Device for testing high-voltage transistors. Scheme, description

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Device for testing high-voltage transistors. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Checking semiconductor devices, especially those purchased from the radio market, is not only desirable, but also necessary, since you can buy a part with a manufacturing defect or a fake. The device proposed by the author is made in the form of an attachment to a digital or analog device and allows you to quickly determine the maximum allowable reverse voltage at a semiconductor junction.

It is known that testing semiconductor devices with an analog or digital multimeter does not give a full guarantee of their serviceability due to the low voltage at which the test takes place. It is clear that probes for testing low-voltage transistors (for example, KT315B, KT815A) are not suitable for testing powerful high-voltage devices.

It should be borne in mind that if the reverse collector current of the transistor under test is significantly higher than that indicated in the reference book, then this may not just be a low-quality copy, but a fake (the so-called "grind"), when something else is sold under the guise of one transistor in the same package .

The proposed device is designed to measure the reverse current of bipolar transistors of any structure at a voltage of 50 ... 600 V. They can also check the reverse current of rectifier diodes, thyristors, triacs and determine the operating voltage of gas discharge lamps, high-voltage zener diodes, varistors.

The schematic diagram of the device is shown in the figure.

(click to enlarge)

Alternating voltage 220 V through the power switch SB1 and resistors R1, R2 is supplied to a half-wave rectifier on diodes VD1, VD2 with voltage doubling. If the slider of the resistor R2 is in the upper position according to the diagram, then each of the capacitors C1, C2 is charged to a voltage close to the amplitude value of the mains voltage, i.e. a little more than 300 V. In this case, the adjustable constant voltage on the left terminals of the resistors R8 according to the circuit , R9 can reach 600V.

Resistors R3, R4 are designed to accelerate the discharge of capacitors C1, C2 after the power is turned off. The neon lamp HL1 lights up at a voltage of more than 100 V. A simple voltmeter with a scale calibrated to 6 V is built on resistors R7, R1 and a microammeter RA600. Resistors R8, R9 limit the short-circuit current of the load to 6 mA; while they dissipate power up to 3,6 watts.

The HL2 LED, which indicates an increase in the reverse current of the p-n junction, begins to glow barely noticeably at a current of 100 μA (it is advisable to select an LED for the highest luminous flux at low current). Silicon diode VD3 protects the connected microammeter from overload.

The device uses constant MLT resistors of appropriate power. Variable resistor R2 - SP-1 1 W with a linear characteristic (group A). The increased power of some fixed resistors is due to their operation at high voltage.

Capacitors C1, C2 - oxide, types K50-7, K50-27 or similar for a voltage of at least 350 V. Non-polar K73-17 with a capacity of 2,2 microfarads per 400 V or similar can be used.

Diodes VD1-VD3 can be replaced by any other low-power silicon diodes with a permissible reverse voltage of at least 700 V.

Neon lamp HL1 - any type with sufficient brightness at a current of not more than 250 μA. Power switch SB1 - push-button (non-latching!) with an operating voltage of at least 250 V.

Microammeter PA1 type M4761 with a frame resistance of 1 kOhm from the level indicator of the reel tape recorder "Saturn 202C-2" It can be replaced by any other with a full deflection current of the arrow 50 ... 300 μA, for example, M68501, M4260, M4204. With such a replacement, a significant adjustment in the resistance of resistors R6, R7 may be required.

Structurally, the device can be placed in a plastic case with dimensions of 100x150x30 mm. On the axis of the resistor R2, be sure to install a plastic handle. With frequent use, the device can be equipped with a polarity switch to test transistors of the "npn" and "pnp" structures.

To avoid electric shock during the measurement, the current in the power circuit is limited, and the operator's hands are busy: you need to simultaneously press the power button and adjust the voltage on the load. Therefore, the design provides for fixing the terminals of the semiconductor device in the terminals or clamps of the connectors that are under voltage.

Before testing a semiconductor device for its maximum voltage, you should still check its main parameters with a conventional ohmmeter, filtering out the defective ones already at the first stage of the test.

When checking the thyristor, it is connected to the terminals for the transistor of the np-p structure, and the control electrode is connected to the terminal for the base. The triac is checked with two options for the polarity of the input voltage, leaving the control electrode unconnected. A microammeter of any design is connected to sockets X1, X2; it is especially convenient to use a digital multimeter. An additional voltmeter can be connected to contacts ХЗ, Х4.

The test voltage should be increased gradually, monitoring the increase in the reverse current with a measuring device or the appearance of the LED.

Due to the variety of types of transistor packages, it is difficult to give a single recommendation for their connection to the device; you can just solder the wires to the transistor terminals. Only two basic requirements must be observed: all connections to the device should be carried out with the power off (the device is not isolated from the mains!) and with completely discharged filter capacitors.

Sometimes it may not be enough to measure the reverse current at room temperature, so the transistor or other semiconductor device can be heated with an electric hair dryer.

Test results may lead to more prudent use of semiconductor devices with sufficient voltage margin.

To exclude the galvanic connection of the measuring circuits with the network, it is advisable to connect the device through an isolating transformer

Author: A.Butov, village of Kurba, Yaroslavl region

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