Abstract:
A digital programmable load measurement device provides a controllable and variable load unit in a system. The variable load unit is connected to a voltage follower and a current follower to measure and figure out dynamic load voltage and load current of a device under test. Selected loads can be switched in a short period to measure the voltage and current values thereof, sampled for saving, and an I-V curve of the system can be depicted.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a digital programmable load (DPL in short hereinafter) measurement device and particularly to a DPL measurement device adopted for use on a power system to rapidly measure and figure out voltage and current parameters. 
     BACKGROUND OF THE INVENTION 
     With advanced development and wide utilization of power energy resources, assessing good or bad condition of a power system becomes increasingly important. For instance, parameters such as voltage and current are the most important reference for solar cells, zinc-air batteries and the like. By measuring variations of the voltage and current of the power system in different loads, an I-V curve can be obtained and served as an important reference to observe energy consumption and element characteristics of the power system. 
     In the past, measuring current-voltage characteristics of the power system mainly uses resistor as a load. By changing resistance the voltage-current characteristics of the power system in different loads can be obtained. However, to do measurement by changing different resistors takes a lot of time. Moreover, variable resistor generally cannot withstand temperature effect caused by great current. This limits measurable power system specifications. Moreover, the number of resistors increases rapidly with accuracy demand but becomes difficult to realize on smaller volume of loads. 
     Since variable resistor load is difficult to implement in practice, some conventional techniques try to get variable loads by incorporating resistors with analog circuits, or employing capacitor charge and discharge approach. However, the two approaches mentioned above need a basic duty frequency which restricts the degree of sampling frequency taken by users. This is because the sampling frequency must be much lower than the basic duty frequency of a simulated load so as to ignore impact of modulation of the basic duty frequency. 
     U.S. Pat. No. 4,456,880 entitled “I-V Curve Tracer Employing Parametric Sampling” employs a switched-capacitor resistor to do charging and discharging, and digital sampling of output voltage and current. However, using the switched-capacitor generates a basic duty frequency in the system. U.S. Pat. No. 5,512,831 entitled “Method and Apparatus for Testing Electrochemical Energy” employs a parallel field effect transistor (FET) as a load, and through a digital feedback approach to control output current of a measurement system. The digital feedback frequency is the basic duty frequency of the system. 
     In order to implement a real variable resistor load and overcome the constraint of the basic duty frequency, the present invention employs an R-2R resistor network to realize the variable resistor load and an operational amplifier incorporating with a power transistor to perform analog feedback control. As the analog feedback control does not need sampling, there is no basic duty frequency and the related bandwidth limitation, hence a greater stable range can be achieved. 
     SUMMARY OF THE INVENTION 
     Therefore, the primary object of the present invention is to provide a DPL measurement device that provides a variable load switching to various desired load values in a short period to measure individual voltage and current signals and prevent energy loss and temperature effect of a device under test. 
     Another object of the invention is to control the measurement device through a digitized approach with digital input to control load switching of entire measurement device and scan to obtain an I-V curve. 
     To achieve the foregoing objects, the present invention provides a digital programmable load (DPL) measurement device which includes an R-2R ladder network, a voltage follower and a current follower. The R-2R ladder network is electrically connected to the voltage follower and current follower. The R-2R ladder network can generate a variable and adjustable load. The voltage follower and current follower form a loop with the variable and adjustable load and a device under test to measure voltage and current values, and also avert great current to protect the measurement device. Thus the invention can separate the device under test in a power system and the measurement device. In the event that a great current is output from the device under test the current in the measurement device can be adjusted lower to avoid damaging elements caused by high temperature resulting from the excessive current, and the problem of thermal effect can be reduced. 
     The foregoing, as well as additional objects, features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying embodiments and drawings. The embodiments serve merely for illustrative purpose and are not the limitation of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an embodiment of the DPL measurement device of the invention; 
         FIG. 2  is a circuit diagram of an embodiment of the R-2R ladder network of the invention; 
         FIG. 3A  is a circuit diagram of an embodiment of the voltage follower of the invention; 
         FIG. 3B  is a circuit diagram of an embodiment of the current follower of the invention; 
         FIG. 4  is a circuit diagram of an embodiment of the common-cathode DPL measurement device according to the invention; 
         FIG. 5  is a circuit diagram of an embodiment of the common-anode DPL measurement device according to the invention; 
         FIG. 6  is a schematic view of a measurement system of an embodiment of the invention in a use condition; and 
         FIG. 7  is a chart showing an I-V curve of a device under test. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Please refer to  FIG. 1 , the DPL measurement device  1  according to the invention comprises a variable load unit  10 , a voltage follower  11  and a current follower  12 . A device under test (DUT in short hereinafter)  13  is externally connected to the variable load unit  10 . By inputing a control signal V CTRL  to the variable load unit  10 , the load resistance of the DUT  13  can be adjusted. The voltage follower  11  is electrically connected to the variable load unit  10 . The voltage follower  11  measures a first signal V 1  of output voltage under the load resistance without increasing additional load. The current follower  12  also is electrically connected to the variable load unit  10 , and a second signal V 2  can be measured without increasing additional load to figure out the corresponding output current of the DUT  13 . 
     Referring to  FIG. 2 , the variable load unit  10  includes an R-2R ladder network  20  which comprises a plurality of resistors R and  2 R and a plurality of switches  21 . The resistors include N−1 first resistors R and N+1 second resistors  2 R. The second resistor  2 R has resistance twice the first resistor R, hence the first resistor is denoted as R and the second resistor is denoted as  2 R. Each first resistor R has two ends electrically connected to two second resistors  2 R (total number is N). The other ends of the N second resistor  2 R are electrically connected to one of the switches  21 . As the total number of the second resistors  2 R is N+1, the last second resistor  2 R is connected to the first resistors R in series and also connected to a measured output signal V M . 
     The R-2R ladder network  20  operates according to the principle as follow: the DUT  13  has one end receiving a first reference signal V REF1  and the other end generating a distal end signal V P . The external control signal V CTRL  (as shown in  FIG. 1 ) controls every switch  21  to selectively whether connect to the distal end signal V P . Thereby the load resistance at the DUT  13  can be changed and a load signal V L  can be determined. The load signal V L  is the difference of the distal end signal V P  and the first reference signal V REF1 , i.e. V L =(V P −V REF1 ). 
     Based on the switch mode of the switch  21 , the load resistance at the DUT  13  can be determined. Assumed that the load resistance is R O  and total current value flowing into the DUT  13  is I, according to current division principle, the following equations can be derived: 
                   I   =         (       V   P     -     V     REF   ⁢           ⁢   1         )     /     R   O       =       V   L     /   Ro               (   1   )                       I   Di     =       ⁢     I   ×     n   /     2   N                     =       ⁢       (     I   -     I   Di       )     ×     n   /     (       2   N     -   n     )       ⁢           ⁢     (   3   )                     (   2   )               
wherein n=2 0 b 0 +2 1 b 1 +2 2 b 2 + . . . +2 N-1 b N-1 ,
 
b i  is 1 representing the ith switch connected to the distal end signal V P ; otherwise it is 0.
 
     On equation (2), I Di  is the load current value flowing from the distal end signal V P  of the DUT  13  through ith and i+1 second resistors  2 R. I subtracts I Di  represents I-I Di  which is the complementary current of current I Di . According to equation (1), the direction of current I flowing through the DUT  13  is determined by the relative voltage of the first reference signal V REF1  and the distal end signal V P . Switching of the switch  21  determines the load resistance of the DUT  13 . According to the equations set forth above, if all the switches  21  are connected to the distal end signal V P , then the current flowing through the 0th second resistor  2 R (most close to the DUT  13 ) is I/2, the current flowing through the 1st second resistor  2 R is I/4, and so forth. The current at the last two sets of the second resistor  2 R (most far from the DUT  13 ) is I/2 N  as denoted on every second resistor  2 R in  FIG. 2 . 
     Referring to  FIGS. 3A and 1 , the voltage follower  11  of the DPL measurement device  1  includes a voltage amplifier  30  which has a non-inverted input end to receive the distal end signal V P  from the variable load unit  10 , and an inverted input end to receive feedback of the first signal V 1  output from the voltage amplifier  30 . By measuring the first signal V 1  output from the voltage amplifier  30 , the value of the distal end signal V P  of the variable load unit  10  can be obtained to further derive the load signal V L . 
     Referring to  FIGS. 3B and 1 , the current follower  12  includes a current amplifier  31  and a current-limiting resistor R 31 . The current amplifier  31  has a non-inverted input end to receive the distal end signal V P  from the variable load unit  10 , and an inverted input end to get feedback of the second signal V 2  from the output end of the current follower  12  and then connect with the current-limiting resistor R 31  in series to input. According to Ohm&#39;s law, through the voltage difference at two ends of the current-limiting resistor R 31 , the current value flowing through the current-limiting resistor R 31  can be figured out, and also the current value is equivalent to the complementary current I-I Di  mentioned above. As shown in  FIG. 3 , the voltages at two ends of the current value resistor R 31  are the second signal V 2  and measured output signal V M . As the load signal V L  has already been obtained from the aforesaid voltage follower  11 , the value of the second signal V 2  can be measured in the current follower  12  to figure out the load current I Di . 
     Moreover, in order to prevent excessive current from flowing into the current follower  12  to cause damage, a protection element  32  may be included and electrically connected to the output end of the current follower  12 , and a second reference signal V REF2  is input to the protection element  32 . 
     Refer to  FIG. 4  for an embodiment of the DPL measurement device of the invention. The first reference signal V REF1  of the variable load unit  10  is connected to a low voltage level, such as ground, and the second reference signal V REF2  of the current follower  12  is connected to a high voltage level Vcc, thus forms a common-cathode DPL measurement device  4 . In the current follower  12 , the protection element  32  may be an NPN bipolar transistor  321  with a base electrically connected to the output end of the current amplifier  31  and with a collector connected to the second reference signal V REF2  at the high voltage level Vcc mentioned above. 
     The DPL measurement device  4  thus formed has a common-cathode circuit to measure the load voltage and load current of the DUT  13 . Adopted the same principle, a common-anode circuit can also be adopted to meet different requirements. 
     Referring to  FIG. 5 , the first reference signal V REF1  of the variable load unit  10  can be connected to a high voltage level Vcc and the second reference signal V REF2  of the current follower  12  can be connected to a low voltage level such as ground to form a common-anode DPL measurement device  5 . In the current follower  12 , the protection element  32  may be a PNP bipolar transistor  322  with a base electrically connected to the output end of the current amplifier  31  and with an emitter electrically connected to the second reference signal V REF2 , i.e. ground. 
     Refer to  FIG. 6  for an embodiment of the measurement system of the invention. It comprises a micro-controller  60 , an I/O unit  61 , the DPL measurement device  1  and an analog to digital converter (A/D in short)  62 . A computer  63  is provided to transmit signals to the micro-controller  60  through a communication interface  64 . The micro-controller  60  receives the signals and outputs an external control signal V CTRL  through the I/O unit  61  so that the DPL measurement device  1  can selectively provide a variable load or a constant load to the DUT  13 . Because of the load, the DUT  13  outputs an analog signal back to the DPL measurement device  1 . The analog signal is converted by the A/D converter  62  to become a digital signal sent back to the micro-controller  60  which in turn sends back to the computer  63  via the communication interface  64 . 
     After the computer  63  has received the digital signal, saving as records and denoting the measured voltage and current signals on the coordinates in an I-V curve at measured points as indicated by black dots in  FIG. 7 . Linking the measured points, an I-V curve of the element can be depicted as shown in  FIG. 7 , which represents linear constant load lines by fine black straights. 
     As a conclusion, the present invention can select a variable load or a constant load, and can automatically measure and depict an I-V curve of an element in a short period without adjusting each variable load individually, and can also record measurement data. The DUT  13  mentioned in the invention may be a general electronic element such as a solar panel, transistor, diode and the like. 
     While the invention has been described by means of specific embodiments, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope and spirit of the invention set forth in the claims.