[time-nuts] FE-5680A thermal management

Dave Brown tractorb at ihug.co.nz
Mon Jun 8 16:49:43 EDT 2009

----- Original Message ----- 
From: "Murray Greenman" <Murray.Greenman at rakon.com>
To: <time-nuts at febo.com>
Sent: Tuesday, June 09, 2009 6:42 AM
Subject: [time-nuts] FE-5680A thermal management

> Leigh,
> I agree with Chuck. I have both an FE-5650A and an FE-5680A. With 
> the
> former I was concerned about the heat, and so ran it only for short
> periods, until I understood what was going on. I had the impression 
> from
> the data sheet that there were different heatsink options for 
> different
> temperature ranges, and I now believe this led me astray.
> With the FE-5680A I had the opportunity to study things in more 
> detail.
> There is no temperature range specification that I could find, and 
> no
> particular advice in the manual regarding installation. I ran the 
> unit
> from regulated 15V DC, and monitored the supply current. With no 
> extra
> cooling, the steady state current was about 700mA. With air blown 
> over
> it, the current increased. With the unit placed in a poly bag, the
> current decreased to about 650mA. By the way, the current also 
> decreased
> when operated from 17V DC.
> This tells me that the whole structure is part of the thermal 
> package,
> and we should not attempt to force down the case temperature just
> because the unit runs hot. My experience with high performance OCXOs
> tells me that the thermal environment is carefully designed, and 
> part of
> the calibration process - if you modify this environment (by cooling 
> or
> extra insulation) you modify the thermal environment, and are at 
> risk of
> modifying the performance.
> I'd leave well alone, and run the unit in an open, breeze-free
> environment.
> 73,
> Murray ZL1BPU

Murray et al,
The package of both units is clearly meant to be mounted on 
something-the real question is how much heat sinking would that 
something have provided?  Changing the external heatsinking to achieve 
the nominal supply current at the nominal supply voltage would appear 
to be the only simple way to operate the units as intended.

 I've not gone through this exercise yet with the 5650, but I suspect 
the amount of heatsinking used is not that critical. Using a variable 
speed fan to determine the 'correct' mounting plate temperature 
(corresponding to nominal supply current/voltage) might be a good way 
to start. The design of a heatsink to achieve the same base plate 
temperature should be a trivial exercise.
 Constraining the 'ambient' air temperature to an appropriate range in 
the vicinity of the unit should certainly help as well.

 DaveB, NZ

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