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Start up circuit for DC powered field instrument

  • US 5,973,942 A
  • Filed: 07/10/1998
  • Issued: 10/26/1999
  • Est. Priority Date: 07/10/1998
  • Status: Expired due to Term
First Claim
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1. An industrial process control transmitter comprising:

  • first and second power supply terminals;

    first and second process control loop terminals;

    an energy storage device coupled between the first and second power supply terminals;

    a switching regulator circuit coupled to the energy storage device and comprising a regulated voltage output;

    a sensor which is powered by the switching regulator circuit through the regulated voltage output for sensing a process variable;

    a microprocessor coupled to the sensor for receiving the sensed process variable, wherein the microprocessor is powered by the switching regulator circuit through the regulated voltage output;

    a memory which is coupled to the microprocessor and powered by the switching regulator circuit through the regulated voltage output;

    a current control circuit coupled between the microprocessor and the first and second process control loop terminals for controlling a current through the first and second process control loop terminals under control of the microprocessor;

    a variable impedance circuit coupled between the energy storage device and the first power supply terminal and having an impedance control input, a first current path with a first impedance and a second current path with a second impedance, wherein the second impedance is less than the first impedance and the second current path is selectively coupled in parallel with the first current path as a function of a voltage applied to the impedance control input; and

    a voltage measurement circuit comprising a comparator having a first comparison input which is coupled to the energy storage device, a second comparison input which is coupled to the first power supply terminal, and a comparison output which is coupled to the impedance control input.

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