Electrical transmission or interconnection systems – Switching systems – Condition responsive
Reexamination Certificate
1999-09-07
2001-09-04
Ballato, Josie (Department: 2836)
Electrical transmission or interconnection systems
Switching systems
Condition responsive
C340S315000
Reexamination Certificate
active
06285094
ABSTRACT:
TECHNICAL FIELD
The present invention relates to electric control circuits and, more particularly, to a method and apparatus for using a self-powered current loop to transmit a process variable using only the current that drives the loop.
BACKGROUND AND OBJECTS OF THE INVENTION
The use of current loops enables the most popular, safe and easy method of transmitting a process variable to a distance limited only by the electromotive force (EMF) that drives the loop. A current loop's simple, two-wire connection allows for fast and simple interconnection to as many devices in the loop (in series) as desired, limited only by the loop's EMF.
Traditional current-loop controllers are externally powered through AC Mains or direct current voltage. They are expensive, complex and bulky. It is desirable, then, to provide a current-loop controller that avoids these shortcomings and that has other advantages as described below. The control circuit should, in addition to driving desired devices, be able to provide, control, and/or indicate outputs and control or manage processes.
SUMMARY OF THE INVENTION
The present invention is directed to an apparatus in the form of a current-loop controller and associated method that is driven by the EMF in the loop. The current-loop controller provides, controls and indicates output to control and manage a process without the need for external power. Since the current industry standard for current-loops is 4-20 maDC, the preferred embodiment is designed to reliably operate devices connected in the loop (in series) within the current operating range of 4-20 mA. This is not, however, intended to limit the present invention to such range.
In the preferred embodiment of the controller electric current is initially applied to the circuit formed by the controller, causing a first light-emitting diode (LED) and a first pair of transistors to tum on. The transistors allow current to flow through a zener diode and a resistor, and out to an −L current loop to close the circuit. A zener diode regulates voltage extracted from the current to power another reference diode and a comparator. If the signal variable monitored by the first resistor drops below the set point value of the comparator, the comparator will switch its output to a high (VCC) level turning the first transistor off and allowing the current to flow through an LED, the opto-isolator's internal LED, another diode, and a second transistor which is on. The opto-isolator's internal LED turns on its phototransistor causing its collector and emitter terminals to have a very low resistance energizing and external device.
When the signal variable exceeds the set point value of the first comparator, it switches to low (ground) turning on the first transistor which turns off the LED and the opto-isolator's internal LED and phototransistor. The same is true for the high-limit second comparator, but in reverse. If the signal in the current loop exceeds the set point, the second comparator will turn off the second transistor, thereby routing the current through its associated LED and opto-isolator turning on the LED and phototransistors in the same manner as in the “low” comparator, but reversed for “high” comparison.
REFERENCES:
patent: 4090121 (1978-05-01), Nelson et al.
patent: 4271505 (1981-06-01), Menot et al.
patent: 4344128 (1982-08-01), Frye
patent: 5295037 (1994-03-01), Dougherty
patent: 5633651 (1997-05-01), Carvajal et al.
patent: 5684451 (1997-11-01), Seberger et al.
Ballato Josie
McCormick Paulding & Huber LLP
Rios Roberto
LandOfFree
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