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Friday, 3 September 2010

This is a setpoint controller circuit using The AD596/AD597. The thermocouple voltage is amplified, compared to an external setpoint voltage, and cold junction compensated. The device temperature can be monitored if the temperature to be controlled is within the operating range (–55°C to +125°C) of the device. Here is the schematic diagram of the circuit:

Setpoint Voltage of AD597 is °C × 10.1mV/°C – 9.1mV and Setpoint Voltage of AD596 is °C × 9.6mV/°C + 42mV. The setpoint pin of the AD596/AD597 has input impedance of about 50k. This resistance has temperature coefficient of ±15ppm/°C. This device has an additional ±1°C degradation of temperature stability over the +25°C to +100°C ambient temperature range due to 100ppm/°C 5k pot.

To increase system reliability and provide noise immunity, this device use Switching hysteresis. to enhanced system reliability, mechanical component wear is reduced by reducing the frequency of on-off cycling. It can be done by adding a single external resistor between Pins 7 and 3 of the AD596/AD597. The hysteresis is determined by following equation:
Rhyst=(Vout/200nA)*(1/°Chyst)

The alarm transistor will be ON regardless of whether there is an open circuit or not because the AD596/AD597 output is saturated at all times. for proper operationof the rest of the circuit, -ALM must be tied to a voltage below (+Vs – 4V) [Circuit's schematic diagram source: analog.com]

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