Abstract:
A current loop input overload protection system for a controller having multiple inputs includes zener diodes that limit the maximum voltage that can be applied to any input. If an electrical short of a temperature, pressure or humidity transducer forces an input to the maximum voltage, an electrical supply monitor immediately disables the transducer&#39;s power supply to protect the input components. After awhile, the overload protection system automatically and periodically attempts to reset the power supply to determine whether the short has been corrected.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The subject invention generally pertains to HVAC controls that receive feedback signals from several thermodynamic sensors and more specifically to protecting the control&#39;s current loop inputs that receive sensors&#39; feedback. 
     2. Description of Related Art 
     HVAC equipment (heating, ventilation and air conditioning equipment), such as heaters, air conditioners, heat pumps, blowers, humidifiers, dehumidifiers and VAV valves (variable air volume valves) typically condition the environment of a room or area within a building in response to sensors or transducers that sense a thermodynamic condition (e.g., temperature, pressure, humidity, etc.) of the area&#39;s environment. With generally large areas, as is the case with many commercial buildings, the area is often divided into separately conditioned comfort zones with each zone having its own sensor (The terms, “sensor” and “transducer” are being considered as equivalent and interchangeable). 
     Since control wiring is often low voltage, the sensors are often serviced while the control and the remainder of the HVAC system is still operating. The supply voltage is usually around 24 volts, and feedback signals from conventional sensors are usually no more than 5 volts or 4 to 20 mA. Leaving the control power on while servicing a sensor not only benefits the occupants of the area being conditioned but is also more convenient for an electrician doing the servicing. Unfortunately, such a practice can create a current overload problem if the electrician inadvertently shorts the sensor and briefly misapplies a full 24 volts across the control&#39;s input terminals. 
     Of course, conventional current limiting practices can be used to address this problem. For example, the control circuit could simply include a conventional fuse or circuit breaker. However, it is difficult to properly size a fuse for a generic, universal control for an indeterminate number of sensors that will be field-connected to the control. More specifically, a common fuse would need to handle the sum of the current delivered to all the sensors. Consequently, each control input would need to have more current carrying capacity than the fuse in order for the fuse to protect an individual input. Moreover, it would be a nuisance to have a brief inadvertent short cause an entire HVAC system to shutdown until manually reset. 
     Another approach is to simply size the control&#39;s input components to handle the excess current from a shorted sensor. But such an approach obviously adds cost and bulk to the control. The problem becomes worse when compact surface-mount input components are used in an attempt to reduce cost and bulk, as one loses the heat sink benefit previously provided by the electrical leads of components having such leads. 
     SUMMARY OF THE INVENTION 
     To protect the input components of an HAVC control system, it is an object of the invention is to limit the maximum voltage and current that can be applied to such input components and further disable the electrical power source should the voltage and current limits be reached. 
     Another object of the invention is to provide an input overload protection system that automatically and periodically attempts to reset itself after responding to an overload fault. 
     Another object is to provide an input overload protection system that can be used with an indeterminate number of sensors. 
     A further object is to provide an input overload protection system that is compatible with a variety of thermodynamic transducers including those that sense temperature, pressure and humidity. 
     A still further object of the invention is to provide an input overload protection system that allows the use of input components that are smaller and less expensive. 
     Yet another object is to provide an input overload protection system that allows the use of input components that have lower current carrying capacity. 
     Another object is to provide an input overload protection system that allows the use of input components that can be surface mounted rather than having to rely upon the heatsinking property of through-hole soldered leads. 
     These and other objects of the invention are provided by an input overload protection system that includes several inputs. The voltage that a current feedback signal can apply to the inputs is limited to a predetermined maximum voltage by a voltage limiter at each input. An electrical supply monitor disables a power source upon detecting that the predetermined maximum voltage has been reached. 
    
    
     BRIEF DESCRIPTIONS OF THE DRAWINGS 
     FIG. 1 is an electrical schematic encompassing at least one embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An HVAC system of FIG. 1 include several thermodynamic sensors  10  that affect the operation of HVAC equipment  12 , which in turn condition the air of a comfort zone  14 , such as a room or area within a building. Sensors  10  are schematically illustrated to represent a variety of sensors, examples of which include, but are not limited to, a temperature sensor or thermostat, a pressure sensor, and a humidity sensor. Equipment  12  is also schematically illustrated to represent a variety of equipment used to condition air. Some examples of equipment  12  include, but are not limited to, refrigeration systems, refrigeration compressors, heat pumps, furnaces, blowers and fans, humidifiers, dehumidifiers, baffles and dampers, rotary heat exchanger wheels, and VAV valves (i.e., variable air volume valves that adjustably throttle airflow through an air duct). 
     Sensors  10  affects the operation of equipment  12  by way of a control system that includes an electrical power source  16 , an electrical supply monitor  18 , and a control circuit comprising a logic circuit  20 , an input circuit  22 , and other ancillary components such as an A/D converter  24 . 
     Power source  16  can assume a wide variety of configurations. In one embodiment, for example, power source  16  receives a generally unregulated 28 to 42VDC at a terminal  26 . The voltage is applied to a collector  30  of a transistor  32  (e.g., an MJD112 by Motorola Semiconductor; of Austin, Texas) whose voltage at base  34  is partially determined by a zener diode  36  (e.g., a Motorola MMBZ5253B). Zener diode  36  is supplied by the unregulated voltage through a resistor 38 (e.g., 22 k ohms) and drains excess voltage (e.g., above 25VDC) to a ground  40 . The voltage at base  34  is modulated by another transistor  42  (e.g., a Motorola MMBT2222A) that includes a collector  44  connected to base  34 , an emitter  46  connected to an output terminal  48 , and a base  50  controlled by an emitter  52  of transistor  32 . Transistor  32 , in conjunction with zener diode  36 , provides a substantially constant voltage of approximately 24 VDC at emitter  52 . Transistor  42  limits the voltage drop across a resistor  54  to no more than 0.6 volts, thereby limiting the output current at terminal  48 . For example, when resistor  54  is 7.5 ohms, the current through output terminal  48  is generally limited to 80 mA. In another embodiment, resistor  54  is 5 ohms to provide a total of 120 mA to serve five transducers (each consuming up to 20 mA, with an extra 20 mA to spare). 
     Through output terminal  48 , power source  16  provides an electrical supply  56  of limited current to sensors  10 . Each sensor  10  provides a feedback signal  58  that varies between and a lower and an upper limit (e.g., 0 to 5VDC or 4 to 20 mA) in response to a thermodynamic condition associated with comfort zone  14 . In one embodiment, for example, sensor  10  is a model MRH-3-OA humidity sensor (by General Eastern of Woburn, Mass.) that provides a current feedback signal of 4 mA for 0% humidity and 20 mA for 100% humidity. 
     Feedback signals  58  are each conveyed to an input  60  of input circuit  22 . Current from each feedback signal  58  passes through a resistor  62  (e.g., 200 ohms) tied to ground  40  to create an analog voltage signal  64  proportional to the feedback current. A zener diode  66  (e.g., a Motorola 1SMB5920BT3) connected in parallel with each resistor  62  serves as a voltage limiter that limits voltage signal  58  to a predetermined maximum voltage of, for example, 6.2 volts. 
     A/D converter  24  converts analog signals  64  to a digital signal  68  that is conveyed to logic circuit  20 . Although logic circuit  20  is schematically illustrated to represent countless equivalent circuits using microprocessors, programmable logic controllers, integrated circuits, discrete components, and combinations thereof; in one embodiment, circuit  20  includes an AT90S1200SC microprocessor by Atmel Semiconductor of San Jose, California. In another embodiment, circuit  20  includes an Atmel AT90S2313SC microprocessor. In response to signals  68 , logic circuit  20  provides control output signals  70  that control the operation of HVAC equipment  12 . The specific algorithm or control scheme of circuit  20  depends on the particular equipment being controlled and its purpose. Such algorithms and control schemes vary widely and are well known to those skilled in the art (e.g., increase cooling in response to a sensed air temperature exceeding an upper limit). 
     To protect zener diodes  66  and resistors  62  from a current overload, electrical supply monitor  18  disables power source  16  when feedback signals  58  are inadvertently shorted directly to electrical supply  56 . When such a short occurs, the affected zener diode  66  will clamp electrical supply  56  to a predetermined maximum voltage (e.g., determined by the characteristics of the zener). In the case of the 1SMB5920BT3 zener, that voltage is approximately 6.2 volts. Output terminal  48  applies the 6.2 volts across a voltage divider comprising resistors  72  and  74  (44 k and 4.7 k ohms respectively) to create, in this example, 0.7 volts at an input  76  of logic circuit  20 . Circuit  20  interprets the 0.7 volt signal as a logic-0, and in response, outputs a logic-1 at an output  78 . A logic-1 at output  78  is conveyed through a resistor  80  to a base  82  of a transistor  84  (e.g., a Motorola MMBT2222A). This turns transistor  84  on to basically clamp base  34  and collector  44  to ground  40 , thereby interrupting electrical supply  56  by lowering its voltage and current to zero. This, in turn, discontinues the current being conveyed to input circuit  22 . 
     After a predetermined time delay (e.g., provided by logic circuit  20 ), circuit  20  provides a logic-0 at output  78  to restore the normal operation of power source  16  in an attempt to automatically reset the control system. However, if the short still exists, the return of a logic-1 at input  76  will again disable power source  16 . If desired, logic circuit  20  can be configured to automatically and periodically check whether the short has been corrected. In one embodiment, a transient voltage suppresser  86  (e.g., a Motorola BAV99) ensures that the voltage at input  76  stays within the limits of zero to 5VDC as determined by ground  40  and the 5VDC applied at a terminal  88 . 
     Although the invention is described with respect to a preferred embodiment, various modifications thereto will be apparent to those skilled in the art. For example the specific circuit components, their specifications and arrangement can vary dramatically and still provide an embodiment that remains within the spirit of the invention. The various circuits, such as power source  16 , supply monitor  18 , logic circuit  20  and input circuit  22  can also be created using a variety of other integrated circuits, discrete electrical components, and combinations thereof to achieve the same general purpose disclosed herein. Therefore, the scope of the invention is to be determined by reference to the claims, which follow.