Crystal Heater Construction Article
By Bob Dengler   NO6B

OK, here is the circuit I used - See image below.  The original National Semiconductor circuit had a 30 K resistor in series with a 400 µF cap., both in parallel with the 100 K resistor on the input of the 1st LM324 section.  I found that combo to actually destabilize the operation of the controller for this particular application, so I removed them & changed the 2 µF cap in series with the 10 megohm resistor to 5 µF.  That capacitor is a non-polarized paper type; I believe a 4.7 µF non-polarized ceramic should work as well.   The 1/4 watt heater resistor & LM34 temperature sensor are mounted on the same side of the crystal but separated as far apart as possible.  Thermally conductive epoxy is used to mount the LM34 & 51 ohm 1/4 watt heater resistor onto the crystal.

Measuring the LM34's output, it was very good: less than 0.2 °F change.  Keep in mind that other applications will probably require a different feedback loop.  The 400 µF cap & 30 K resistor I removed across the 100 K resistor were probably for controlling much larger systems with longer time lags between the heater & sensor.
 
 

HTML © Copyright January 11, 2007, Kevin K Custer   W3KKC
Text, concept, images, etc., Copyright Bob Dengler  NO6B
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