Abstract:
A circuit arrangement includes a current measuring or temperature measuring device, used for triggering a shunt device. Said shunt device produces a short-circuit, which directly leads to the blowing of a fuse. The arrangement is modular and allows the operation of several current measuring devices on a common shunt device. Said arrangement is specifically adapted to prevent a protection resistor from warming up to too high a surface temperature in the case of an error, because the current is flowing for too much time.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to electrical safety circuits, and more particularly to power limiting circuits for preventing excessive current draw and overheating of circuit components due to power dissipation. 
   Electrical devices often have to be deployed in hazardous areas where potentially combustible gas is present, and measures have to be taken to prevent explosions. According to the specifications for explosion protection, an electrical load is deemed an “intrinsically safe load” the maximum current that this load can draw from the power source cannot exceed a certain limiting value. The maximum values of current and voltage depend on environmental conditions. In one case the maximum power value derived from these specifications is 40.29 W at 12.1 V and 3.33 A. 
   BACKGROUND OF THE INVENTION 
   The specifications assume that in the intrinsically safe device, short-circuit faults could occur, which could possibly lead to a greater current drain, causing the device to lose its property as intrinsically safe. 
   To ensure that such an operating state cannot occur due to a malfunction of components in the device, the specifications demand a current limiting resistor in the power supply line. In the short-circuit case, up to 36 W of power dissipation can appear across this current limiting resistor. The specifications require that under no circumstances may this 36 W cause the temperature on the surface of the resistor to go over 135° C. To ensure that this temperature requirement can be maintained under unfavorable conditions, protective resistors with large surfaces and correspondingly large physical sizes are needed. Also the device housing must be able to dissipate the maximum power dissipation of 36 W in the interior without unacceptable heating. 
   With today&#39;s demands for device miniaturization, the large size of the protective resistor has proven to be a significant obstacle to further size reduction. Ultimately, the current limiting resistor determines the volume of the housing based on the required maximum surface temperature. 
   The same issue exists likewise for simple safety barriers. A safety barrier is a protective circuit that absolutely ensures that power-carrying lines that exit the safety barrier and run into the hazardous zone cannot carry a current or voltage that exceeds the permissible values for intrinsically safe power circuits. 
   The simplest type for such safety barriers uses an ohmic resistor in the series branch of the power path, which, if the safety barrier is to be arranged in the hazardous zone, must meet the same safety requirements for intrinsically safe devices as explained above. Thus, the safety barrier typically provides additional protective circuits in order to prevent over-voltages. 
   OBJECTS AND SUMMARY OF THE INVENTION 
   In view of the foregoing, it is an object of the invention to provide a power limiting circuit that allows a resistor with a smaller cooling surface and thus with a smaller physical size to be used as the protective resistor that produces the current limiting effect. 
   This task is accomplished according to the invention by the circuit arrangement with the features of claim  1 . 
   The power limiting circuit arrangement according to the invention has two series branches lying in the supply line to the corresponding load, with the protective resistor being included in one of the series branches. A sufficiently fast-acting current measurement device detects the current in one or both series branches and generates a signal, which is applied to a controllable shunt device between the two series branches. As soon as the shunt device is activated, it generates for a short time a low-dissipation current draw, which activates a non-linear currently limiting element. In the simplest case, this non-linear element is a safety fuse, which is blown due to the current draw generated by the shunt device. Alternatively, this element could also be a PTC resistor with a strongly non-linear characteristic curve, which abruptly enters the high-resistance state if the trigger-point temperature is exceeded, wherein the resulting low power dissipation keeps the resistor in the high-resistance state. 
   By means of the current measurement device in connection with the shunt device, a rapid shutdown or at least a strong reduction of the current in the series branch is provided, if the current strongly exceeds the limiting value or is in the limiting region over a long time. 
   This rapid shutdown or current reduction has the effect of preventing the surface of the protective resistor of the power limiting circuit from being overheated. As a result, the protective resistor can be dimensioned smaller correspondingly. It only has to ensure that the current measurement device and the shunt device become active just before the impermissible surface temperatures of the protective resistor are reached. 
   Advantageously, both the current measurement device and the shunt device preferably have redundant configurations, because they contain semiconductors that tend to have defects according to the assumptions of the specifications. 
   A very simple way to implement the current measurement device is to use at least one NTC (negative temperature coefficient) resistor, which is thermally coupled to the protective resistor. If the protective effect of the protective resistor is triggered and the resistor begins to heat up, the NTC resistor also begins to heat up. Just before the protective resistor reaches impermissible surface temperatures, the resistance of the NTC resistor becomes sufficiently small that it can trigger the shunt device. 
   The current measurement device may also be implemented using a current sensor resistor in one of the two series branches. The current sensor resistor can have a very low resistance, so that under no circumstances does it dissipate a large amount of power. The current value measured at this point is used as an input to the control circuit, which generates from this value the control signal for the shunt device. The current measurement device can have at least one controlled semiconductor device, preferably a transistor, whose control segment is connected in parallel to the current sensor resistor, for detecting the voltage drop across the resistor. 
   Another implementation lays the main segment of a circuit that operates as a type of parallel regulator parallel to this current sensor resistor. 
   If the voltage appearing across the current sensor resistor is too small to generate the control or power supply current for the shunt device, the control voltage for the shunt device can be formed by combining the voltage drop across the current sensor resistor with a constant voltage. The constant voltage can be obtained with the help of a Zener diode or a comparable integrated circuit from the voltage between the two series branches. 
   The power limiting circuit arrangement according to the invention can be used both on the power supply side of intrinsically safe devices and also as a stand-alone safety barrier. In addition, it permits the new circuit to be used to build a “multi-channel” safety barrier. For this purpose, the shunt device is only provided once, while on the “output side” the series branch forks or splits into a corresponding number of parallel series branches, with another current measurement device being allocated for each pair of additional series branches. Here, for instance, a bus-rail type power supply can be created, which has a shunt device on the side of the power supply and goes out by a selected number of power supply paths to the corresponding devices. Thus, each power supply path contains its own current measurement device, with all the current measurement devices switched in parallel and acting in common on the one shunt device. If a dangerous condition appears in only one of the power supply paths, the shunt device is activated and all commonly powered current paths are turned off. 
   Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic circuit diagram showing an embodiment of a power limiting circuit for providing intrinsic safety of electrical power supply in accordance with the invention; 
       FIG. 2  is a schematic circuit diagram showing a multi-channel configuration of the power limiting circuit; 
       FIG. 3  is a schematic circuit diagram showing another embodiment of the power limiting circuit in accordance with the invention; and 
       FIG. 4  is a schematic circuit diagram showing the use of an integrated circuit for replacing a Zener diode circuit in the circuit of  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a circuit arrangement  1 , as it can be arranged, e.g., in the power supply line to an intrinsically safe device. The protective circuit  1  has a first and a second series branch  2 ,  3 , each of which galvanically connects, respectively, a first input point  4  to a first output point  5  and a second input point  6  to a second output point  7 . A load is attached to the output points  5  and  7 . In the series branch  2 , there is a protective resistor  8 , which is required according to the specifications of explosion protection for the “intrinsically safe” type of protection. The resistor  8  should limit the current in the series branch  2  and thus also in the series branch  3  to typical, intrinsically safe values if a short circuit occurs between the output points  5  and  7 , i.e., on the sides of the load, which is attached to the output points  5  and  7 . 
   In the failure case, a power dissipation of ca. 36 W appears on the protective resistor  8 , which must be converted into heat. Here, the surface temperature of the resistor may not exceed 135° C. 
   This surface temperature is not reached instantaneously because the protective resistor  8  exhibits a certain thermal inertia due to its structural size and surface and also the materials that are used. By dimensioning the protective resistor  8  appropriately, the surface temperature can always remain below the acceptable limiting value. However, such dimensioning leads to a mechanically very large protective resistor  8 . 
   The mechanical size of the protective resistor  8  can be reduced if the current is directed into the two series branches  2 ,  3  in the fault case in order to turn off the current to the load just before the protective resistor  8  heats up to an impermissible degree. For this purpose, the protective circuit contains a current measurement device  9 , which detects the current in the series branch  2  and thus to the load, as well as a shunt device  10 , which leads to the current to the load being shut off in connection with a fuse  11 . 
   A current sensor resistor  12 , which lies in the series branch  2  between the fuse  11  and the protective resistor  8 , belongs to the current measurement device  9 . Its resistance is small relative to the value of the protective resistor  8 . The voltage drop across the current measurement resistor  12  is detected with the help of a bipolar transistor  13 , whose emitter is connected to the end of the resistor  12  facing the node  4  and whose base is connected via a decoupling resistor  14  to the node between the protective resistor  8  and the current sensor resistor  12 . The collector of the transistor  13  forms a signal output  15  of the current measurement device  9 . Instead of the ohmic resistor, a non-linear resistor can also be used as the current measurement resistor  12 . This can be formed by a diode poled in the direction of throughput, if only a very small current is drawn off as a rule at the output points  5 ,  7 . 
   The current measurement device  9  has a redundant configuration and contains additional bipolar transistors  16  and  17 , whose emitters are connected together galvanically to the emitter of the transistor  13 . The collectors of the transistors  16  and  17  are likewise attached to the signal output  15 . The bases of the transistors  16  and  17  are each connected via separate decoupling resistors  18  and  19  to the node between the current sensor resistor  12  and the protective resistor  8 . 
   The shunt device  10  likewise has a redundant configuration and contains two thyristors  21  and  22  connected in parallel, whose anodes are connected as shown to the series branch  2  and whose cathodes are connected to the series branch  3 . The control electrodes of the thyristors  21  and  22  are each connected via separate decoupling resistors  23  and  24  to the signal output  15 . The node between the cold ends of the two resistors  23  and  24  forms, so to speak, a control input  25  of the shunt device  11  [sic;  10 ]. 
   Finally, for discharging unavoidable leakage currents, there is a discharge resistor  26 , which connects the control input  25  to the series branch  3  at a high resistance. 
   To increase the safety of the entire arrangement, there is an NTC resistor  27 , which connects the control input  25  to the series branch  2 . Another NTC resistor  28 , which is connected in parallel to the NTC resistor  27 , is thermally coupled to the protective resistor  8  and directly measures its surface temperature, while the NTC resistor  27  detects the ambient temperature. 
   Finally, a Zener diode  29  is provided, which lies between the series branch  2  and the control input  25  and which ensures that the voltage between the series branches  2  and  3  cannot exceed a predetermined limit. 
   Based on its function, the signals supplied by the current measurement device  9 , the NTC resistors  27  and  28 , and the Zener diode  29  are OR-gated to the control input  25 , i.e., to trigger the shunt device  10 , it is sufficient if one of the signals exceeds a permissible limit. 
   To explain the function, it is assumed that a power source is attached to the input points  4  and  6 , while the output points  5  and  7  are connected to an arbitrary load. From the viewpoint of the nodes  4  and  6 , the arrangement should be intrinsically safe in the sense of the specifications of explosion protection. 
   As long as no short circuit appears on the load side, the current flows over the two series branches  2  and  3  to the load and then back from the load, respectively. Here, a small voltage drop results across the current measurement resistor  12  and the protective resistor  8 . The voltage drop across the protective resistor  8  is too small in normal operation for significant power dissipation, which would lead to unacceptable heating, to appear there. 
   The voltage drop across the current sensor resistor  12  is smaller than that corresponding to the threshold voltage of the PN junction of the transistors  13 ,  16 , and  17 . These transistors  13 ,  16 ,  17  remain blocked. 
   The voltage between the two series branches  2  and  3  is smaller than the Z voltage  29 , so that this also remains blocked. The temperatures are low, which means the NTC resistors  27  and  28  each remain at a high resistance. Consequently, there is also no current in the discharge resistor  26 , which could lead to a voltage drop, which would be over the gate trigger voltage of the thyristors  21  or  22 . 
   If a short circuit appears in the load, which leads to a large current in the series branch  2 , the limiting effect by the protective resistor  8  is activated. Simultaneously, the voltage drop across the current measurement resistor  12  increases. The voltage drop reaches values that can make at least one of the transistors  13 ,  16 , and  17  conductive. Therefore, the transistors become low resistance and the potential on the control input  25  shifts to the value on the series branch  2  minus the saturation voltage of the transistors  13 ,  16 , and  17 . In each case, the voltage is sufficient to trigger the thyristors  21  and  22  via the protective resistors  23  and  24 . Because the thyristors  21  and  22  are attached directly to the cold side of the fuse  11 , a current, which immediately burns through the fuse  11 , is produced through the fuse  11 . Thus, within a short time, the load carries no current. 
   The two resistors  8  and  12  lie beyond the circuit formed by the fuse  11  and the thyristors  21  and  22 . Thus, these two resistors  8  and  12  have no effect on the current flowing through the thyristors  21 ,  22  and also through the fuse  11 . 
   The current measurement device  9  reacts very quickly and thus, in connection with the thyristors  21  and  22 , it can completely turn off the current path within a very short time after the appearance of the limit current. 
   The transistors  13 ,  16 , and  17  are connected in parallel to each other both on the input side and also on the output side, which produces the redundancy mentioned above. If one of the transistors can no longer be set into the conductive state because of a fault, the remaining, still functional transistors can generate the necessary current for triggering the thyristors  21 ,  22 . 
   The resistors  14 ,  18 , and  19  should prevent a feedback effect on the function of the remaining transistors  13 ,  16 ,  17 , if the base-emitter path on one of the transistors  13 ,  16 ,  17  breaks down. Without the decoupling resistor  14 ,  18 , or  19 , this failure would lead to a short circuit of the current sensor resistor  12 . Also, the functional transistors would no longer receive a control signal. 
   For the shown circuit arrangement  1 , if none of the transistors  13 ,  16 , or  17  reacts, e.g., because the current sensor resistor  12  is defective, the protective resistor  8  heats up. The temperature of the protective resistor  8  is measured by the thermally coupled NTC resistor  28 . At a sufficient distance below the impermissible surface temperature, the resistance of the NTC resistor  28  decreases to a value that generates a current to trigger the thyristors  21 ,  22 . 
   With the help of the NTC resistor  28 , failure states, for which impermissible surface temperatures could be achieved due to a very small dimensioning of the protective resistor  8 , can also be detected before the voltage drop across the current sensor resistor  12  is sufficient to control one or more of the transistors  13 ,  16 , or  17 . Thus, the NTC resistor  28  would react to a long-lasting slight overload, while the current sensor device  9  reacts quickly to a very large overload. 
   The NTC resistor  27  monitors the general ambient temperature and provides a turn off if the ambient temperature has risen so far that ordinary cooling of the components is no longer guaranteed. 
   The Zener diode  29  monitors the voltage between the two series branches  2  and  3 . As soon as the voltage exceeds an impermissible value, the Zener diode  29  becomes conductive and delivers a gate trigger current for the two thyristors  21  and  22 . 
   The circuit arrangement shown in  FIG. 1  is suitable not only as a component of intrinsically safe circuit devices, but also as a circuit for a safety barrier. In addition, on the basis of this basic circuit, a multi-channel safety barrier can be built, as shown schematically in  FIG. 2 . 
   The difference between the circuit arrangement from  FIG. 2  and the arrangement from  FIG. 1  primarily concerns the lack of the protective resistor  8 . Limiting the current to values required according to the specifications of intrinsic safety happens exclusively with the help of the current monitoring device  9  in connection with the current sensor resistor  12 . In general, the circuit from  FIG. 2  contains the circuit from  FIG. 1  completely, whereby the same reference numbers are used for repeating components. The change of the circuit in the direction towards a multi-channel safety barrier consists in the connection of additional branch lines  2 ′ and  2 ″ or  3 ′ and  3 ″, which branch off from the two series branches  2  and  3 . In an electrical sense, the branching point lies on the cold side behind the Zener diode  29 . The Zener diode likewise has a redundant configuration in the case of the arrangement from  FIG. 2 , i.e., there is another Zener diode  29 ′. 
   Because the branch lines  2 ′ . . .  3 ″ branch between the current measurement device  9  and the Zener diode  29 , the monitoring functions by the Zener diodes  29  and  29 ′, the NTC resistor  27 , and also the shunt device  10  are the same for all circuit parts lying to the right from this point. The branch lines  2 ′ . . .  3 ″ are, so to speak, extensions of the series branches  2  and  3  leading in the direction of the load up to the Zener diode  29 . 
   Between the branch lines  2 ′ . . .  3 ″ there are corresponding current measurement devices  9 ′ and  9 ″, which are configured in the same way as the current monitoring device  9 , whose function is explained in more detail above in connection with  FIG. 1 . 
   Because the monitoring device  9  behaves like an open-collector circuit at its output  15 , it can be connected at the control input  25  to other similarly configured circuits, wherein the above mentioned OR linking is used. Thus, for each channel of the circuit arrangement shown in  FIG. 2 , the current to a load is monitored individually, as indicated by  31 ,  31 ′,  31 ″. If a fault produces power consumption to a degree that is greater than that permissible according to explosion-protection specifications for the intrinsically safe type of protection, transistors  13 ,  16  contained in the affected current measurement device  9  become conductive and trigger the thyristors  21  and  22 , which are common for all channels. Therefore, all loads attached to the multi-channel safety barrier, thus also those operating fault-free, are turned off. 
     FIG. 3  shows the principle circuit diagram for an arrangement featuring a precise switching threshold. The switching threshold, for which the shunt device  10  from  FIG. 1  is triggered, depends on the component tolerances of the transistors  13 ,  16 , and  17 . On top of everything, the characteristic curve is relatively flat because the amplification is relatively small. A higher switching accuracy can be achieved if the voltage on the current sensor resistor  12  is detected not only with the help of a single transistor, but also with the help of an integrated circuit exhibiting the characteristics of a Zener diode. An integrated circuit that is suitable for this purpose is available under the model designation TL-431 from Motorola. 
   A series circuit consisting of a Zener diode  33  and a limiting resistor  34  leads from the node between the current sensor resistor  12  and the protective resistor  8  to the other series branch  3 . In parallel with the series circuit to the Zener diode  33  and the current sensor resistor  12 , there is an ohmic voltage divider consisting of two resistors  35  and  36 . In parallel with the resistor  35 , there is the control path of an integrated circuit  37  with higher amplification and a sharp turn in the characteristic curve. Its control input  38  is attached to the node between the resistor  35  and the resistor  36 , while the common low point  39  is connected to the resistor  34 . 
   A third connection  41  is connected via the series circuit of two resistors  42  and  43  to the series branch  2  between the fuse  11  and the current sensor resistor  12 . The integrated circuit  37  behaves like a threshold switch between the two connections  39  and  41 . Below a predetermined threshold, the path between the connections  41  and  39  has a high resistance. No current flows through the resistors  42  and  43 . 
   For power amplification, a bipolar transistor  44  is used, whose emitter lies on the series branch  2 , and between the fuse  11  and the current measurement device  12 . The base is connected to the node between the resistors  42  and  43 , while the collector lies on the trigger electrode of the thyristor  21  via a decoupling and protective resistor  45 . 
   The circuit arrangement from  FIG. 3  operates in the following way: 
   The operating voltage for the integrated circuit  37 , which exhibits the characteristics of a threshold switch, is generated with the help of the Zener diode  33 , to which the voltage drop across the current sensor resistor  12  is added. 
   The voltage drop across the current sensor resistor  12  changes depending on the current to the load, which is considered attached to the output points  5  and  7 . As long as the current to the load remains below dangerous values, the sum of the voltage drop across the current sensor resistor  12  and the Zener diode  33  is too small to generate a voltage, which would be greater than that voltage necessary to change the integrated circuit  37  between the connections  39  and  41  into the conductive state, on the input  38  by means of the voltage divider consisting of the resistors  35  and  36 . Therefore, the voltage component from the resistors  42  and  43  remains without current and the transistor  44  is blocked. 
   If the current through the current sensor resistor  12  rises, the sum from this voltage and the voltage drop across the Zener diode  33  increases correspondingly. When a certain value is reached, the voltage on the resistor  35  consequently also rises over the reference value, after which the integrated circuit  37  becomes conductive. This change-over occurs due to the high inner amplification of the integrated circuit  37  with a sharp bend, so that the current through the resistors  43  and  42  is turned on abruptly correspondingly, with the result that the transistor  44  is also set. Thus it delivers the gate trigger current for the thyristor  21  over its base-emitter path. 
   If the approximately flat characteristic curve of the Zener diode  33  or its temperature path changes, the circuit from  FIG. 4  can also be used as a Zener diode replacement. Here, the integrated circuit ZHT- 431 , which is connected in parallel with two resistors R 1  and R 2 , can be used. R 2  lies between the control input  48  and a ground connection  49 , which simultaneously corresponds to the anode of the modified Zener diode. The second power input  51  is also connected to the control input  48  via the resistor R 1 . The power connection  51  corresponds to the cathode of a Zener diode. 
   The voltage between the two connections  51  and  49  satisfies the following equation:
 
V z =(1+R 1 /R 2 )×Vref
 
   One circuit arrangement has a current or temperature measurement device, which is used to trigger a shunt device. The shunt device generates a short circuit, which leads directly to the burn-through of a fuse. 
   The arrangement is modular and permits several current measurement devices to be operated on a common shunt device. In particular, in the fault case the arrangement prevents a protective resistor from heating up to high surface temperatures due to the current flowing over too long a time.