Abstract:
The described circuit is an intrinsically safe circuit configured for supplying intrinsically safe power from a power supply in a hazardous environment. The intrinsically safe circuit reduces a number of node connections by reducing a number of components. The intrinsically safe circuit comprises an opto-coupler circuit configured for receiving a control signal, a resistor, and a single voltage limiting circuit. The reduction in components and node connections results in a reduction of board costs and area, while maintaining a desired level of protection.

Description:
FIELD OF THE INVENTION 
     This invention relates to transmitting data from intrinsically safe circuits to non-intrinsically safe circuits. More particularly, this invention relates an opto-coupler used to transmit data between the circuits. Still more particularly, this invention relates to circuitry connected to a phototransistor to prevent excess energy from being applied to the opto-coupler. 
     PROBLEM 
     Opto-couplers comprising an LED and an associated phototransistor fabricated together as a single device are known and are widely used in applications in which DC and AC isolation must be maintained between a signal source and a signal output representing an amplified or processed input signal. Opto-couplers are used in instances in which the isolation provided by capacitance coupling between an input and an output circuit is not sufficient. 
     One such instance in which capacitance coupling is not sufficient occurs when input and output circuits are operated in hazardous locations. In such instances, it is well known to use opto-coupler circuitry in which an LED receives an input signal to be amplified or otherwise processed and, in so doing, generates an optical output signal representative of the received input signal. The LED is part of a device package that also includes a phototransistor that has a base that receives the optical output of the LED and, in turn, controls the collector current of the phototransistor. The phototransistor may be advantageously operated as an amplifier whose output is developed across a collector resistor. 
     An opto-coupler operated in this manner provides increased isolation compared to that provided by the capacitor connected transistor amplifier. The isolation provided by an opto-coupler is needed to connect intrinsically safe circuitry to non-intrinsically safe circuits. Intrinsically safe circuits are circuits that operate under a certain low energy level. By operating under a certain energy level, the circuitry is ensured not to generate a spark or sufficient heat to cause an explosion of material in a hazardous environment even if the circuitry fails in some manner. The power level needed to make circuitry intrinsically safe is determined by regulatory agencies such as the UL in the United States, CENELEC in Europe, CSA in Canada, and TIIS in Japan. 
     In order to be intrinsically safe, the intrinsically safe circuit must have a galvanic isolation barrier between the intrinsically safe circuit and circuitry that is not intrinsically safe. In order to transmit digital data across the galvanic isolation barrier, a large number of safety components are needed. In an opto-coupler, either the LED or the phototransistor is part of the intrinsically safe circuit. 
     In a common emitter configuration, the phototransistor saturates and an output voltage from the phototransistor is equal to a saturation voltage of a collector of the phototransistor when the phototransistor is activated by the LED. When the phototransistor is inactive, the output voltage is equal to a power supply voltage. In operation the terminals connected to the phototransistor must not exceed two thirds (⅔) of the maximum current, voltage, or power rating of the opto-coupler. To prevent the terminals connected to the phototransistor from exceeding the maximums, a zener diode and a suitably rated fuse connected to a terminal is typically considered sufficient protection for the terminals. A resistor is also commonly added to prevent against the breaking capacity of the fuse. In this configuration, the diode and the fuse are connected to the collector of the phototransistor and the power supply, thereby increasing the number of components. However, the on all three connections increase circuit size. As the cost of components increases and the board size of circuits decreases, there is a need to configure such circuits in a manner that reduces board cost. 
     SOLUTION 
     The above and other problems are solved and an advance in the art is made by the protection circuitry of this invention. The circuitry of this invention reduces the number of node connections for a phototransistor to two nodes. A single Zener diode, a fuse, and a resistor provide protection. This reduces the number of components needed, which in turn reduces board cost and board area needed. 
     In accordance with this invention, the power supply connection to a collector of the phototransistor is reduced to a signal node. Collector-emitter current is conducted to an output resistor to toggle a data line. The power supply connection and collector are reduced to a single node using a resistor circuit connected between the power supply connection and collector. The use of the resistor allows the opto-coupler to remain powered by a dedicated supply current. 
     In a first aspect of this invention, circuitry that prevents an opto-coupler from exceeding a particular power rating is comprised of the following components. A resistor circuit connects a collector of a phototransistor to a power supply. A fuse connects the power supply to the resistor circuit. A diode connected to the fuse and to the resistor. Finally, a fuse resistor is connected to and between the fuse and the diode. 
     In another aspect of this invention, a resistor may connect the power supply at a first terminal and to the output resistor between the power supply resistor and the diode. 
     In another aspect of this invention, an output resistor may be connected between the emitter and ground. The output resistor may be a 604 ohms resistor. 
     In another aspect of this invention the power supply may provide 14 volts to the circuitry. 
     In another aspect of this invention the resistor circuit may be a 1.0 kilo ohms resistor. 
     In another aspect of this invention the fuse may be a 63 milliamp fuse. 
     In another aspect of this invention, the ground resistor may be a 49.9 kilo Ohms resistor. 
     In another aspect of this invention the fuse resistor may be a 6.2 Ohms resistor. 
     In another aspect of this invention, the diode is a 16.0 Volts Zener diode. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The above and other aspects of this invention are described in the detailed description and the following drawings: 
     FIG. 1 illustrating a prior art opto-coupler and protection circuitry; and 
     FIG. 2 illustrating an opto-coupler and protection circuitry in accordance with this invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates prior art circuitry  100  that includes intrinsically safe circuit  101  and non-intrinsically safe circuit  102 . Galvanic isolation barrier  103  provides a separation between intrinsically safe circuit  101  and non-intrinsically safe circuit  102  in accordance with safety specifications. Light Emitting Diode (LED) L 1  and phototransistor Q 1  form an opto-coupler  105  that passes an optical signal from intrinsically safe circuit  101  to non-intrinsically safe circuit  102 . 
     Opto-coupler  105  is a conventional opto coupler such as a Hewlett Packard HCNW 4506 opto-coupler. In order to transmit data, LED L 1  transmits light signals control phototransistor Q 1 . When Q 1  is “turned on”, phototransistor conducts current. Output voltage, Vo, is equal to a collector-emitter voltage of phototransistor Q 1 . When phototransistor Q 1  is “turned off”, the output voltage, Vo, is pulled to Vdd. A power supply connection to Q 1  supplies a bias voltage, Vdd. 
     In accordance to standards, such as Cenelec document EN 50020 titled “Electrical Apparatus for potentially explosive atmospheres-intrinsic Safety,” the non-intrinsically safe terminal of an opto-coupler must have protective circuitry that ensures the opto-coupler does not exceed two-thirds of a maximum current, maximum voltage, or maximum power rating. In the prior art, two of three nodes of the phototransistor are protected by fuses. 
     In order to protect a phototransistor Q 1 , fuse F 1  is connected between a power supply and the has a cathode connected to phototransistor Q 1  and to fuse F 1  and an anode connected to receive a ground reference potential. Resistor R 1  is connected between fuse F 1  and diode Z 1  to protect the breaking capacity of the fuse. 
     A collector node of phototransistor Q 1  supplies Vo. The collector node is connected to the power supply node by resistor R 2 . Fuse F 2  is also connected to the collector node. Diode Z 2  has a cathode connected to the collector node of phototransistor Q 1  and an anode connected to receive a ground reference potential. Resistor R 3  is connected between the collector node and fuse F 2  and protects the breaking capacity of the fuse. An emitter node of phototransistor Q 1  is connected to ground and to the anode of Diode Z 2 . 
     FIG. 2 illustrates an exemplary embodiment of circuitry having protection circuitry in accordance with the present invention. The circuitry of this invention reduces the number of nodes in order to reduce the number of components needed to provide protection against the opto-coupler exceeding two thirds of the maximum voltage, maximum current or maximum power of the opto-coupler. Circuit  200  includes intrinsically safe circuit  201  and non-intrinsically safe circuit  202 . Galvanic isolation barrier  203  separates intrinsically safe circuit  201  and non-intrinsically safe circuit  202 . LED L 1  and phototransistor Q 1  form opto-coupler  205 . An exemplary opto-coupler is the HCNW 4506 opto-coupler produced by Hewlett Packard. Opto-coupler  205  transmits an optical control signal through the galvanic isolation barrier  203  located between intrinsically safe circuit  201  and non-intrinsically safe circuit  202 . 
     Phototransistor Q 1  has a power supply node, a collector node and an emitter node. A power supply is connected to phototransistor Q 1 . A fuse F 1  is in path  210  between the power supply and fuse resistor Rf. Fuse resistor Rf in path  210  is connected between Fuse F 1  and phototransistor Q 1 . The breaking capacity of fuse F 1  is protected by fuse resistor Rf. Diode Z 1  connects path  210  to path  215  with a cathode that is connected to the fuse resistor Rf and an anode that is connected to the output of the circuit. Diode Z 1  regulates a voltage. Power supply resistor Rdd is connected to the cathode of diode Z 1  and the phototransistor Q 1 . A resistor Rg is connected to path  210  and path  215  between power supply resistor Rdd and diode Z 1 . 
     Power supply resistor Rdd is connected between phototransistor Q 1  and the fuse resistor Rf in path  210 . The collector of phototransistor Q 1  is connected to resistor Rd. Resistor Rd connects the collector to path  210  via power supply resistor Rdd. The connection of the collector of phototransistor Q 1  to path  210  allows a single fuse and a single diode to provide protection to opto coupler  205 . 
     Path  215  connects an emitter of the phototransistor Q 1  to the output of the circuit. Output resistor Ro is connected between the emitter of the phototransistor Q 1  and ground. Resistor Rg is connected to path  215  between the emitter of the phototransistor Q 1  and output resistor Ro. Diode Z 1  is connected to path  215  in parallel to resistor Rg between the emitter of the phototransistor Q 1  and output resistor Ro. 
     According to Cenelec document EN 50020, opto-coupler  205  cannot exceed two thirds of the values shown in the following table. 
     
       
         
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Parameter 
                 Symbol 
                 Maximum 
                 Units  
               
               
                   
                   
               
             
             
               
                   
                 Output Current 
                 Io 
                 15 
                 mA 
               
               
                   
                 Resistor Voltage 
                 Vr 
                 Vss 
                 Volts 
               
               
                   
                 Output Voltage 
                 Vo 
                 30 
                 Volts 
               
               
                   
                 Supply Voltage 
                 Vss 
                 30 
                 Vo  
               
               
                   
                   
               
             
          
         
       
     
     The following table includes values of the components of non-intrinsically safe circuit  201 . The values will be used below to determine the values for voltage and current outputted by the circuit. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Component 
                 Symbol 
                 Value 
                 Units  
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Power Supply 
                 Vdd 
                 14.0 
                 Volts 
               
               
                   
                 Fuse 
                 F1 
                 63 
                 mA 
               
               
                   
                 Fuse Resistor 
                 Rf 
                 6.2 
                 Ohms 
               
               
                   
                 Supply Resistor 
                 Rdd 
                 1.21K 
                 Ohms 
               
               
                   
                 Divider Resistor 
                 Rd 
                 1.0K 
                 Ohms 
               
               
                   
                 Ground Resistor 
                 Rg 
                 49.9K 
                 Ohms 
               
               
                   
                 Output Resistor 
                 Ro 
                 604 
                 Ohms 
               
               
                   
                 Zener Diode 
                 Z1 
                 16.0 
                 Volts  
               
               
                   
                   
               
             
          
         
       
     
     When phototransistor Q 1  is turned off, current from the power supply, I cch , is limited to 0.6 mA. As a result, the output voltage can be determined from the following equation.              Vo   =       (       Vdd     Rf   +   Rg   +   Ro       +   Icch     )     *   Ro             (   1   )                                
     When the values from the table of component values are inserted into Equation 1, the output voltage when the transistor Q 1  is inactive can be determined to be 0.53 volts. This is well under the values of 20 volts required. 
     When phototransistor is receiving signals from LED L 1  and is active, phototransistor conducts current and the output voltage is determined by the following equation.              Vo   =     Vdd        (     Ro     Rf   +   Ro   +       Rg        (     Rdd   +   Rd     )         Rd   +   Rdd   +   Rg           )               (   2   )                                
     By inserting values from the above table of component values, the output voltage when phototransistor Q 1  is turned on is 3.10 Volts which is well under two thirds of the maximum output voltage given in the table of maximum ratings for opto-coupler  205 . 
     The voltage of power from the power supply, Vss, varies according to whether phototransistor Q 1  is turned on or off. When phototransistor Q 1  is receives optical signals from LED L 1 , phototransistor Q 1 turns on and Vss is equal to the current flowing through resistor Rd. From equation 2, the current flowing through resistor Rd is:              IRd   =       (     Vdd   -   Vo     )         Rg        (     Rdd   +   Rd     )         Rg   +   Rdd   +   Rd                 (   3   )                                
     By inserting values from the table of component values and the table of maximum values, the current flowing through the resistor Rd is 5.15 mA which gives a supply voltage Vss of 5.15 volts. 
     In this embodiment, supply voltage Vss, resistor voltage Vr, and output voltage Vo are limited by diode Z 1 . Therefore diode Z 1  must limit to less than two-thirds of the maximum supply voltage from the table of maximums, or 20 Volts in the preferred embodiment. Fuse F 1  limits the current from the power supply to prevent diode Z 1  from exceeding the two-thirds power of the maximum supply voltage. 
     In the preferred embodiment, Diode Z 1  has a rating of 3.0 watts therefore the maximum power through the diode is limited to 2.0 watts. The power through diode Z 1  may be calculated in the following manner. 
     
       
         Pzener=1.7*IF1*Vzmax  (4) 
       
     
     Where Vzmax is a voltage rating of diode Z 1  plus the tolerance of the diode Z 1  (5%) and a multiplier of 1.7 as required by Cenelec document En 50020 for circuits that include a fuse. Diode Z 1  has a rating of 3.0 Watts which means that the maximum voltage power through the diode must be limited to 2.0 Watts. In the preferred embodiment, the maximum power through Diode Z 1  is 1.8 Watts. 
     The breaking capacity of the fuse must also be protected. In a preferred embodiment, the breaking capacity of fuse F 1  is 50 A and fuse F 1  is rated at 250 Volts. Since the value of fuse resistor Rf is 6.2 Ohms as given in the table of component values, the current of fuse F 1  is limited to 250 Volts divided by 6.2 Ohms or 40.3 A. 
     The last parameter sought is current through the collector of phototransistor Q 1 . Maximum current is reached when phototransistor Q 1  is turned on. The maximum current can be determined by dividing the result of equation 4 by output resistor Ro value.              Icollector   =       1.7   *   IF                 1   *   Vz                 max     Ro             (   5   )                                
     In a preferred embodiment, output current is 5.13 mA when values from the tables of maximums and component values are inserted. This is less than two-thirds of the maximum value for an output current of 10 mA. 
     The above is a description of circuitry for providing protection for an opto-coupler. Those skilled in the art are expected to design alternative circuits that infringe this invention as set forth in the claims below either literally or through the Doctrine of Equivalents.