[time-nuts] Using a common power supply among few time standards

[Bob kb8tq]

Hi

As you have already noted, things do die from time to time. Fuses are a good idea, along
with protection diodes. Still, none of it is 100%. The more heat / temperature you have in 
a part, the quicker it is likely to fail. 

One often overlooked failure mode is loosing one supply on a +/-12V pair. If you go back in
the archives (yes that’s not easy) there are reports of significant damage on things like DMTD’s
from this sort of problem. An open ground also can do the same sort of thing. 

If everything is AC coupled then a cascade failure is less likely. We run stuff like PPS, serial 
com, and USB that are very much DC coupled. Having a supply die and kill the entire bench 
is *not* a good thing. Protection on these signals is worth thinking about. 

The “honking big supply” comes with great big cables protected by great big fuses. Bump this
here or there and a *lot* of energy is involved before the fuse does its thing. Work on 
trucks or aircraft and you will come up with a lot of data on the sort of damage this can do.

These days small power supplies are dirt cheap. DC/DC converter boards also can be had 
for next to nothing. For a bit more money, fairly good versions can be purchased. For a lot more
money, very good ones are out there. Running a bunch of switchers has its drawbacks. Filtering 
*is* possible. 

Not every device needs a supply that is at the <1 nv / Hz level. Most devices are fairly tolerant
of supply noise. There are a *lot* of systems out there that run with supplies in the 10 mv (rms) 
range. One size fits all is great for inventory management. It isn’t so great on the credit card.

Does a $5 board “deserve” a $500 power setup? At least around here, it’s going to get a $1
wall wart. A $5,000 device may get treated a bit better. If a shiny new / in warranty piece of
gear shows up with this or that for power …. it stays that way (at least through the warranty
period). If HP “back in the day” thought that this approach was ok for a box I power up < once
a month, it’s fine now. 

The “best answer” to energy savings is still to turn things off. That’s the compelling reason
to have external reference inputs on test gear. It’s also a compelling reason to run gear that 
does not take hours (or days) to warm up and stabilize. With fun stuff like controlled outlets, 
you can let Alexa manage power to this or that device. Get up in the morning and tell her to 
warm up the bench. Yes, setting this all up takes some thought and some planning. 

You may decide you want to have a *lot* of sources on at the same time. Watch out for cross talk.
The more sources you have, the more trouble you will get into. Indeed this can become nearly
impossible to deal with as you get into the “many thousands” of devices all on the same frequency.
We toss around numbers like 120 db of isolation. That’s only adequate at a specific offset and
a desired impact on stability……

Lots of directions this can go and lots of choices …..

Bob


> On Feb 7, 2020, at 10:01 PM, Taka Kamiya via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> I recall S-100 scheme had an issue of per-board regulator failing short and dumping 8V into bus killing rest of the system....
> I am thinking of making use of 28V (aircraft power supply?) regulated power supply, and have local linear regulator to created needed voltages.  One concern is, should I need large amount of 5V then loss will be tremendous.  I just came across an article that discussed magnetic field sensitivity of LPRO-101.  The author ended up relocating the transformer off-board.  (in a different case)  A very timely information as this is what I am actually working on.
> Further comment will be appreciatedTakaTime-nut in training
> --------------------------------------- 
> (Mr.) Taka Kamiya
> KB4EMF / ex JF2DKG
> 
> 
>    On Thursday, February 6, 2020, 6:51:38 PM EST, jimlux <jimlux at earthlink.net> wrote:  
> 
> On 2/6/20 3:14 PM, Will Kimber wrote:
>> Hi,
>> 
>> Usual (best?) is supply slight over voltage and use LDO/RC filtering in
>> each unit as Bob suggested. But that would require either supplying
>> 24v/12v/5v/-12v (with some margin 10%?) or converting some single supply
>> of 24v or 48v  in each unit to required voltages at each piece of
>> equipment.  The later requires switching supplies to get all voltages.
> 
> 
> Well that was the strategy used on S-100 boards. Bulk 8V unregulated 
> supply regulated down to 5V on the board.
> 
> 
> 
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