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Posted on November 28th, 2010

circuit, microcontroller, old

Crystal Oven Experiments

Now that I've finished my 6-channel data logger (previous post), it's time to put it to the test! I'm using a handful of LM335 temperature sensors to measure temperature, and a 20 Ohm resistor to act as a heater. When 1A of current passes through it, it gets quite toasty! First, I'll make some temperature probes...

UPDATE: Those photos show a partially completed sensor. Obviously the third wire is required between the resistor and the LM335 to allow for measurement! Here's a more completed sensor before the shrink tube was massaged over the electrical elements:

Then I mounted the sensors on a block of steel with the heater on one side. This way I can use one temperature to measure the heater temperature, and the other to measure the temperature of the metal chassis. I then put the whole thing in a small Styrofoam box.

When I fire the heater, that sucker gets pretty darn hot. In 40 minutes it got almost 250F (!) at which time I pulled the plug on the heater and watched the whole thing cool. Notice how the metal chassis lags behind the temperature of the heater. I guess it's a bit of a "thermal low-pass filter". Also, yes, I'm aware I spelled chassis incorrectly in the graphs.

But how do we use this to build a thermo-stable crystal oven for a MEPT (radio transmitter)? I tried a lot of code, simply "if it's too cold, turn heater on / if it's too hot, turn heater off" but because the chassis always swung behind the heater, and even the heater itself had a bit of a delay in heating up, the results were always slowly oscillating temperatures around 10F every 20 min. That's worse than no heater! My best luck was a program to hold temperature stable at 100F with the following rules:

1.) If heater > 155F, turn heater off (prevent fire)
2.) If chassis < 100F, turn heater on
3.) if (heater-target) > (target-chassis), turn heater off

What a great job! That thing is practically stable in 20 minutes. The advantage of this over an analog method is that I can set the temperature in software (or provide an interface to change temperature) and my readings are analytical, such that they can be conveyed in a radio message. Again, my best results came when I implemented rule 3 in the code above. More experiments to come!