[time-nuts] hammond 77589

Ed Breya eb at telight.com
Tue Jan 15 17:41:12 UTC 2013


I just noticed one more thing - if the NTC thermistor in the schematic 
is the actual one that senses the oven temperature, then the oscillator 
and bridge are one in the same. If this is the case, then the oscillator 
amplitude could be proportional to bridge imbalance - if the NTC is 
being balanced by the 1414 ohm resistor. I doubt that this is the case 
though, since the DC resistance of the inductor would swamp the others, 
so it would need to be AC-coupled into the bridge to isolate its 
resistance, but the schematic shows direct connection. Also, you would 
need the phase information to know which direction is needed to correct 
it, so there would have to be another signal pickoff from this circuit 
to get the demodulator phase info.

If the thermistor is just sensing ambient temperature of the control 
circuit, then it's probably compensating for circuit drift and 
oscillator amplitude. I think this is the more likely setup, given the 
schematic info.

Ed




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