Transistor withstand voltage measuring instrument circuit a

In order to ensure that the transistor can work normally in the circuit, it is often necessary to measure the reverse breakdown voltage of the transistor. Especially for
tubes that require a relatively high reverse breakdown voltage, such as the tubes of the differential amplifier in the OCL power amplifier, the reverse breakdown voltage is required to be greater than tens of volts. It can be tested with a transistor
withstand voltage test circuit. The TV amplifier output and line output tubes are required to be greater than one hundred and dozens of volts, so they need to be measured. Figure 5-23a
is a simple transistor withstand voltage test circuit. It can measure the X-axis breakdown voltage of crystal diodes and transistors, and can also test
the voltage stabilizing characteristics of crystal regulators.
    Circuit description: The reverse current of the diode plus the reverse voltage is very small. But when the reverse voltage increases to a certain value, the reverse current
increases sharply and the tube enters a reverse breakdown state, as shown in Figure 5-23b. After the tube enters the breakdown state, the current flowing through the tube mainly depends
on the external circuit. If the external circuit has measures to limit the current, the current will be very large and the tube will be in danger of burning out.
    The reverse breakdown voltage of a transistor reflects the breakdown characteristics. It refers to the reverse voltage when the reverse current reaches a certain specified value. The specified value of this
reverse current can be found from the transistor characteristics table test conditions.
    In the circuit in Figure 5-23, the transformer converts the 220V AC voltage into a 250V AC voltage. After diode rectification and capacitor filtering,
a 300V DC high voltage is obtained. The voltage is lower when there is a load.
    The voltmeter is used to measure the reverse breakdown voltage of the diode or transistor, and the ammeter is used to monitor the reverse current of the diode or transistor.
    (1) Measure the BVceo of high-power tubes. When the single-pole double-throw switch S is set to "high power",
the reverse breakdown voltage of high-power diodes or transistors can be measured. For example, to measure the collector-emitter reverse breakdown voltage BVceo of 3AD30, set the ammeter to the 50mA range, and the circuit
is as shown in Figure 5-22c. R1 and RP1 form a current limiting resistor. It can be found from the transistor characteristics table '. The BVceo test condition of 3AD30 is reverse
current 20mA. Adjust RP1 so that the ammeter pointer falls at 20mA. At this time, the reading of the voltmeter is the collector-emitter
reverse breakdown voltage of the tube. If you measure the reverse breakdown voltage BVceo. A specified resistor is connected between the base and the emitter. For example, to measure the BVceo of 3AD30
, you need to connect a resistor of about 20fz.
    (2) Measure the BVceo of low-power tubes. When the single-pole double-throw switch S is set to "low power",
the reverse breakdown voltage of the low-power diode or transistor can be measured. For example, to measure the reverse breakdown voltage of the collector-emitter of 3DG6, the ammeter in the circuit should be set to 500riA . The circuit
is shown in Figure 5-23d. Ruler 2 and RP2 form a current-limiting resistor, and the voltmeter should use a high internal resistance voltmeter. It can be found from the transistor characteristics table that
the BVceo test condition of 3DG6 is a reverse current of 100yA. Adjust RP2 so that the ammeter pointer falls at 100VA. At this time, the reading of the high internal resistance voltmeter
is the collector-emitter reverse breakdown voltage of this tube.
    (3) Measure the voltage stabilizing characteristics of the voltage regulator tube. When measuring the voltage regulator tube, S is also set to "high power". In addition to reading the voltage stabilization value of the voltage regulator tube from the voltmeter
, by adjusting RP1, the voltage stabilization characteristics of the voltage regulator tube from a few milliamps to 20mA can also be measured.
    (4) Commercially available power transformers can be used for component selection transformers. The rectifier diode can be
a 2CP tube with a withstand voltage greater than 300V and a forward current greater than 100mA.
    When measuring high-power tubes, the voltage range of a simple multimeter can be used as the voltmeter. However, when measuring low-power tubes,
you must use a high internal resistance voltmeter, such as a 20kfl per volt voltmeter or a simple transistor voltmeter, otherwise it will not be measured.