High voltage probe. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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In recent years, the journal "Radio" describes several interesting devices [1-5], assembled using parts of electronic ballasts from failed so-called energy-saving compact fluorescent lamps (CFLs). I bring to the attention of readers another design using these parts - a high-voltage probe.

The device (its diagram is shown in Fig. 1) is a flyback converter of the supply voltage 9 V to 1000 V with a maximum output current of up to 1 mA. This is quite enough to measure the breakdown voltage of electronic components without damaging them. The disadvantage of the device is the inadmissibility of a short circuit at the output, as this will lead to the failure of the protective diode VD5, and then the measuring head PA2.

Rice. 1. Diagram of the device (click to enlarge)

The probe contains a rectangular pulse generator with a repetition rate of about 20 kHz and an adjustable duty cycle on the elements DD1.1, DD1.2, a buffer stage of the elements connected in parallel DD1.3, DD1.4, an electronic switch on a transistor VT1, loaded by a transformer T1, a half-wave rectifier on a VD4 diode with a filter capacitor C2, an output voltage and current control unit (PA1R4, PA2R5VD5). The device is powered by a GB1 battery with a voltage of 9 V (the key on VT1 is directly, and the DD1 microcircuit is through a parametric stabilizer R3VD3).

With the power on (the element under test - suppose it is a diode - must be connected in advance in the correct polarity), the pulse generator starts to work. As the duty cycle decreases, the voltage at the probe output increases, the value of which is shown by the PA1 microammeter. As soon as it approaches the maximum reverse voltage of the diode under test, the needle of the PA2 microammeter will begin to deviate. The readings of the PA1 device in this case will correspond to the measured voltage value.

The device is not critical to the parameters of the parts. Capacitor C1 - ceramic or film, C3 - oxide, diodes VD1, VD2 - any low-power silicon, VD5 - low-power germanium. Diode VD4 must be designed to operate at a frequency of 20 kHz with a maximum reverse voltage of at least 1 kV. The rated voltage of the zener diode VD3 must be within 6 ... 7 V. The K561LA7 microcircuit is interchangeable with an analogue from the K176 series or a foreign analogue.

Transformer T1 is wound on a ferrite W-shaped magnetic circuit from a CFL choke with a power of 15-20 W (to disassemble it, you need to put the choke in acetone for a while). The primary winding contains 20 ... 30 turns of copper winding wire with a diameter of 0,2 ... 0,3 mm, and the secondary - 200 ... 300 turns of wire with a diameter of 0,1 mm. Insulation is required between the layers of the secondary winding. The author used scotch tape for this purpose. Capacitor C2 can be made up of several parallel-connected capacitors from CFLs with a rated voltage of at least 1,2 kV and a total capacitance of at least that indicated in the diagram, or one K78-2 capacitor can be used. As measuring heads PA1, PA2, the author used pointer indicators of the M476 series, which were used to control the recording level in old domestic tape recorders. Instead, any small-sized indicators of the magnetoelectric system with a full deflection current of the arrow of 100 ... 200 μA will do.

Set up the device as follows. By connecting a voltmeter (preferably digital) with a measurement limit of at least 1 kV to the probes instead of the element under test, the voltmeter readings are set at the level of 1 kV with the variable resistor R1 slider, and then, selecting the resistor R4, the maximum deviation of the arrow of the device PA1 is achieved. Further, instead of the element under test, a resistor with a resistance of 100 kOhm is connected in series with the milliammeter to the switched off device. Having set the current to 1 mA with the variable resistor slider, the resistor R5 is selected, achieving the maximum deviation of the arrow of the PA2 device.

Rice. 2. Parts of the device in the case of a faulty digital instrument

The author placed the details of the device in the case of a faulty digital device, mounting the low-voltage part on the printed circuit board, and the high-voltage part - in a hinged way (Fig. 2). The appearance of the device is shown in fig. 3.

Rice. 3. Appearance of the device

Literature

1. Nechaev I. From the details of energy-saving fluorescent lamps. - Radio, 2012, No. 6, p. 26-28.
2. Nechaev I. Triac power controller and automatic lighting control. - Radio, 2012, No. 9, p. 31, 32.
3. Nechaev I. From the details of the CFL. LED flasher for a New Year's toy. - Radio, 2012, No. 11, p. 36, 37.
4. Nechaev I. From the details of the CFL. Control of a network lamp on two wires. - Radio, 2013, No. 8, p. 34, 35.
5. Zakharov D. From the details of the CFL. The second life of transformers and chokes. - Radio, 2016, No. 12, p. 24, 25.

Author: V. Staroverov

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