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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Portable version of the meter Uke.max. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology In [1], the Uke.max meter was described for the selection of powerful UMZCH transistors. This article describes a device for a similar purpose, but the new device is not tied to mains voltage, you can take it with you to the radio market to test transistors. And this, you see, is a very important advantage of the new meter. The device, which will be discussed, was manufactured even before the appearance of the article [1]. The meter [1] serves me to this day. It is often necessary to check transistors according to the Uke.max parameter after a standard check with a conventional M41070/1 pointer ohmmeter. By the way, this ohmmeter is better suited for testing transistors than the popular digital 830 series ohmmeters, etc. But real numbers can be obtained only under conditions close to the operating modes of transistors. To ensure that the transistor under test does not fail, care must be taken to build a system close to non-destructive testing. And, of course, the device must be portable. It was decided to abandon the galvanic cells, they were replaced by a battery. Experimenting with various voltage converter circuits, I came to the circuit of Fig. 1.
The device turned out to be small-sized - the mass of the device was mainly determined by the masses of the battery and the case. It managed to obtain an output DC voltage of more than 4 kV! Therefore, a resistor R6 is introduced into the circuit, limiting the range of high voltage regulation from above. Such a high voltage, by the way, allows you to check capacitors and diodes. To check the transistors are connected in parallel with an adjustable voltage source. Thanks to the resistor R15 (R16), when the load is closed, the circuit operates in the mode of a stable current generator. This protects both the circuit and the transistors under test. As the practice of measurements with the device [1] has shown, in the vast majority of cases there is no need to include a resistor between the base and emitter of the transistor under test. If the transistor is serviceable with a shorted base with an emitter, then it can be installed in the equipment without any doubt (verified by many years of experience). For this reason, in the circuit of Fig. 1, the terminals of the base and emitter of the transistors are short-circuited by mounting jumpers already in the connectors. But those who wish can turn on variable resistors, as is done in the device [1]. In order not to switch the type of conductivity (npn or pnp), the connectors have separate contacts for transistors of different conductivity. This practically eliminates the possibility of connecting a voltage of reverse polarity to the transistor under test (this immediately disables the transistor). This instrument has a voltmeter with a "stretched" scale to indicate the condition of the battery. The voltmeter is made on the elements VD3, VD4, R11 and pointer meter RA2. The same meter also monitors the health of the measured transistors. In the position of the SA2 switch shown in the diagram, the current through the transistor is measured. When the SA2 contacts are closed, the RA2 meter is connected through the elements R11, VD3, VD4 to the positive terminal of the battery. The "stretching" of the scale is carried out by the Zener diode VD4 and the diode VD3. This improves the accuracy of the battery status indicator, which means that a cheap measuring head can be used. In order to reduce the probability of failure of the PA2 meter with defective transistors or accidental short circuits of the collector-emitter terminals, elements VD5 and R10 are installed in the circuit. The "highlight" of the circuit is an electronic kilovoltmeter, made on the VT3 assembly of the KPS104 type and the RA1 meter. The traditional design of similar devices provides for a pointer current meter (usually 50 or 100 μA) and an additional resistor. To measure voltage up to 3 kV with a 100 μA meter, an additional 30 MΩ resistor is required. The high input impedance of the field effect transistor VT3.1 allows you to install a resistor R8 with a resistance of 100 MΩ. This allows you to turn on a cheap PA1 meter from a 500 μA tape recorder. With R8=100 MΩ and a voltage at the output of the voltage multiplier of 3 kV, the current consumption is only 30 μA. If the user has a more sensitive meter at his disposal, then R8 can be increased even up to 500 MΩ, which will improve the overall weight and size of the device. Somewhat unusual in the device under consideration is the regulation of the output voltage, produced by changing the voltage on the collector of the transistor VT1 by the potentiometer R5. Such an inclusion guarantees the adjustment of Uke from zero to the maximum value, the latter is limited by resistor R6. Other methods do not guarantee stable operation of the circuit for small Uke. The generator is made on the elements DD1.1, DD1.2 according to a well-proven circuit with diodes, thanks to which it is possible to separately set the pulse duration and pause duration. The pulse frequency is determined by the capacitance of the capacitor C1. In this circuit, it is equal to 20 kHz. Increasing the frequency makes sense when sectioning the transformer T1 (in this case, it is made unsectioned). The generator is decoupled by two buffer elements DD1.3, DD1.4. A transistor VT1 with a high base current transfer coefficient (KT3102E) was used as a current amplifier. In the terminal stage VT2, the KT903A transistor gives good results (although the KT801B, KT815B, KT940A, KT805A, KT819G, etc. transistors were also used). From the secondary winding of the transformer T1, voltage is supplied to the voltage multiplier (elements VD13 ... VD20 and C5 ... C12). The device has terminals for connecting the charger. To charge the battery, switch SA1 is moved to the position shown in Fig. 1. Diode VD12 prohibits the supply of reverse polarity voltage to the battery. To indicate the inclusion of the device, the VD21 LED is used. Thus, switch SA1 is also a power switch. Details. Instead of the K561LE5 chip, the K561LA7 is also suitable. Instead of the KT3102E transistor, you can use KT3102D or KT342. It has already been said about the VT2 transistor, but I will add that if you do not need a voltage of 3 kV, then the range of transistors used becomes very wide - medium power transistors are also suitable. But in this case, you will not be able to check television transistors of types KT838A, KT872A and the like. To test most high-voltage transistors, a voltage of 1,5-2 kV is sufficient. As VT3, you can use any single field-effect transistors, but the assembly is still more convenient. You can use KPS104 with any letter index. Instead of diodes KD521A (B), KD522 is suitable. Diodes D220 and D223 can be replaced by any similar ones, including KD521, KD522. Instead of series-connected diodes VD6 ... VD9, zener diodes were originally installed, but they have large leakages, which introduced errors when measuring high voltages. High-voltage diodes of the 1N4937 type (600 V; 0,1 μs) are quite replaceable by domestic types KD226(G-E), KD243(DZh), KD247(D-Zh). Zener diode VD4 is selected during commissioning (see below). Switches SA2, SA3 type MT-1 or any other small. Switch SA3 type MT-3. High-voltage resistors R8, R15, R16 type KEV-1. The remaining resistors are types MLT and MT. The following types of capacitors were used: KD (C1), K73-17 (C3 ... C12, C14), K50-16 (C2, C13). Meter RA2 type M476 / 3 (100 μA), I can’t indicate type RA1, I took it from an old tape recorder, it is convenient in that it has a large scale (56x56 mm). The pulse transformer T1 is wound on a ferrite ring of size K45x23x8. Ferrite brand M2000NM1. The choice of this standard size is justified by the fact that it is necessary to wind the windings for a long time and carefully. The secondary winding is wound first - 1000 turns of PELSHO-0,25 wire. A primary winding is wound on top of it - 27 turns of the same wire, but folded into 7 cores. Design. The meter is placed in a polystyrene case measuring 215x148x55 mm (ready-made from some apparatus). The front panel is made of white plastic, it is easy to write on it with a black ballpoint pen, which can then be sealed with adhesive tape. The case also includes an eastern-made battery (6 V, 4 Ah, 640 cycles), its dimensions are 107x69x47 mm. Such a battery has a low self-discharge, so you can not charge it for months. Recently, a change was made to the device circuit - the SA2 switch was replaced by a two-section one. The second section of the switch is turned on according to the diagram in Fig.2. This allows you to more smoothly adjust Uke in the range of 0 ... 600 V and eliminate the off-scale indicator PA2 in the range of 3 kV. The device is made block by block. The converter with the terminal transistor VT2 and the transformer T1 is placed on the printed circuit board (Fig. 3).
The voltage multiplier is assembled on a separate printed circuit board (Fig. 4).
The electronic voltmeter is assembled on the third printed circuit board (Fig. 5). The remaining elements of the circuit are soldered to the fixed parts on the device case. Transistor VT2 is installed without a heat sink.
Adjustment. All radio components used must be carefully checked. First of all, it is necessary to calibrate the scales of the PA1 kilovoltmeter. There are two of these scales (600 V and 3 kV). It is important to carefully disassemble the microammeter without damaging the head. To do this, make cuts along the clearly visible connecting junction of the body halves with a sharp scalpel. The scale is made of white paper using a compass and scissors. About the voltage divider R10 and R11. First you need to select R10, since R11 has more effect on the voltmeter readings. You can calibrate with the same circuit (from point "B"), using a meter with a scale of 50 μA and a 100 MΩ resistor. Having closed the contacts of the SA3 switch, we select the R10 resistor for the 3 kV range, only after that we select the R11 resistor for the 600 V range. We start the adjustment of the voltage converter with the generator. Capacitor C1 select the frequency in the range of 20-30 kHz. Instead of resistors R1, R2, you must first solder the potentiometers and set the duty cycle to 2. The slider of the resistor R5 must be in the leftmost position (according to the diagram). Then we begin to move this engine, while the voltage at point "B" should increase. If this is not the case, the installation and details must be carefully checked. During these works, the device must be powered by a voltage regulator with a current limit of up to 1 A. Otherwise, it is easy to disable the transistor VT2. Set the voltage at point "B" to 200 V. After that, we select the capacitor C1 according to the maximum increase in this voltage. Then we select resistors R1, R2 for the same purpose. After that, potentiometer R5 sets the maximum voltage value at point "B". If necessary, you can reduce the resistance of the resistor R6. You should not reduce the resistance of the resistor R3 (you can damage the microcircuit). About the "stretching" of the voltmeter scale on RA2. A chain of elements VD3, VD4, R11 and PA2 is connected to an adjustable stabilized power supply. The voltage control zone of this circuit is within 5 ... 8 V. Thus, it is possible to monitor the state of the battery both during operation and during charging. By setting the output voltage of the power supply to 5 V, we achieve the deviation of the arrow of the PA2 meter. This is achieved by selecting a Zener diode VD4. After that, we select the resistor R8 for the maximum deviation at a voltage of 8 V. The modernization of the device consists in sectioning the transformer T1 to increase the efficiency of the circuit. You can also install a 1 µA head as a PA50 meter, which will reduce the current drawn from the high-voltage rectifier, and hence the power of the circuit. References:
Author: A.G. Zyzyuk
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