[time-nuts] Re: Help needed with a UPS for my GPSDO

[Poul-Henning Kamp]

Can I just interject, based on the subject line, that using a UPS
for a GPSDO is a bad idea ?

The UPS you have scored are designed to keep a 240W load running
for a few minutes and it is not very good at keeping a 10W load
runing for a long time, because most of the energy in the batteries
end up as heat in the UPS DC/AC converter.

According to the spec for the "APC backup-ups 400", holdover is
never going to be over 90 minutes, no matter how light the load.

Ditch the UPS, float-charge the same battery, and run your
GPSDO from that, possibly using a small DC/DC converter with a
wattage well-matched to the GPSDO consumption.

To take an example:  A Trimble Thunderbolt uses 12W from cold, we
need to design for that, so a 15W DC/DC converter is prudent.

That will have an efficiency of around 80% at the 8W which the
Thunderbolt uses steady state.

That means that the DC/DC converter will draw 8W/.80 = 10W from
the battery.

That means that the exact same 12V/7Ah VRLA Lead-Acid battery will
keep your GPDSO running for at least 6-8 hours.

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
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