[NLRS] Efratom Lpro 101 Rubidium Standard

Sun Nov 18 02:17:57 EST 2012

How much total power goes into the LPRO? A few milliwatts comes out the 
signal port, so all is dissipated IN the standard and has to be removed 
through that surface. The only time a heatsink is too big is when it 
won't go through the door. Lots of times heatsinking is called for that 
has no engineering basis, just its sure to be plenty big. Heatsinks need 
to be bigger for unfriendly environments, like poor air circulation and 
a high ambient temperature.

I went though a bit of this experimentally with 10 G 3 watt DEMI amp a 
couple months ago. For August I borrowed and uses Donn's spare. Lots of 
fins in the heatsink. Never did detect a temperature rise. When I built 
my own before the September weekend (eyes were getting better all the 
time) I tested it without a heat sink keeping key down periods short. 
Didn't detect a rise with the case bolted to a U of 1/16" aluminum 
sticking up past the 1/4" thick base plate And that U is bolted to the 
dish feed support flat tube. So I had just two fins. For safety sake I 
bent a couple more U's and bolted them to the base plate. I was running 
key down full carrier and blew the power fuse. 5 amps did eventually 
take out the 4 amp fuse. But in that minute or so of key down I still 
didn't detect a serious temperature rise. Yet Steve wants lots of fins 
and maybe even a fan which I do have but haven't mounted either since I 
can't see the need for it even if it was on a FSK beacon running 24/7. 
Now that amp draws about 3 amps at 12 volts, and puts out 10% of the DC 
input. Its not quite as much heat as a PC CPU dissipating 100 watts in 
about the same space where the fan on the well finned heatsink is 
required to keep the CPU temperature rise reasonable.

Figuring heat transfer isn't something I can do, but I can experiment. 
Dig up some power resistors that bolt to the deck with total power 
dissipation (and resistance to match an available power supply) of the 
standard and bolt them to the box, inside or outside. Measure the box 
temperature, apply power and see how much the temperature rises with 
that heat applied. If its too much, add fins either heat sink extrusion 
or simply sheet stock bent into a U channel shape. Repeat the 
experiment. Then check the effect of painting with Krylon Ultra Flat 
Black for a bit better radiation (out but unfortunately also in when hit 
by the sun, it was the best coating I found for a solar collecttor some 
38 years ago, better than special solar collector paint).

Commercial finned heatsinks are often rated in watts per degree 
temperature rise or watts per 50 degrees temperature rise so you can 
figure out what heatsink and how much of it you need. The box doesn't 
have that rating until you experiment.

There will be two components of temperature rise, transient and steady 
state. Transient will show as a slow rise in heat sink temperature due 
to the thermal inertia of the hunk of metal. So the temperature rise 
test needs to run until the temperature stops rising and levels off. 
With the 1/8" thick wall and 1/4" thick base my DEMI 3 watt amp probably 
for ordinary operations is working purely on thermal inertia and getting 
away with it. The Rubidium Standard is something that will tend to run 
continuously.

The orientation of the box or fins will significantly affect temperature 
rise for any given amount of heat. That can be included in the 
experiment if you wish.

73, Jerry, K0CQ

On 11/17/2012 4:49 PM, tosca005 at umn.edu wrote:
>
>
> I am in the process of building up a rubidium standard into a die-cast
> aluminum box. The box will contain an Efratom Lpro 101 rubidium standard,
> power supply (24vdc direct input or 12vdc->24vdc voltage doubler boost
> regulator), a Down East Microwave 10-4 (four-port filter/splitter for the
> 10 MHz output), the rubidium oscillator lock signal ->  LED circuit, and
> some voltage monitoring using digital panel meter LED blocks.
>
> I am puzzled about meeting the heat sinking requirements. Plan A was to
> simply drill 6 holes through the bottom of the aluminum box in the
> positions of the 6 mounting screw holes of the Lpro101, and bolt the
> Lpro101 to the inside bottom of the box (which will become the top of the
> box during operation, as I will put rubber feet on the "lid" and flip the
> whole assembly over to make that the base. One web site that discussed
> building up a standard like this claimed that this would be more than
> satisfactory at carrying away the excess heat of the rubidium
> lamp/oscillator. Then at the last meeting of the Roadrunners Microwave
> Group, one of the members (sorry, I'm bad with names and callsigns, I'll
> need a few repetitions to remember everyone) brought his homebrewed 10 MHz
> standard which also used the Lpro 101 but he added a heat sink on the wall
> of the box that the rubidium standard was bolted to. I didn't get a chance
> to discuss it with him in depth. But I got to thinking if a heat sink is
> really needed, I need to have excellent flatness on the inside of the
> aluminum box and heat sink compound between the Lpro101 and the box, plus
> between the box and the heat sink.
>
> Is this really necessary? Do I need to have the inside of the aluminum box
> milled to extreme flatness? (I am sure that without some work, it is not
> terribly flat now.) The outside of the aluminum box looks "flat enough" to
> me, but not the inside.
>
> Anyone with any insight on this that they'd like to share?
>
> John P. Toscano, W0JT/5
> EL09ro October-May
> EN34js May-September
>
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