[time-nuts] UPS for my time rack

[Bob Camp]

Hi

One option for the “fuse” part of the DC supply system are PTC resettable 
fuses. You *do* indeed need to be careful about voltage and current ratings 
on these gizmos. If the only objective is to “not have smoke” when there is a short, 
that can reduce the variables to a manageable level. If your wire will handle 20A for 
long enough to get the gizmo to limit and your load is typically < 5A there are 
parts you can find. 

A few cautions: 

The trip points are very temperature dependent. If you need to handle -20C in the winter 
and +40C in the summer, that will narrow things down quite a bit. 

Mounting matters quite a bit. If you go with SMD parts, be careful of traces that act as
heatsinks. This is one case that the part needs to get hot. 

There are to many variables on most of the spec sheets to simply pick one and move on. 
The only good way to do it is to get several and run repeated tests on them Min carry current
will occur at your highest temperature. Worst case trip will occur at your lowest temperature. 

Consider that the “carry current” may not be the real limitation. Resistance in the supply lead 
of an OCXO is a bad idea. This may limit your current well below the point that the fuse actually 
trips. 

This sounds like a pretty scary list. To some degree it is. These parts do have their place. That
does not mean they work everywhere and anywhere. 

Bob

> On Oct 13, 2015, at 2:54 PM, Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:
> 
> --------
> 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.