There's an interesting circuit in National's app note AN-460, page 8.
Here's a link to the PDF of AN-460:


http://www.national.com/an/AN/AN-460.pdf

Mike Hardwick
Decade Engineering


>I am building a 10mhz xtal frequency standard and am looking for a
>circuit diagram for a simple temperature regulator. I have the
>oscillator running in a small cast aluminum box and my idea was to bolt
>a power transistor to the case and use that as the variable heating
>element. I would then mount the whole unit inside a block of foam
>insulation. I would like to hold the crystal at a non critical
>temperature between 35-50C. Any ideas?

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