[time-nuts] Power supply for OCXO using "USB power blocks"

[jimlux]

On 3/9/19 7:42 PM, Gerhard Hoffmann wrote:
> 
> Am 10.03.19 um 01:38 schrieb jimlux:
>> Has anyone tried a +5V to +15V DC/DC to run an OCXO, getting the 5V 
>> from one of those rechargeable USB power bricks.  I was thinking about 
>> portable operation.
>>
>> In my case, the OCXO is something like a Wenzel streamline. I would 
>> think that the DC/DC probably has some noise, but maybe DC/DC to a 
>> higher voltage, then a good linear regulator to clean it up (or would 
>> the radiated noise just leak around).
>>
>> How quiet is the output from those USB battery things..I've used one 
>> to run a RTL-SDR, but that's hardly a ultimate low noise receiver.
>>
> That's impossible to predict. These boxes normally contain one Lithium 
> cell and a DC/DC converter to 5V.
> 
> Its output noise depends on the skill of the designer and the mood of 
> the bean counter. It is probably
> 
> not a high priority. Also, there my be "effects" from the interaction of 
> their switcher and your's.
> 
> And the efficiencies multiply.
> 
> BUT:
> 
> 3 of these Li cells deliver 12V in ultimate quality:

True.. but there are a plethora of the USB 5V power blocks around - in 
general, there are lots of USB 5V (noisy, I'm sure), e.g. Cars now have 
5V USB jacks, so I was thinking about designing with that in mind.

The question is really more one of "how much filtering do I need to 
design into the downstream power supply circuits"





> 
> < 
> https://www.flickr.com/photos/137684711@N07/39972841815/in/album-72157662535945536/ 
>  >
> 
> The squares on the paper are 5mm.
> 
> 
> Noise density is better than 1nV / rtHz above 100 Hz. You definitely 
> need a fuse.  Ishort is >> 30A.

Oh ho.. yes, as long as you stay with cylindrical cells.. if you go to 
pouch cells, they can catch fire internally with no load needing to be 
attached. We had a bit of excitement on lab a couple summers ago, 
luridly captured in this nice headline from the Daily Mail

  "Nasa reveals shocking video of secretive military 'RoboSimian' 
EXPLODING as its batteries catch fire"  (I *am* disappointed that they 
didn't work aliens into it somehow)

Or from everyone's favorite, RT: "Shocking video shows NASA military 
robot exploding due to battery failure"


https://media.wired.com/photos/5926a837cfe0d93c47430f84/master/w_582,c_limit/robot-explosion.gif


> 
> This Panasonic type is OK. There are lots of others on the market that 
> claim twice the power capacity
> 
> but it's a blatant lie.

for all practical purposes any legitimate 18650 sized Li-ion cell is 
around 3.5 Ah.


> 
> 
> regards, Gerhard
> 
> (who thinks about using a dozen of these for portable EME tests on 432 
> :-) )

You're going to run your kilowatt amp off 36 cells in series? Why not..