[time-nuts] float chargers for oscillator backup power

Poul-Henning Kamp phk at phk.freebsd.dk
Sun Oct 26 18:03:21 EDT 2014


--------
In message <CAGVVbuFAM=u5Uhdf+wgTUNuoeMZ+=PzWQ4JpugjNJdd60FGp0A at mail.gmail.com>
, Brian Lloyd writes:

>> There are a variety of inexpensive wall-wart packaged float chargers for
>> lead acid batteries around. Might be easier to just get something off the
>> shelf.
>
>Some of these things are *extremely* noisy.

Well, yes and no.

It's amazing how much noise doesn't make it past the lead-acid battery
due to its low internal resistance.

The real issue is not the chargers noise in float mode, where it is
barely loaded, but in bulk-charge mode, where it works full bore.

Unless you want to burn a LOT of heat charging your batteries with
a linear regulator, you are better of shaving the noise after the
battery.

For OCXO's, the 14.5-11.6V supply range is going to be much more
important than the noise from the charger.

In other words, you will need some kind of regulation between
the battery and the OCXO, and that is where you should cope with
the noise.

A couple of other concerns should worry you too:  Isolation and
short-circuit current.

The design I will suggest, is to find a good charger which takes
care of your battery, and have it do only that.

Driving the load with the charger almost invariably means the
battery doesn't get optimal conditions which is why it will
croak in 5 years instead of the 20 years in the datasheet.

Find another power supply for your load, at a voltage slightly
above the chargers bulk-mode (14.5 V) voltage, and use two
solid diodes to "or" the battery and the "production supply"
onto your "DC-bus".

Don't skimp on the ATO fuses, put one right next to the battery.

Your OCXO should be driven using a small isolated DC/DC converter
from this DC bus.

It is important that the converter is isolated:  It prevents
ground-loops, but it also allows you to common-mode filter the
supply to the OCXO to get rid of any noise from the DC/DC
converter.

Yes, it is alot more complex, but the result is also much better.


-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.


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