New magnetic amplifier improves EMF to current converter

The all solid-state millivoltage to current converter described meets the exacting dependability, accuracy, and stability demanded of industrial process controls. It has a minimum input span of 2 mV direct current, an output current range of 10-50 mA direct current that is virtually independent of load resistance, and has complete electrical isolation between input and output. It is intended primarily for use with thermocouples. A new design of the input magnetic amplifier stage results in a very low overall noise figure, a high current gain, and a high effective input impedance (as much as 100 kΩ) relative to thermocouple resistance. The converter consists of two separate precision amplifiers in cascade, each stabilized by a large amount of feedback. Zero elevation and cold junction compensation are provided as needed with the aid of a precision Zener diode regulator circuit.