[time-nuts] UPS for my time rack

[Florian Teply]

Am Tue, 13 Oct 2015 18:54:58 +0000
schrieb "Poul-Henning Kamp" <phk at phk.freebsd.dk>:

> --------
> In message <4FD0F30EBAEF49609DF207E3EE61C15B at pc52>, "Tom Van Baak"
> writes:
> 
> 
> >I used to rely on one massive UPS (along with natural gas generator)
> >for my entire lab. Eventually I found it more reliable and convenient
> >to have localized power backup. By local I mean backup for a single
> >shelf, or even a single instrument.
> 
> The big gain is avoiding the DC->AC conversions.
> 
> AC->DC conversions are covered under EnergyStar and similar
> programmes, so they're generally 90% efficient or better.
> 
> But DC->AC conversions, for instance in inverters and UPS's are not
> covered, the argument going that they are only run very seldomly,
> and therefore capital cost would be wasted.
> 
Definitely, avoiding unnecessary conversions is key to achieve good
efficiency. And in most cases equipment actually converts to DC
internally, so converting AC to DC (for the battery), then back into AC
to power the equipment, which in turn converts to DC to power internal
circuitry is bound to be inefficient. DC-DC converters with very
good efficiency exist for quite some time now. I've converted most of
the stuff I regularly use to run off common 12V and/or 48V rails
generated by two "central" power supply blocks.

> 
> The argument against running stuff on 12 or 24V DC is the short
> circuit currents, and the absense of an affordable standarized
> connector.
> 
> For the short circuit currents the only cure is fuses and caution,
> and for connectors there seems to be no hope of a standard - ever.
> 
Umm, around here, at least for ham radio operators, it seems many
standardize now on PowerPole connectors for 12V DC. They are pretty
affordable, running at below 2 Euro for a single pair. Sure, it's not
as cheap as an USB connector, but they are designed to handle
significant currents (15, 30, 45 amps which are freely interchangeable,
versions rated for 75 or 120 amps exist also).

Now of course if you want to mix voltage levels, things might become a
bit more complicated, as most 12V equipment doesn't like to be supplied
with 24 volts, so it might actually not be the brightest idea to use
identical connectors in such circumstances. Don't ask how I know... ;-)

Of course, short circuit currents are the same as before, so properly
rated fuses and/or circuit breakers are a must, but that would be
recommended for mains powered equipment as well.

Best regards,
Florian