[time-nuts] 5680A update (temperature stabilization)

beale beale at bealecorner.com
Tue Jan 17 15:44:22 EST 2012


I tried a simple bang-bang controller (LM35 temp sensor+comparator+pass transistor to drive a small fan) with the LM35 taped to the center of a large finned heatsink. The FE-5680A+heatsink are sitting upside down, so heatsink fins point up. LM35 leads wired with 34 gauge wire, which is taped along heatsink surface. The fan cycle time was about 20 seconds, and the peak variation measured at the LM35 was about 0.2 degrees. Now, of course what I am controlling is the top surface of the heatsink. What's the temperature variation inside? Well, another temperature sensor, a thermistor full bridge attached to the center top surface of the 5680A (which is underneath, in my setup) indicates that surface varies only 4 millidegrees C in the short term (one fan cycle) due to the thermal mass of the assembly. However, long term it drifts much more along with ambient, because I do not have good insulation around the body of the 5680A. I plan to put it in a well-insulated box with just the heatsink exposed, so nearly all the heat transfer happens at the heatsink fin surface, which I am controlling. I am guessing I can manage 0.01 C stability inside the box that way.

-John Beale

>  -------Original Message-------
>  From: WarrenS <warrensjmail-one at yahoo.com>
>  To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
>  Subject: Re: [time-nuts] 5680A update
>  Sent: 17 Jan '12 11:52
>  
>  to Chris
>  What I've seen is that holding 0.1 C AT the SENSOR is pretty easy,
>  (Lady Heather will hold the TBolt's sensor to 0.01 deg using just a fan),
>  AND if you blow a lot of air around, then keeping the air gradients inside a
>  closed 'oven box' below 0.1 deg is also NO problem.
>  
>  to Bert
>  Have you measure what the Temperature coeff is over normal room changes with
>  and without the addition of the temp controller?
>  What is the best configuration to keep the fe5680 freq constant?
>  For the LPRO, what I found by experiment worked best for me is to place the
>  unit upside down so the heat sink was on top.
>  If any air was blown on the non heat sink side, that would greatly effect
>  the frequency stability in a bad way.
>  
>  A way to get around the compromise of where the best place is to put the
>  sensor, either close to the heat source or close to the device.
>  Best answer is BOTH. The way to get high end control and a much more stable
>  control loop, is to use TWO temperature sensors.
>  Put one temperature sensor near the Heat source and a second one at the
>  place you want to hold constant.
>  Then in effect 'AC couple' the heat source sensor, so that it does the
>  course temperature control.
>  One way to do this is to set it up so that the heat source sensor is the
>  feed-forward or "D" input for the main sensor PID control loop.
>  Another way to set it up is so that the device sensor's error slowly changes
>  the temperature set-point of the heat source's temperature control loop.
>  
>  ws
>  
>  ***************
>  >I am, as I reported previously using a SMD LM335 away from the fan and held
>  > down with a screw and a small bracket and I get consistent .1 C. I do not
>  >think that I would get 1 E-12  over weeks when my lab has seen more than
>  >5C temperature changes if my temperature readings are not correct.
>  >Do not forget this was a quick and dirty setup, the final product will look
>  > more professional.
>  >Bert Kehren
>  
>  *************
>  albertson.chris at gmail.com writes:
>  
>  On Tue,  Jan 17, 2012 at 8:38 AM,  <EWKehren at aol.com> wrote:
>  >>> I am  using a fan that holds it within .1 C Its been month since I
>  >>>measured it but I did report it here and I think it is 42.7C.
>  
>  >>0.1C is very good  for just using a fan.     What is the  fe5680
>  >>mounted  to?  just the heat sink or is there a thick metal plate.
>  >>Also what are  you using as a heat sensor.  Is the sensor press fit to
>  >>the heat sink  or.    I do remember reading about your temperature
>  >>controlled  fan but not the 0.1C part.   I'd have guessed you could only
>  >>do  about 2.0C with a setup like yours.
>  
>  >>Chris Albertson
>  >>Redondo  Beach,  California
>  
>  
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