Abstract:
A system and method for isolating the input and output of a self-powered current loop includes coupling a voltage to frequency converter to the input terminals and connecting the output of the voltage to frequency converter through an optical isolator, either directly or through other circuitry, to the output terminals depending upon the nature of signals which are to be provided from the circuit.

Description:
RELATED APPLICATION/PRIORITY DOCUMENT 
   This patent application corresponds to, and claims the benefit of priority under Title 35 U.S.C. §119(e) of co-pending provisional application Ser. No. 60/709,754 filed Aug. 22, 2005, incorporated herein by reference. 

   BACKGROUND 
   This invention relates to electric control and monitoring circuits, and more specifically, to circuits using self-powered current loops to transmit process variables to remote locations. For many applications, due to safety regulations, the connections to the loop must be earth grounded, creating a ground loop and/or defeating the “hazardous to safe area” requiring isolation. 
   Products have been provided which provide “galvanic” (transformer) isolation which, due to its inherent design, provides a limited isolation to typically 500 VDC/RMS. Due to the transformer oscillator driven design, such isolation products inject a “chopper” noise on the source, creating problems for other electronic components in the loop, as well as being subject to extreme temperatures and humidity. This limits the use of such transformer oscillator driven isolators in many modern process monitor/control environments. 
   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. The simple two-wire connection of the current loop allows for fast and simple interconnection to as many devices in the loop (in series) as desired, limited only by the EMF of the loop. 
   Traditional current loop isolators are externally powered through AC mains, current loops or direct current voltage. Such traditional isolators are expensive, complex and bulky. In addition, the conventional isolators use galvanic isolation (transformers) that, due to their nature, have limited voltage breakdown and generate noise due to the oscillator technology used, which introduces errors to equipment in the input loop. 
   It is desirable to provide a current loop isolator which overcomes the disadvantages of the isolation devices mentioned above, and which eliminates the inherent chopper/switching electrical noise generated by existing galvanically (transformer) isolated technologies. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The sole FIGURE of the drawing is a schematic diagram of an embodiment current loop isolator according to the present invention. 
   

   DETAILED DESCRIPTION 
   Reference now should be made to the circuit diagram illustrated in the sole FIGURE of the drawing. A current loop isolator circuit  10  of an embodiment of the invention is disclosed. The +L current is applied to a first input terminal  12 . Two light-emitting diodes  14  and  15  are connected in series with a resistor  16  to develop the voltage to power the loop circuitry and to provide a visual display indicative of current flow in the loop. The diodes  14  and  15  also operate as a voltage regulator to protect the circuitry from over-voltages, which may result from inadvertent connections. 
   The resistor  16  in series with the diodes  14  and  15  passes the current through the circuitry and to the −L current loop in terminal  24  from the instant current is applied to the terminals  12  and  24 . The voltage developed across the light-emitting diodes  14  and  15  and the resistor  16  is used to power a voltage to frequency converter  20  and an optical isolator circuit  22 . The voltage to frequency converter circuit U 1  is a National Semiconductor LM331M or its equivalent. The variable voltage across the resistor  16  is fed into the comparator input of the voltage to frequency converter circuit  20  through a resistor  26  at the junction of that resistor with a capacitor  27  connected to ground. This comparator input voltage is internally compared to the oscillator reference of the circuit  20  to determine the output frequency at the FOUT lead  28 . 
   The selection of the design and parameters of the components comprising the voltage to frequency converter circuit  20  is made to ensure linear current to voltage to frequency conversion and operation at input current loop variations from under 4 mA to over 20 mA to cover under and over range conditions. A resistor  30  and capacitor  32  establish the R/C time constant for the circuit oscillating range, as determined by a threshold set by a resistor  34  and a capacitor  36  connected in parallel with one another and in series with a resistor  38 . 
   A resistor  40  sets the reference for the current input to the circuit  20 . The resulting frequency output of the voltage to frequency circuit  20  at output  28  turns on the internal LED of an optical isolator circuit  22 , which is powered by VCC through the resistor  42 . As the internal LED of the isolator  22  turns on and off, the mating opto-transistor of the isolator  22 , driving a coupler capacitor  46 , through a resistor  47  sharpens the pulses from the circuit  22 , which triggers a frequency to a voltage converter circuit  48 . The frequency to voltage converter circuit  48  also may be a National Semiconductor LM331M or its equivalent. 
   As noted above, the circuits  20  and  48  both are the same parts; but they are connected differently for voltage to frequency conversion for the circuit  20  and frequency to voltage for the circuit  48 . The current output of the circuit  48  at its output lead  50  is directly proportional to the frequency input at the input (threshold) pin  52 , as determined by the value of a resistor  51  and capacitor  53 , along with a resistor  58  connected between the lead  50  in parallel with a capacitor  57  and ground. The output  50  of the circuit  48  is a pulse width modulation (PWM) current output, which is converted to voltage by the capacitor  57  and resistor  58 . This is buffered by an operational amplifier  60 . 
   The output of the amplifier  60  is converted back to current through a resistor  61  and a potentiometer  68  to be summed with current produced by the resistor  67  and a potentiometer  66 . These two currents are fed into the input pin  62  of a voltage-to-current converter circuit  64 . The circuit  64  may be a Texas Instrument XTR116 or its equivalent. The circuit  64  applies the two input currents supplied to its input  62  for subsequent conversion to 4-20 mA output. The potentiometer  66  sets the zero offset of the voltage-to-current converter  64  (typically 4 mA) when the input current loop is 4 mA (or as required by the process). The potentiometer  68  adjusts the output of the voltage-to-current converter  64  for 20 mA (SPAN) when the input to the voltage-to-frequency converter  20  is 20 mA (or as required by the process). 
   An LED  69  coupled to the +L input  70  and to the V+ terminal of the circuit  64  serves two purposes: one of these purposes is to provide a visual indication of the output power “on” and the other is to prevent reverse polarity of applied power. A transistor  72  buffers the output of the voltage-to-current converter  64  to minimize self-heating effects. 
   Finally, a resistor  74  connected across the output terminals  76  and  78  converts (if required) the current into a voltage output at the terminals  76  and  78 . 
   Additionally, and as a byproduct of the circuit shown in the drawing, the circuit can be used to convert loop powered signals to a frequency output by using jumpers between terminals  80  and  82 . This then would delete the circuit  48 , 64 , 60  and the associated components providing an “open collector” output of the frequency. For voltage input signals, the resistors  84  and  16  form a voltage divider to operate the voltage to frequency circuit  20 , as described previously, and obtain an isolated voltage, frequency or current loop output, as explained above. For frequency input applications, jumpers between terminals  86  and  88  are used to drive the opto-isolator  22  directly and are outputting either frequency, voltage, or current loop outputs, as previously explained. 
   The system, through the use of a voltage-to-frequency solid state converter and the other circuit components shown between the input terminals  12  and  24  and the output terminals  76  and  78 , provides an effective isolator driven by the EMF in the loop without requiring any additional internal or external power source to re-transmit the loop signal. This is done in an optically isolated electronic manner without the use of any galvanic transformers. The circuit  10  is capable of an extended isolation range greater than that of galvanic systems used in the past. 
   The foregoing description of an embodiment of the invention is to be considered as illustrative only and not as limiting. Various changes and modifications will occur to those skilled in the art to achieve substantially the same result, in substantially the same way without departing from the true scope of the invention as defined in the appended claims.