[time-nuts] Ublox 8T - breakout board

[Matthias Welwarsky]

On Dienstag, 15. Dezember 2020 19:22:14 CET paul swed wrote:
> Hello to the group Nigel is correct the way the schematic is drawn the
> Regulator will feed the battery and most likely cause it to burst.
> It should be 1 diode in series with the regulator and 1 in series with the
> battery so that the regulator can not charge the battery.

Yes, that's what I discussed with Nigel off-list. The two diodes in series 
create a voltage difference of 1.4V, so as long the battery voltage is above 
1.9V, no current will flow through the diodes at all and the battery is safe.

However, there is a greater-than-zero probability that a board will just be 
dropped into a drawer with the battery in place, fully draining it eventually. 
If it's then powered up again, chances are that a substantial current will 
flow into the battery and potentially cause a fire, explosion, etc.

I've therefore updated the design and put a small Schottky diode in series 
with the battery. I selected a part with small leakage current ( 0.5µA 
typical). But of course a regular silicon diode like 1N4148WS will work as 
well, at the expense of wasting some of the energy of the coin cell.

> Nice design you have created.

Thanks - Hopefully it will also work :-)

> Regards
> Paul
> WB8TSL
> 
> On Tue, Dec 15, 2020 at 11:55 AM Nigel gm8pzr via time-nuts <
> 
> time-nuts at lists.febo.com> wrote:
> > Hi Mathias,
> > I'm assuming you're using a coin cell for back up, so wondering if one of
> > the diodes shouldn't be in series with the battery, with both cathodes to
> > V_BCKP?
> > Nigel GM8PZR
> > 
> > 
> > I've updated the repository on github with the latest design files. The
> > breakout board is now final as far as I'm concerned.
> > 
> > I managed to implement all the requested features, this includes the
> > backup
> > battery and also the buffered PPS output with SMA footprint.
> > 
> > Next step on my side will be to order a small batch of PCBs. There's not
> > much
> > on them and I guess I will end up with around 25? for 5 boards fully
> > assembled, shipping not included.
> > 
> > I eventually removed the I2C pull-ups again, thanks to Patrick Tudor for
> > the
> > hint. The footprints are still there, I only removed them from the BOM.
> > 
> > Schematic and a screenshot of the top and bottom sides attached.
> > 
> > BR,
> > Matthias
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