyone's 
>>> toes.
>>> I planned to change the layout of the pcb to as much smd as 
>>> possible, but
>>> that was just my preference for my own assembly. In any case I will 
>>> have
>>> probably 20 pcb's made, so don't mind sharing them around.
>>>
>>> Regards,
>>> Bruce Phillips
>>> Australia
>
>    Feel free to use the design. That's why I put all the files on Github.
>
>    Blank boards are the easy part, and cheap. I have mine made by
> Seeed, in Shenzhen.  It's the assembly that's hard.
>
>    Notes:
>
>    - An all-SMD design would be nice, but there are some problems.
>      The CP2102 USB to serial converter is packaged in a QFN28 package.
>      Those mount via pads under the part, 0.4mm apart.
>      While it's possible to hand solder such parts, the technique
>      is complicated. See it done:
>
>      https://www.youtube.com/watch?v=lhOKSqipwqg
>      https://www.youtube.com/watch?v=ljyGMOi5JPo
>
>      Data sheet:
> https://www.silabs.com/documents/public/data-sheets/cp2102n-datasheet.pdf
>
>      That's why the daughter board for the USB to serial converter.
>      I'd like to get rid of that, and if you're going for assembly
>      using a solder paste stencil and pick and place machine,
>      go for it.
>
>    - C1 and C2, the 1uf ceramic capacitors, are through-hole
>      because the new multilayer surface mount large-value
>      ceramic capacitors (MLCC) on't work. Their capacitance
>      decreases with voltage, down to about 20% of normal
>      value. They're OK for filters but not for energy storage,
>      which is how they're used here. Watch this tutorial on
>      MLCC capacitors.
>
>
> https://www.digikey.com/en/ptm/m/murata-electronics-north-america/high-capacitance-hi-cap-mlcc/tutorial 
> (see slide 7).
>
>   - The layout around the switch controller U1 is critical. Trace
>     lengths must be very short in that area, or it won't work at all.
>     See the data sheet.
>
> http://cds.linear.com/docs/en/datasheet/3750fa.pdf
>
>     This is a switching power supply operating around 300KHz with
>     2ns rise times. Keeping those spikes out of the computer that's
>     powering the thing, or generating RF noise, requires
>     tight layout and various bypass caps and ferrite beads on the
>     board. Steve Garrison says the thing is RF-quiet; he's not
>     hearing it on his receivers or seeing noise on a spectrum
>     analyzer.
>
>   - The board could be made shorter. There's a price break if you
>     get all dimensions below 100mm.  I just used that size because
>     there's a nice case available for 125mm x 75mm boards.
>
> It took seven revisions of the board to get this working. It's
> non-trivial.  If you redesign the board, be prepared to debug it.
>
>                 John Nagle
>
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