[time-nuts] Ancient OCXO in scope calibrator.

[David C. Partridge]

Out of curiosity, I tried to find something that might do the job.

The only PTC thermistor I could find was from Digikey (mfr GE
Infrastructure) with a transition temperature in the right range was 70C and
rated at only 25 volts, and that was 50 Ohms at 25C :-(

D.

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of David C. Partridge
Sent: 10 September 2008 18:12
To: 'Discussion of precise time and frequency measurement'
Subject: Re: [time-nuts] Ancient OCXO in scope calibrator.

Now that's an interesting thought.   

How should one go about specifying such a part?

Do you have any experience of doing this?  We'd be looking at (I guess) a
turnover from almost short circuit to darn near open circuit over a range of
a few degrees centred on 75 celsius.   Do such beasties exist?

If it's relevant there are two heater windings, one for 230V, one for 115V
though both do go through the thermostat.

This rather surprised me as I would have expected two windings and only use
one for 115V, and switch the second in series for 230V, but if I'm reading
the circuit diagram right (not g'teed) this isn't what happens (it's on the
last schematic page which is the power supply for those who've donwloaded
the manual from BAMA).

There's also something connected between the two windings which looks a bit
like two cup-hooks - what this is I'm not sure.
I don't recognise the symbol at all, it could just represent the bimetal
strip part of the thermostat.

I also don't quite understand the point of the capacitor at the top end of
the 230V winding.

Cheers
Dave