Abstract:
A multi-port power monitor is provided which indicates whether power is being supplied to one or more of a plurality of ports connected to external devices. The multi-port power monitor comprises a plurality of serially connected indicators and a plurality of controllable switches which supply power to the individual indicators when one or more of the corresponding ports are active. Because the indicators are connected in series, rather than in parallel, the same current passes through each illuminated indicator thereby avoiding the need for a large power resistor or a large AC capacitor for each individual port

Description:
FIELD OF THE INVENTION 
     The present invention relates to a multi-port power monitor for indicating whether power is being provided to one or more ports through which power is provided to external devices connected to the ports. More particularly, the present invention is directed to a power monitor comprising a plurality of controllable switches for providing power to one or more of a plurality of indicators when power is provided to one or more of a plurality of ports. 
     BACKGROUND OF THE INVENTION 
     Multi-port units are used in machine controls to provide power to controllable devices on an assembly line. In particular, a controller transmits a control signal to provide power through a power port to an external controllable device. Typically, a monitor circuit is connected in parallel with the power port in order to indicate whether power is being properly supplied to the power port to which the external device is connected. As shown in FIG. 1, a conventional monitor circuit  10  for indicating whether power is being provided through a power port (not shown) to an external device (not shown) comprises a light emitting diode (LED)  1 , a diode  2 , an alternating current (AC) power source  3 , a capacitor  4  and a resistor  5 . However, the capacitor occupies a large amount of valuable space. Further, in the case of multiple ports, a separate monitor circuit is required for each port with the respective LEDs indicating the power supply status for each port connected in parallel. As a result, a large aggregate current is required in order to simultaneously illuminate the LEDs. For example, in the case of eight ports with eight corresponding monitor circuits, resulting a total of eight LEDs each requiring 5 mA for illumination, a total current of 40 mA is required to simultaneously illuminate an eight LEDs. 
     Accordingly, it is an object of the present invention to provide a multi-port monitor with reduced power and space requirements. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a multi-port power monitor is provided which indicates whether power is being supplied to one or more of a plurality of ports connected to external devices. The multi-port power monitor comprises a plurality of serially connected indicators and a plurality of controllable switches which supply power to the individual indicators when one or more of the corresponding ports are active. Because the indicators are connected in series, rather than in parallel, the same current passes through each illuminated indicator thereby avoiding the need for a large power resistor or a large AC capacitor for each individual port. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention are described in detail below with reference to the attached drawing figures, in which: 
     FIG. 1 is a circuit diagram illustrating a conventional power monitor circuit; 
     FIG. 2 is a circuit diagram illustrating a power monitor circuit in accordance with a first embodiment of the present invention; and 
     FIG. 3 is a circuit diagram illustrating a power monitor circuit in accordance with a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 2, the monitor circuit of a preferred embodiment of the present invention will be described for a four port assembly. The monitor circuit comprises AC input compatible photocouplers (optical couplers)  31 - 34 , LEDs  41 - 44  which indicate whether ports  11 - 14  are active, an LED  70  which indicates whether the monitor circuit as a whole is operating, zener diodes  51 - 58 , a rectifier bridge  60 , and resistors (resistive elements)  21 - 24  and  91 - 94  for limiting power. Ports  11 - 14  are receptive to a three-wire plug or jack assembly (not shown) through which power is supplied to an external device. A twelve pin connector  100  provides AC power (120 volts) to the monitor circuit and the respective ports  11 - 14 . That is, pins 1-4 of the connector  100  provide A/C power to the respective ports  11 - 14  while pin 11 of the connector provides AC power to the monitor circuit. Further, each of the ports  11 - 14  comprises a first terminal which receives a common ground from pin 9 of the connector  100 , a second terminal which receives AC power from one of pins 1-4 of the connector, and a third terminal which connected to pin 11 of the terminal which provides common AC return for the ports  11 - 14 . The second terminals of the ports  11 - 14  are respectively connected to the photocouplers  31 - 34  through the resistors  21 - 24 . The zener diodes  51 - 58  and the LED  70  are connected in series between the positive and negative output terminals of the rectifying bridge  60 . The LED  70  indicates whether the monitor circuit as a whole is in an “on” or “off” operating state. 
     The AC input compatible photocouplers  31 - 34  incorporate an input circuit comprising two internal LEDs and an output circuit comprising a phototransistor. The two LEDs are connected in reverse parallel so that when the positive and negative polarities of the AC input signal are changed, either of the two LEDs emits light so that a full-wave rectified output signal is generated by the phototransistor. Further, the photocouplers  31 - 34  provide a current amplification or gain which is typically on the order of two or more times the input current. An example of a photocoupler which may be used in the preferred embodiment is a PS2505-1 photocoupler manufactured by NEC Corporation. 
     In operation, the photocouplers  31 - 34  function as controllable switches to illuminate the corresponding LEDs  41 - 44 . In particular, when power is supplied to one or more of the ports  11 - 14  through pins 1-4 of the connector  100 , power is also supplied to the corresponding photocoupler(s) through resistors  21 - 24 . As a result, the photocoupler is turned on so that a current required to illuminate the LED is conducted through the LED and bypassed or diverted from the zener diode connected the LED. For example, when the port  11  is active, the photocoupler  31  is turned on, providing a conductive path through LED  41  which diverts the current necessary to illuminate the LED from the zener diode  51  and illuminates the LED  41 . When power is not supplied to the ports  11 - 14 , current bypasses the corresponding LEDs  41 - 44  and passes through the corresponding zener diodes  51 - 54 . 
     An important feature of the monitor circuit of FIG. 2 resides in the serial connection of the LEDs  41 - 44  through the photocouplers  31 - 34 . The zener diodes  51 - 54  permit the LEDs  41 - 44  to be connected in series by selectively bypassing current flow from the individual LEDs  41 - 44  when power is not supplied to the corresponding ports  11 - 14 . Because the LEDs  41 - 44  are connected in series, rather than in parallel, the current required to simultaneously illuminate two or more of the LEDs  41 - 44  is equal to the current required to illuminate any single one of the LEDs  41 - 44 . Thus, if 5 mA is required to illuminate any one LED, then only 5 mA are required to simultaneously illuminate all four of the LEDs rather than 20 mA. 
     In addition to the series resistors  91 - 94  and resistors  21 - 24  which serve to limit the power supplied to the rectifier bridge  60  and the photocouplers  31 - 34 , respectively, a metal oxide varistor (MOV)  80  is connected to the rectifier bridge  60  to provide power surge protection. Although the four port monitor circuit requires only four zener diodes (i.e., one for each port), each connected to one of the LEDs  41 - 44  and one of the optical couplers  31 - 34  to cause the LEDs  41 - 44  to emit light when power is provided to the ports  11 - 14 , additional zener diodes  5558  are provided for current control purposes. 
     In the preferred embodiment, the rectifier bridge  60  is a full-wave rectifier bridge. However, it should be noted that a half-wave rectifier may be used in place of the full-wave rectifier bridge without affecting operation of the monitor circuit. Similarly, one of ordinary skill in the art will recognize that the AC input compatible photocouplers  31 - 34  may be replaced with DC input compatible or one-direction input photocouplers (i.e., only one LED is used in the input circuit of the photocoupler) and a diode bridge or CR circuit which full- or half-wave rectifies the AC input signal. Further, transformers may be used as controllable switches in place of the photocouplers. 
     FIG. 3 illustrates a second embodiment of the present invention for an eight port assembly wherein similar elements used in the embodiment of FIG. 2 are designated with the same reference numerals. The monitor circuit comprises AC input compatible photocouplers  31 - 38 , LEDs  41 - 48  which indicate whether ports  11 - 18  are active, an LED  70  which indicates whether the monitor circuit as a whole is operating, zener diodes  51 - 58 , a rectifier bridge  60 , and resistors (resistive elements)  21 - 24  and  91 - 94  for limiting power. The monitor circuit of FIG. 3 operates in the same manner as that of FIG. 2, with the exception that zener diodes  55 - 58  are utilized to bypass the additional LEDs  45 - 48  when power is not supplied to ports  15 - 18 . 
     Although the present invention has been shown and described with respect to preferred embodiments, various changes and modifications within the scope of the invention will readily occur to those skilled in the art. For example, although the multi-port monitor of present invention is particularly useful for multiple AC power ports, its application is not limited thereto and it may be used with DC power ports as well.