Digital meter of transistor parameters. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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In the magazine "Radio", 1998, No. 8, p. 62-65 describes the KR572PV5 chip and options for its use, including non-standard ones. Here we give a brief description of the digital transistor meter as an example of using this microcircuit in differential switching.

The device allows to measure the base current transfer coefficient h21E in three ranges with upper limits of 200, 2000, 20 at a collector current set discretely by 000; 0,1; 0,3, etc. up to 1 mA. In addition, it is also possible to measure the reverse collector current on three ranges with upper limits of 300 and 20,2 μA, the resolution at the lowest is 0,2 nA. The determination of h0,1E is carried out at a collector-base voltage of about 21 V, and the reverse current of the collector junction is at 1,5 V.

The h21E measurement principle is illustrated in fig. 1. The VTX transistor under test is connected according to the common base circuit. Its emitter current is determined by the relatively large resistance of the current-setting resistor installed in the emitter circuit (one of the resistors R15 - R23) and the power supply voltage. A current-measuring resistor (R11-R14) is also included in the emitter circuit. A resistor is installed in the base circuit in the diagonal of the VD1 diode bridge, the voltage drop across which is proportional to the base current (R1-R6).

The ratio of the voltage across the resistor in the emitter circuit to the voltage across the resistor in the base circuit is proportional to the current transfer coefficient in the common collector circuit, it is one more than the same coefficient in the common emitter circuit. This ratio is measured by the ADC on the KR572PV5 chip. Current-measuring resistors are selected such that the drop on the emitter resistor is about 50 or 150 mV, on the base resistor - 25 ... 1500 mV, depending on the base current transfer coefficient h21E and range. The diode bridge is necessary in order to be able to check transistors of various structures without switching the inputs of the UOBR ADC. In addition, the voltage drop across the bridge diodes ensures the collector-base voltage at the indicated level of 1,5 V. The voltage at the input UIN of the ADC can change sign, so a diode bridge is not required in the emitter circuit.

When measuring the reverse current of the collector junction IKo, a voltage of 5 V is applied from the divider R7R15 between the collector and emitter of the tested transistor VTX (Fig. 2). The voltage drop across the current measuring resistors R8-R10 is proportional to the measured current. In this mode, a voltage of 100 mV is applied to the ADC UOBR input. The role of the divider is not only to reduce the voltage supplied to the transistor to 5 V and limit the current in the event of a faulty transistor being installed, but also to bring the common-mode voltage at the UIN inputs of the ADC to half the supply voltage. Naturally, in this mode, you can also check the reverse currents of the diodes.

The diagram of switching circuits of the meter is shown in fig. 3. Switch SA1 is used to select the emitter current of the tested transistor and enable the measurement mode of the reverse collector current IКо, switch SA2 determines the measurement ranges h21Э and IКо, the position of SA3 is determined by the structure of the transistor. Capacitors C1 and C2 are needed to eliminate the generation that sometimes occurs when testing high-frequency transistors, C3 eliminates mains interference when measuring the reverse current of the collector junction. Chains R24C4, R25C5, R26C6, R27C7 serve to protect the inputs of the KR572PV5 chip from static electricity.

(click to enlarge)

The measuring part of the device is assembled according to the scheme of Fig. 3 [1] (the R7C6 circuit is excluded), the values ​​of the elements and the divider to obtain a voltage of 100 mV are borrowed from [2]. Clock generator frequency - 40 kHz (R46 in [2] - 110 kOhm). The common wire of the device is the connection point of output 32 of the KR572PV5 microcircuit with capacitors C9 and C28 in [2].

It is desirable to select resistors R1-R6, R8-R14 with an accuracy of at least 2%, in extreme cases, resistors with a tolerance of 5% can be used without selection. In the described design, resistors of the C2-29V type with a power of 0,125 W were mainly used. Resistor R14 is made up of two C2-29V 1 ohm 0,125 W connected in parallel. Resistors R7, R15-R23 are used of the MLT type with a tolerance of 5%, R23 is composed of two connected in series with a resistance of 12 and 15 ohms with a power of 2 watts. The KTs407A diode bridge can be replaced by four silicon diodes with an operating current of at least 100 mA. Switch SA1 type PG7-35-16P5N, SA2 - PG2-11-6P6N, SA3 - PG2-13-4P3N. The circuit diagram shows the numbering of the contacts shown on the switches.

When setting up the device, it is desirable to set the frequency of the ADC clock generator equal to 40 kHz by selecting the resistor R45 [1]. To do this, an oscilloscope synchronized from the network controls the pulse frequency at the output F of the KR572PV5 microcircuit (pin 21). The image of the pulses on the screen should be practically stationary, while their frequency is 50 Hz. You also need to calibrate the current meter. The easiest way is to set a voltage of 26 mV relative to the common wire on the engine of the tuning resistor R1 [100], controlling it with an accurate voltmeter with an input resistance of at least 1 MΩ.

Literature

1. Biryukov S. Application of ADC KR572PV5. - Radio, 1998, No. 8, p. 62-65.
2. Biryukov S. Digital multimeter. - Radio, 1996, No. 5, p. 32-34; No. 6, p. 32-34; 1997, No. 1, p. 52, no. 3, p. 54.

Author: S. Biryukov

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