Using NI Data acquisition card to achieve simple electronic test 3--drawing diode v-i characteristic curve

This article from my 163 blog moved to this.

Next, we use USB-6009 and LabVIEW to test and draw the "v-i characteristic curve" of the most important characteristic curve of the diode.

First, what is the diode v-i characteristic curve

Kang Hua optical version of the "Electronic Technology Fundamentals-Simulation Part" This introduces the v-i characteristics of the diode: The starting part of the diode forward characteristic (the characteristic of n positive p negative voltage at both ends of the PN Junction), because the forward voltage is small, the external electric field is not enough to overcome the internal electric field of the PN junction, so the forward current is almost The diode presents a large resistor, as if it were a threshold. Silicon Tube Gate Voltage Vth (also known as the Dead Zone voltage) is about 0.5V, germanium tube Vth about 0.1V, when the forward voltage is greater than Vth, the internal electric field greatly weakened, and thus the rapid growth of the current. The so-called "v-i characteristic curve" is the relation between the forward voltage and the forward current.

Second , the test circuit

The v-i characteristic curve is the relationship between the diode current I and the voltage drop V, with only one independent variable and one dependent variable, which is the curve on the evaporating plane. The test circuit is required to control the current flowing through the diode through the USB-6009 DAC and to draw the v-i curve by measuring the voltage drop on the ADC. As the output characteristic curve of the transistor is plotted, the forward current flowing through the diode can be controlled by the feedback control circuit composed of the op-amp, but when testing the v-i curve of the ordinary diode, the test current is generally required to reach more than 100mA, which is not directly produced by the OP amp, which needs to be borrowed from the transistor amplification output current. I have designed the test circuit shown in Figure 1.

Figure 1 Diode v-i characteristic test circuit

where Di_ctl is the current control voltage output by the USB-6009 DAC. The d_test is a test diode that uses a common rectifier diode, 1n4007, whose ends are connected to AI3 and AI7 are a pair of differential inputs to the USB-6009 ADC for testing d_test VDT at both ends. The RD1 is a current-test resistor that converts the current ID flowing through the tested diode into a voltage VD1, resulting in a negative current feedback path. To prevent damage caused by excessive power dissipation on the RD1, the RD1 value is small and only 0.1 ohms. Under 500mA current conditions, the pressure drop on it is only 50mV, it is difficult to compare with the control voltage of the USB-6009 DAC output, so it is necessary to op_da the test voltage VD1 on the Rd1 into VD2 by the co-amplifier circuit composed of op amp. Its gain g= (RD3+RD2) is/rd2=31.3 times, which converts the 500mA current flowing through Rd1 to 1.56V output voltage. The op amp op_db constitutes the error amplification of the entire negative feedback circuit, which amplifies the error between the actual current and the control current voltage of the USB-6009 DAC output, and uses the amplified error to control the output current of the power P-Channel MOS tube MOSP. According to the analysis method of negative feedback system, the working principle of Figure 1 is analyzed.

1, to the OP amp op_db application "virtual short" principle: Op_da output VD2 should be equal to the USB-6009 output control voltage Di_ctl, when the change di_ctl, you can change the current ID flow through d_test. The relationship between the current ID and the voltage Di_ctl is:

ID=vdi_ctl/rd1 (1)

2, one of the premise of the above circuit using virtual short is that the whole system is the current negative feedback system: but I D Increase when V D1 and V D2 will also increase, which will increase the op_db voltage at the same phase end, thereby increasing the gate voltage of the P-channel MOS tube and reducing the degree of its opening so that I D and ultimately constitute a negative feedback system.

3. The above-mentioned circuit uses the virtual short premise second is, the entire system stability. CD1 has the function of guaranteeing closed-loop stability, it can reduce the open-loop gain of high-frequency signal, so the whole system has self-excited oscillation in high frequency segment.

In addition, this circuit is worth noting is that this is a single-supply circuit, but the reference point of the circuit is the "ground potential." Specifically reflected in:

1, from the upper end of the Rd1 VD1 is a very small voltage, may have only a few MV, to use only a single-supply op amp pick this voltage is difficult, but the LM358 common-mode voltage can reach the negative supply voltage, so can pick up and enlarge VD1.

2, because the current test resistor Rd1 installed between the measured diode and the ground, it is not possible to use a single-ended measurement of the voltage drop on the diode, just USB-6009 provides a differential measurement, the two differential input terminals are connected to the two ends of the object being measured, the common-mode voltage between the two inputs can be suppressed, Thus the potential difference of the two ends of the object is directly obtained.

Third, the test procedure

1, the test program Flowchart 2 is shown

Figure 2 Test program flowchart

2. Test current I D the Settings

DACDI_CTL Implementation of the test current I D uses the formula node to achieve the conversion between Di_ctl and ID, 3 shows:

Figure 3 The Formula node that implements the test current ID and control voltage conversion

3, differential mode acquisition diode voltage drop VDT USB-6009 Differential Mode configuration 4, note that at this time the input voltage range can be modified to improve the PGA in the USB-6009 gain, so as to achieve higher voltage resolution.

Fig. 4 program block diagram of differential mode acquisition voltage

4. Test Process ControlIt is not possible to constantly repeat the measurement process after the start of the program, so the "event structure" is used to manage a "start test" button, and the test process will begin only if you click "Start Test". 5. Program Block DiagramFigure 5 and Figure 6 Show the program block diagram of the USB-6009 control test current and read diode pressure drop.

Figure 5 Setting the test current program block diagram

Figure 6 Read Diode pressure drop program block diagram

iv. Results of program Operationtest the 1n4007 "diode v-i characteristic line" of the SMD package and get the result shown in Figure 7.

Figure 7 Actual test results of the Diode VI characteristic curve

It can be seen that the v-i characteristic curve of IN4007 is in the form of exponential curve, which is consistent with the theoretical derivation. To be continued ...

Using NI Data acquisition card to achieve simple electronic test 3--drawing diode v-i characteristic curve