[time-nuts] Ublox 8T - breakout board

[paul swed]

Yes you did the right thing. I am a drop it in the drawer person.
Hate when those batteries make a bang.

On Tue, Dec 15, 2020 at 5:06 PM Matthias Welwarsky <time-nuts at welwarsky.de>
wrote:

> 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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>
>
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