[volt-nuts] Temperature controller for ovenizing and temperature cycling
jan at 41hz.com
Thu Jan 30 14:10:40 EST 2014
I just built a small heater / regulator board for ovenzing components. I
should also be usable for temperature cycling. The heater outputs up to
around 0.6W, runs on 12V.
It is designed with a voltage sensor and opamp that compares the sensor
output to a fixed (setpoint) voltage divider. The opamp output drives a
small power transistor. Pretty much like the LTZ1000 datasheet circuit but
with a few tweaks.
I have used the LM35 temperature sensor as sensing element, and compared it
to a reference voltage, derived from a zener.
The LM35 has an output of 10mV / C. I gave it a separate buffered output,
amplified it by a factor 10, so I can easily monitor the temperature with a
voltmeter (ie 5.0V = 50C)
The heater resistors give about 0.4W output on average and the regulator
loop switches on/off about once per second at best.
It has been running since last night. Short term stability has been about
0.002 - 0.004C peak to peak (2-4 milli Celcius (mC) ) over a few minutes,
at a nominal temperature of 33C. The temperature has varied by around
10mC as I have tried different component values.
I have not really measured the drift so far, as I have been tinkering too
much with the circuit. It seems to be no more than one or two mC so far.
I am prettu sure that If I place the sensor and heater behind copper plate
or heatsink, the other side of the plate would consistently see less than 1
mC rms oscillations. Long term drift should only depend on the LM35, a
resistor pair and the zener reference.
If someone is interested, I will get back when I have built a few more
boards, tuned the component values, made some longer tests.
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