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finding short on motherboards with a shorty (with display)

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jdobry:
Incomplete PCB you can see on the photo.
https://www.eevblog.com/forum/testgear/finding-short-on-motherboards-with-a-shorty-(with-display)/msg4618300/#msg4618300

timeandfrequency:
Hi jdobry,

Thanks for your comprehensive work and the sharing of that interesting project.

May you provide some explanation on how you choose the value of L2 (6.3 nH) ?
I checked against kripton2035s schematics and the power supply on the bottom is new in the design, because I guess, of using much smaller AAA batteries. kripton2035 used a 9V battery.


I understand that U2 is a boost switcher that clocks around 2 MHz to create the '3V3' digital power supply rail.
A second filtering around L2, C1, C3 and C4 leads to the analog power supply rail labeled 'A3V3'.
The latter powers the MCP3421 which is a really low power device (typically 145µA during conversion @ Vdd = 3V) and the AD8628 whose supply is 1mA.


By using the Thompson formula, the 2nd order LPF built around L2, C1, C3 and C4  (6.3 nH and 10.2 µF) has a calculated corner frequency of 627.8 kHz

6.3 nH is a really small value : it corresponds in air/vacuum to the inductance of a 1 mm diameter free hanging wire having a length of 1 cm.

What is the reason for keeping the corner frequency well above 500 KHz rather than setting it to a much lower value, that would filter out all of the switcher and digital noise coming from the µC, down to a few kHz ?

By changing L2s value to 47 µH (RDC= 5.8 Ohms Imax = 90 mA), which is about the biggest possible inductor value available in the 0603 size, the corner frequency becomes 7.2 kHz.

jdobry:
About L2:
I want to use low resistance inductor just to reduce main noise source from step up (2MHz). I prefer low-resistance because I have bad experience with standard feritte pearls - unwanted feedback caused by variable current to power line voltage on another project.
But to be honest this value I just test and it works perfectly. You are right, selected inductor looks like piece of wire on ceramic substrate. Few additional mm of path on the PCB could made same service.
But I want to use 0603 part to allow change in problem case. I thing that "0R" resistor could work on this place too, but I don't test it.
Similar story is R17. I just want to have 0603 part on this place to define single point of connection between signal ground and main ground. A am using 0R, but i thing that 10R (for ex) will work too.

PS: all devices and PCBs are reserved or sold. For this moment I don't have plan to make more series (it is not my business, but just hobby project). I could co-operate with some company on the production, but made it manually is fine only for prototypes.

natman69:
Very interesting project!

@jdobry: maybe you can manage to sell a kit of components and a pcb, making a small profit!!!

Or we can organize in this thread a group buying of all components, avoiding making multiple single orders on aliexpress.

jdobry:
Buy & sell isn't technical problem for me. I can order for ex batch of 50 PCBs. Price will be less than $30 each include all parts (one side fully assembled, side with buttons and OLED in package as parts) include 3D printed case, 1m of cable etc. And sell it as DIY kit with price around $40.
But I have problem to do it legally (taxes, custom declaration, invoices, accounting etc). And of course worldwide shipping.
By my mean it need business partner in Cina a send it directly from China (they have experience and possibilities how to make this business). And I don't have trustful contacts for it.
But I thing that market for this device is too small to try something like kick starter and make business by this way.

Jiri

PS: series production can need some minor changes. For ex replace D2 by 2 diodes (BAS40-05-TP isn't available in JLCPBC SMT assembly service now), change type of Q1, change U1 by another chip (TPS7A20185PDBVR isn't in JLCPBC SMT assembly service now) and minimize parts in second side of PCB.

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