OPTOCOUPLER FOR A NON-INTRINSICALLY SAFE CIRCUIT

Abstract: Circuitry (200) that prevents an opto-coupler (205) from exceeding a particular power rating. A resistor divider (Rd) in the circuitry connects an emitter of a photo transistor (Q1) to a power supply for the photo transistor (Q1). A fuse (F1) connects the power supply to a power source and to the resistor divider. A diode (Z1) is connected between to the power source and ground to control voltage. A fuse resistor (R1) between the fuse, the power source of the photo transistor, and the resistor divider (Rd) limits voltage between the components.

Description: OPTOCOUP ER FOR A NON-INTRINSICALLY SAFE CIRCUIT 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 photo transistor to prevent excess energy from being applied to the opto-coupler. Problem Opto-couplers comprising an LED and an associated photo-transistor 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 is in applications in which the input and output circuits are operated in hazardous locations in which the safety provided by capacitance coupling alone is not sufficient. 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 photo-transistor whose base receives the optical output of the LED and, in turn, controls the collector current of the photo-transistor. The photo-transistor 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 coupled transistor amplifier. The isolation provided by an opto-coupler is needed to connected 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 photo transistor in part of the intrinsically safe circuit. In a common emitterconfiguration, the photo transistor saturates and an output voltage from the photo transistor is equal to a saturation voltage of a collector of the photo transistor when the photo transistor is activated by the LED. When the photo transistor is inactive, the output voltage is equal to a power supply voltage. In operation the terminals connected to the photo transistor must not exceed two thirds (2/3) of the maximum current, voltage, or power rating of the opto-coupler. To prevent the terminals connected to the photo transistor from exceeding the maximums, a zener diode and 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 fuse are connected to the collector and power supply for the photo transistor. The result is that no path from the signal pins to ground goes through the fuse. Therefore the fuse is not needed on all three connections. 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 photo transistor to two nodes. Therefore, protection can be provided by a single Zener diode, fuse, and resistor. 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 the photo transistor and a collector of the photo transistor are reduced to a signal node. Collector-emitter current is then impressed across an output resistor to toggle a data line. The power connection and emitter are reduced to a single node using a resistor divider between the power connection and emitter. The use of the resistor divider 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 divider connects an emitter of a photo transistor to a power supply for the photo transistor. A fuse connects a power supply to the power source and the resistor divider. A diode connects to the power source and to ground. Finally, a fuse resistor is connected between the fuse resistor and the power source of the photo transistor and the resistor divider. In another aspect of this invention, a power supply resistor may limit a voltage of power applied to the power source of the photo transistor and the resistor divider. This regulates the voltage of current applied to the photo transistor. The power supply resistor may be connected between said power source and said fuse resistor. In another aspect of this invention, a ground resistor may connect the power source and ground between the power supply resistor, the diode, and the fuse resistor. 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 of electricity to the circuitry. In another aspect of this invention the resistor divider 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 that separates the two circuits for purposes of safety ratings. Light Emitting Diode (LED) L1 and photo transistor Q 1 form an opto-coupler 105 that passes digital signals 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 L1 transmits light signals that are received by photo transistor Q1 and that activate and deactivate Q1. When Q1 is activated, photo transistor saturates with power from power supply V αt) . When saturated an output voltage Vo is equal to a collector-emitter saturation voltage of photo transistor Q1. When photo transistor Q1 is deactivated, output voltage Vo is pulled up to Vdd. A power supply connection to Q1 remains biased to Vdd. In accordance to standards, such as Cenelec document EN 50020 titled "Electrical Apparatus for potentially explosive atmospheres-Intrinsic Safety," non-intrinsically safe terminal of an opto-coupler must have protective circuitry that insures the opto-coupler does not exceed tow thirds of a maximum current, voltage, or power rating. In the prior art, two of three nodes of photo transistor Q1 must be protected. As long as two of the three are protected there is no path the current can flow through that does not go through the fuse. In order to protect a power supply node for photo transistor Q1 , fuse F1 is connected between a power supply and the power supply node. Fuse F1 is a fuse having a rating that is suited for sufficient protection. Diode Z1 is connected to ground and between the power supply node and fuse F1. Resistor R1 is connected between fuse F1 , the power supply node, and diode Z1 to protect against the breaking capacity of the fuse. A collector node of photo transistor Q1 supplies Vo. The collector node is connected to the power supply node by resistor R2. Fuse F2 is also connected to the collectornode. Diode Z2 connects the collector node of photo transistorOI toground. Resistor R3 is connected between the collector node and fuse F2 and protects against the breaking capacity of the fuse. An emitter node of photo transistor Q1 is connected to ground and to Diode Z2. From the above discussion, it can be seen that the power supply node and collector node are protected in non-intrinsically safe circuit 102. 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, current or energy of the 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 L1 and photo transistor Q1 form opto-coupler 205. One exemplary opto-coupler is a HCNW 4506 opto-coupler produced by Hewlett Packard. Opto-coupler 205 transmits data over the galvanic isolation barrier 203 between intrinsically safe circuit 201 and non-intrinsically safe circuit 202. Photo transistor Q1 has a power supply node, a collector node and an emitter node. A power supply is connected to the power supply node. A fuse F1 is in path 210 between a power supply and fuse resistor Rf. Fuse Resistor Rf in path 210 is connected between Fuse F1 and the power supply node. The breaking capacity of fuse F1 is protected by fuse resistor Rf. Diode Z1 connects path 210 to ground via path 215. Diode Z1 is connected to path 210 between power supply resistor Rdd and fuse resistor Rf. A resistor Rg connects path 210 and path 215 between power supply resistor Rdd and fuse resistor Rf. Path 210 may also include power supply resistor Rdd that regulates the amount of electricity supplied to opto-coupler 205. Power supply resistor Rdd is connected between the power supply node and the fuse resistor Rf in path 210. The collector of photo transistor Q1 is connected to divider resistor Rd. Divider resistor Rd connects the collector to path 210 between the power supply node and power supply resistor Rdd. The connection of the collector of photo transistor Q1 to path 210 allows a single fuse and a single diode to provide protection to photo coupler 205. Path 215 connects an emitter node of photo transistor to ground. Output resistor in path 215 is connected between the emitter and ground. Ground resistor Rg is connected to path 215 between the emitter and output resistor Ro. Diode Z1 is connected to path 215 in parallel to ground resistor Rg between the emitter and output resistor. In a preferred embodiment, opto-coupler 205 is a HCNW 4506 produced by Hewlett Packard. According to Cenelec document EN 50020, this opto-coupler 205 cannot exceed two thirds of the values shown in the following table. The following table includes the values of the components of non-intrinsically safe circuit 201 for a preferred exemplary embodiment. These values will be used in the description below to determine the values of voltage and current being applied to opto-coupler 205 and being output be photo transistor Q1. The output voltage is determined in the following manner. When photo transistor Q1 is not receiving light signals from LED L1 and is inactive, current from the power supply is limited to .6 mA. As a result, the output voltage can be determined from the following equation. When the values from the table of component values are inserted into FIG. 1 , the output voltage when the transistor Q1 is inactive can be determined to be .53 volts. This is well under the values of 20 volts required. When photo transistor is receiving signals from LED L1 and is active, photo transistor conducts current and the output voltage is determined by the following equation. By inserting values from the table of component values, the output voltage for a preferred embodiment when photo transistor Q1 is active 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 photo transistor Q1 is active or inactive. When photo transistor is receiving signals from LED L1 and is active, Vss is equal to the current flowing through divider resistor Rd. From equation 2, the current flowing through divider resistor Rd is: By inserting values from the table of component values and table of maximum values, the current flowing through the divider resistor 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 Z1. Therefore diode Z1 must limit to less than to two third the maximum of the supply voltage which from the table of maximums may be determined to be 20 Volts in a preferred embodiment. Fuse F1 limits the current from the power supply to prevent diode D1 from exceeding the two thirds power of the maximum supply voltage. In the preferred embodiment, Diode D1 has a rating of 3.0 watts which means that the maximum power through the diode must be limited to 2.0 watts. The power through diode D1 may be calculated in the following manner. Pzener = 1.7 X IF \ X Vz max (4) Where Vzmax is a voltage rating of diode Z1 plus the tolerance diode Z1 (5%) and a multiplier of 1.7 as required by Cenelec document En 50020 for circuits that include a fuse. Diode D1 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 Z1 is 1.8 Watts. The breaking capacity of the fuse must also be protected. In a preferred embodiment, the breaking capacity of fuse F1 is 50 A and fuse F1 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 F1 is limited to 250 Volts divided by 6.2 Ohms or 40.3 A. The last parameter which must be protected is the current through the collector of photo transistor Q1. The maximum current is reached when photo transistor Q1 is active. The maximum current can be determined by dividing the result of equation 4 by the output resistor Ro value. 1. / X Ifuse X Vz max 1 collector — (θ) In a preferred embodiment, output current lo 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 output current lo which is 10mA. 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.

Claims: What is claimed is: 1. Circuitry (200) that prevents an opto-coupler (205) from exceeding a particular power rating comprising: a resistor divider (R d ) connecting an emitter of a photo transistor to a power supply for said photo transistor; a fuse (F1) connecting a power supply to said power source and said resistor divider (R d ); a diode (Z1 ) connected between to said power source and ground; and a fuse resistor (R1 ) connected between said fuse resistor and power source of said photo transistor and said resistor divider. 2. The circuitry (200) of claim 1 further comprising: power supply resistor (R M ) to limit a voltage of power applied to said power source of said photo transistor (Q1 ). 3. The circuitry of claim 2 wherein said power supply resistor (R dd ) is connected between said power source and said fuse resistor (R1 ). 4. The circuitry of claim 2 further comprising: a ground resistor (R 0 ) connecting said power source and ground between said power supply resistor (R dd ), said diode (Z1 ), and said fuse resistor (R1 ). 5. The circuitry of claim 4 further comprising: an output resistor (R6) connected between said emitter and ground. 6. The circuitry of claim 5 wherein said output resistor (R6) is a 604 ohms resistor. 7. The circuitry of claim 4 wherein said power supply provides 14 volts of electricity. 8. The circuitry of claim 4 wherein said resistor divider (R d ) is a 1.0 kilo ohms resistor. 9. The circuitry of claim 4 wherein said fuse (F1 ) is a 63 milliamp fuse. 10. The circuitry of claim 4 wherein said ground resistor (R 0 ) is a 49.9 kilo Ohms resistor. 11. The circuitry of claim 4 wherein said fuse resistor (R1 ) is a 6.2 Ohms resistor. 12. The circuitry of claim 4 wherein said diode (Z1 ) is a 16. Volt diode.

Inventors: MANSFIELD WILLIAM M, MCANALLY CRAIG B, MANSFIELD, WILLIAM, M., MCANALLY, CRAIG, B.
Applicants: MICRO MOTION INC, MICRO MOTION, INC.
Application year: 2001
Main classification: H04B10
IPC codes: H01L31, H03K17, H04B10