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

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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A schematic diagram of a device for testing transistors is shown in fig. 1. It contains a symmetrical multivibrator assembled on transistors VT1 and VT2 according to the standard scheme. The multivibrator generates rectangular pulses with a frequency of less than 10 Hz. From the collector of the transistor VT2, the pulses through the capacitor C3 are fed to the voltage divider R5, R6 and to the socket X2 "B", to which the base of the transistor under test is connected. To sockets X1 "K" and X3 "E", respectively, connect the collector and emitter of this transistor.

(click to enlarge)

When checking, the switch SA1 is set to the position corresponding to the structure of the transistor. The diagram shows that switch SA1 is in the position for testing n-p-n transistors. The transistor under test is turned on according to the circuit with a common emitter in the amplifying mode. Its collector circuit includes a current-limiting resistor R7 and indicator LEDs NI, HL2.

When the SB1 button is pressed, power is supplied to the multivibrator and the transistor under test, and the multivibrator pulses entering the base of the transistor under test periodically open and close it. If the transistor is working, then the HL2 LED flashes at a frequency equal to the frequency of the multivibrator. If the HL2 LED is constantly on, then the collector junction is broken at the transistor, if the HL2 LED does not light up at all, then this indicates a break in one of the junctions or a breakdown of the emitter-base junction.

To check the transistors of the pn-p type structure, switch SA1 is switched to the "pn-p" position, while if the transistor is in good condition, the HL1 LED flashes, and if the transistor is faulty, it either shines constantly or does not shine at all.

The device receives power from three elements (AA batteries), but batteries can also be used. The device can also be powered from an external source with a voltage of 3 to 9 V, X4 sockets are designed for this. An external source cannot be connected when the batteries are installed in the device, however, in the case of using batteries, it is permissible to connect a device to the sockets of the X4 connector to charge them. Other low-power devices can also be powered through these sockets.

A multivibrator can be made on almost any low-power transistors, even p-n-p structures, by changing the polarity of the power supply and capacitors C1-C3. In this case, the inscriptions on the switch SA1 and on the LEDs HL1, HL2 also need to be swapped. The switching frequency of the multivibrator can be changed by selecting capacitors C1, C2. The multivibrator may not be symmetrical if you install capacitors of different capacities, while the nature of the blinking of the LEDs changes.

On fig. 2 shows the printed circuit board of the device, which can be made of one-sided foil fiberglass or getinaks. It is quite acceptable to use another insulating material, having made holes in it and connecting the parts, according to the figure, with their leads threaded into the holes, and with additional wires.

(click to enlarge)

Sockets X1-X3 for connecting the transistor under test are half of the panel for microcircuits with 14 pins, the middle contact of which is removed (see Fig. 1), and the pins of the same name are connected to each other. You can also use any other XS1 five-pin connector (fig. 3) that matches the diameter of the holes. It is mounted on a small printed circuit board, fixed from the inside to the front panel of the device. The designs of homemade connectors have been cited more than once in the Radio magazine.

Any button SB1 is suitable, for example, KM1-1, P2K without fixing, etc., the SA1 switch is also any (MTZ, P2K with two groups of contacts). Resistors - MLT, capacitors - K50-35 or other similar ones. The mounted board of the device is fixed in any case of suitable size (Fig. 4), made of insulating material. The front panel contains LEDs HL1 and HL2, under which there is an inscription indicating the structure of the transistor under test, an XS1 connector for connecting the transistor, a SA1 switch indicating the structure, and an SB1 button to turn on the device. On the side wall of the housing there is an X4 connector for connecting an external power supply with a voltage of 3 to 9 V.

When checking transistors of high power, but with a small current transfer coefficient, especially germanium ones, the LEDs blink weakly, although the transistor is working. In this case, instead of the resistor R5, it is useful to turn on a circuit of series-connected variable and constant resistors R5 'and R5 ". By changing their resistance, the LEDs get the best glow.

It is also useful to select resistor R6 by increasing its resistance, otherwise, when checking some powerful transistors, the HL1 and HL2 LEDs will glow at any position of the SA1 switch, even if this transistor is working. This is especially true for high-gain power transistors and germanium power transistors.

Author: A.Slichenkov, Ozersk, Chelyabinsk region

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