Multi-speed current test circuit of transistor HFE

The DC gain factor HFE of a transistor is an important parameter that indicates its amplification capability. Selecting the HFE of a transistor according to the working current requirements of the transistor in the circuit is conducive to the full utilization of the transistor and the normal operation of the circuit. The HFE of a low-power transistor can be measured relatively easily, but it is not easy to measure the HFE of a medium- and high-power transistor. The author has made an adjustable voltage-stabilized power supply and a HFE measurement circuit under multiple IC currents, which has successfully solved the HFE measurement problem of large, medium and small power transistors, especially high-power transistors, and provided convenience for the selection of transistors in electronic production. The main features of the circuit are: (1) The IC current is divided into seven gears from 3mA to 3A, and the current span between gears is 1:3 to 3.3, which improves the uniformity of the IC current. (2) When the full-scale HFE of the meter is 300, the power supply voltage is 10V; when the power supply voltage is 5.3V (silicon tube) and 5.1V (germanium tube), the full-scale HFE of the meter is 600; when the power supply voltage is 19.4V (silicon tube) and 19.8V (germanium tube), the full-scale HFE of the meter is 150. (3) By measuring the size of the HFE of the same crystal triode under multiple current levels, the quality of its linearity can be roughly judged. (4) The dial scale of the meter is 30 or 300, which is conducive to FE and current readings. The circuit principle is shown in Figure 1. It mainly consists of three parts: power supply part, HFE measurement circuit and base bias resistor. They are introduced below.

　　1. Stabilized power supply part

1. Make a voltage-stabilized power supply board according to Figure 2 and assemble the components as required.

　　2. The power transformer has a power of 100W, a core cross-section of 28mm×40mm, a primary 220V winding of 990 turns of φ0.47mm~0.51mm enameled wire, and a secondary 30V winding of 143 turns of φ1.2mm enameled wire.

　　3. The three-terminal voltage regulator integrated circuit LM317 and the high-power tube are installed on the heat sink, and the LM317 is installed on the copper foil surface of the printed circuit board.

HFE measurement section

　1. To measure the internal resistance of the meter, you can use a high-precision digital multimeter to measure the internal resistance of the meter, or you can use the shunt method shown in Figure 3 to measure its internal resistance. First, adjust the potentiometer RP to make the meter indicate the full scale, then close the switch S, and turn the variable resistance knob on the resistance box to make the pointer of the meter under test stop at 1/2 of the full scale value. The resistance value of the resistance box at this time is the internal resistance Ri of the meter under test.

　　2 Measure the sensitivity of the meter head and connect it as shown in Figure 4 to measure the meter head sensitivity IM.

　　3. For the convenience of calculation, the closed-circuit tap method is used to divide the current into sections. Here, the meter sensitivity IM=1mA, i.e. 0.001A, the total resistance RM=400Ω, and the current is divided into 7 sections: 3A, 900mA, 300mA, 90mA, 30mA, 9mA, 3mA, respectively represented by I1~I7. ​​The deer circuit composition is shown in Figure 5, and the calculation method is as follows:

　　Because I7=IM+ISIS=I7-IMIN×RM=RS(I7-IM)(1)Rs=IM×RM÷(I7-IM)=200ΩR1(I1-IM)=IM(RM+RS-R1)I1RI=IMRM+IMRS (3)Substitute (1) into (3):

　　I1R1=RS(I7-IM)+IMRSI1R1=RSI7R1=(RS×I7)÷I1=200Ω×0.003A÷3A=0.2ΩR1+R2=RS×I7÷I2=0.6÷0.9=0.67ΩSimilarly, the resistance values ​​of R3, R4, R5, R6, and R7 can be calculated as follows:

　　So R2=0.47Ω R3=1.33Ω R4=4.67Ω R5=13.3Ω R6=46.7Ω R7=Rs-(R1+R2+R3+R4+R5+R6)=133.3Ω 3. Determination of base bias resistance Take silicon tube VBE=0.6V Germanium tube VBE=0.2V When the meter is full scale HFE=3003A, measure silicon tube Rb=(VC-VBE)÷IB=10-0.6÷3/300=940Ω(IB=I B/HFE) The rest of the gears are 3.13k, 9.4k, 31.3k, 94k, 313k, 940k. The Rb of the germanium tube is (VC-VBE) ÷ IB = 10-0.2 ÷ 3/300 = 980Ω. The rest of the gears are 3.26k, 9.8k, 32.6K, 98k, 326k, 980k. There are no special requirements for the components of the voltage-stabilized power supply. You can select them according to the diagram. The HFE measurement part and the ammeter shunt resistor should be wound with manganese copper resistance wire. It is best to use a multi-purpose bridge to measure its resistance value to ensure that the current error of each gear is less than 3%. Each transistor bias resistor should use a 1/4W metal film five-ring precision resistor. The gear selection switch should be a three-pole seven-position rotary switch.

　　The PNP and NPN conversion switches use double-pole double-throw button switches. The silicon and germanium tube conversion uses single-pole double-throw button switches. In actual measurement, small power tubes are measured with small power blocks, and high power tubes are measured with large current blocks. If small power tubes are measured with large current blocks, the transistor will burn out. When high power tubes are measured with small current blocks, the correct HFE value will not be obtained. The circuit sets the ICEO measurement button switch S3. This switch is a normally closed switch. When measuring ICEO, press and hold the switch and the reading is ICEO. Since the ICEO of the germanium tube is very large, its value needs to be subtracted from IC when accurately measuring HFE. To simplify the circuit, when measuring NPN tubes, the meter head is connected in series with the positive end of the power supply, and the displayed current is IC+IB. When measuring PNP tubes, the meter head is connected in series with the emitter, and the displayed current is IE. Because IE=IC+IB, the measurement results of NPN and PNP are consistent. No matter how large the measured HFE value is, the reading can be subtracted by 1.

　　When used as a voltage-stabilized power supply, it can directly display the circuit current. At this time, be sure to pay attention to the appropriate position of the current range switch to prevent the meter from burning out (generally, the maximum current range should be set first).