Electronics > Metrology

LM399 with external oven

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I don't have 24 h data (only shorter time) for the stability for my analog temperature regulator. Given that it is only a LM358 (inside the oven) with a diode, I would expect something in the 0.05 K range. For the long term the plastic case of the transistor used as a sensor may be an issue. For me this is good enough with a relatively well adjusted TC voltage reference (JFET based) and the temperature regulation mainly to fight the square law part.

Edit: I found a stability test for a bit over 2 hours:  This is the sensor also used for the regulation and may this be misleading. Chances are much is LM358 noise. Some 5-10 µV_pp noise over 1 hour sounds plausible.

As you wrote thermal stability is a physics problem, if one does everything right with the digital controller and with the temperature sensors. For example i used the aluminum oven for heat distribution and the heat transfer to the pc board inside and its components constitutes a low pass filter. The on-board (actually on-chip) sensor is used for integration and the Peltier element has its own glass NTC to achieve rapid response against ambient temperature changes entering through the Peltier element.
I used a fairly large heatsink on the other side of the Peltier element and the whole setup is inside an outer case made of plastic. The heatsink is protected from ambient air draft.
I don't have access to the logs right now, but 24h stability is better than 1 mK (RMS). In the winter there used to be a 10 mK excursion when the heaters in the lab turn on at 7:00 in the morning. After that ambient air tempature rises from 16 °C to 20 °C within 45 minutes.

Regards, Dieter

LM399 are interesting parts and i have quite a few, i was thinking to adjust the zener current and try to find the zero t.c temperature for a couple of unheated samples. My plan is to use my old calibration equipment to get the zero t.c zener current.

The plan.
1: Use my Tek Know TC400 dry well and ice bath to set the temperature.
2: Using my process calibrator i can generate different zener currents for the references to find the optimal value.
3: Read/plot zener current, zener output voltage and temperature using the process calibrator and a Datron 1071 for precision voltage measurements.

The equipment specifications.

TEK know TC-400
TEMPERATURE RANGE: ~Ambient to 400°C

Process calibrator specifications: Voltage range 60V with 0.01mV resolution, Current generation range 0…25 mA uncertainty about 1.1-3uA

The Datron 1071 is 7.5 digits and quite stable over time so it should do alright in these tests.

Soon when i get some vacation time of work i will start working on this :D

The LM399 reference part has more than just a zener. The actual zener current is regulated internally and unless going too low the TC does not change much with changing the current to the reference part.

AFAIK the datron 1071 has a LM399 or similar reference. So it would be a comparison of two comparable references only.

Yep, the LM129/399 zener's current is set to 250uA internally, and stabilized, afaik. I doubt you could force the zener's current more than, say, +/- 20uA from that value.
Have a look at the 399's schematics in its datasheet.
The other suitable part for experimentation will be the ADR1001, when it gets finally available. You may set the zener's temperature with an external resistor, as well as the zener's current..


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