Probe for commissioning and electrical work. Scheme, description

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Probe for commissioning and electrical work. Encyclopedia of radio electronics and electrical engineering

Free technical library

Encyclopedia of radio electronics and electrical engineering / Electrician's Handbook

When carrying out electrical installation and commissioning, the simplest probes are often used, similar to the circuit shown in Fig. 1. When the SB1 button is released, they can determine the presence of an alternating voltage of 100 ... 400 V with a frequency of 50 Hz (mainly when searching for a phase wire), while the neon lamp HL1 is lit. When the button is pressed, the probe can roughly estimate the resistance of the tested circuit to direct current ("continuity"). If it is within ten ohms, then the HL2 incandescent lamp is on. Unfortunately, very often, when the SB1 button is pressed, the probe is mistakenly connected to circuits under mains voltage, as a result of which the HL2 lamp instantly burns out ...

Probe for commissioning and electrical work
Rice. 1. Scheme of the simplest probe

The proposed probe (its block diagram is shown in Fig. 2) is free from this disadvantage. The function of the SB1 button in it is performed by the trinistor VS1, equipped with a control device (CU). As in the simplest probe, the HL1 lamp indicates the presence of an alternating voltage, the HL2 lamp lights up with a low resistance of the controlled circuit.

Probe for commissioning and electrical work
Rice. 2. Scheme of the proposed probe

UU works as follows. If there is an alternating or direct voltage of any polarity on the probes of the probe X1 and X2, then block A2 issues a blocking signal to block A3, which performs the function of a logic element 2I, and the signal to open the trinistor VS1 is not received. In this case, the neon lamp HL1 and one (at a constant voltage) or two (at a power frequency voltage of 50 Hz) LEDs in block A2 are lit (they also indicate the polarity of the applied voltage).

If there is no voltage on the probes X1 and X2, block A2 issues an enabling signal to block A3, and if there is active resistance between the probes of the measured circuit, then block A1 is triggered and, with a time delay t = 0,5 s, issues an enable signal to the second input of block A3. As a result, a signal appears at the output of the latter, which is amplified by block A4, and a signal is output from its output to the control electrode of the trinistor VS1. The trinistor opens, and if the resistance between the probes X1 and X2 is small enough (no more than a dozen ohms), then the HL2 incandescent lamp lights up. By the degree of its incandescence, one can approximately judge the value of the resistance of the circuit (recall that the probe is mainly focused on use in electrical work on branched electric lighting networks). By the brightness of the LEDs in block A2, you can also estimate the amount of voltage applied to the probes.

We will consider the operation of the probe according to its schematic diagram, shown in Fig. 3. Block A1 is made on the transistor VT1. When probes X1 and X2 are connected to a circuit under test with a resistance of less than 10 ohms, on which there is no voltage, transistor VT1 opens along the circuit plus power batteries GB1 - probe X2 - measured R_x- probe X1 - fusible link FU1 - resistor R2 - emitter junction of transistor VT1 - minus battery GB1. After a time delay t = 0,5 s, determined by the elements R5, C1, the opening signal is applied to the base of the transistor VT5, which acts as a power amplifier. If at the same time the transistors VT2, VT4 are closed, then the transistor VT5 opens and an opening signal is applied to the control electrode of the trinistor VS1. The latter opens, and if the resistance of the tested circuit R_x does not exceed ten ohms, the HL4 lamp starts to glow.

Probe for commissioning and electrical work
Rice. 3. Schematic diagram of the probe

Let now a voltage act at the input of the probe, the minus of which is applied to the probe X1, and the plus - to X2. In this case, the HL3 LED lights up, indicating the polarity of the applied voltage.

If the polarity of the input voltage is reversed (minus - on the X2 probe, and plus - on the X1 probe), the HL2 LED lights up, indicating the polarity of the applied voltage, and the transistor VT3 opens. Its collector current opens the transistor VT4, which shunts the emitter junction of the transistor VT5 with its collector-emitter section, prohibiting the passage of the signal to open the trinistor VS1.

In order for transistors VT2 and VT4 to open at approximately the same voltage on the probes, regardless of its polarity, a zener diode VD2 is included in the base circuit of the first of them, the voltage drop across which is approximately equal to the voltage of the GB1 battery. When an AC voltage is applied to the probes X1 and X2, both LEDs light up, transistors VT2 and VT4 open alternately, keeping the transistor VT5 closed.

Since the current consumed by the probe in standby mode is only about 2 µA, there is no power switch.

The probe does not contain scarce parts. Resistors - any corresponding dissipation power, capacitor C1 - imported oxide, C2 - ceramic KM or similar, transistors - KT315, KT312, KT3102 and KT3107, KT361 with any letter index (taking into account the structure and pinout). Increased requirements only for the transistor VT1: its static base current transfer coefficient h_21Э must be at least 90 (preferably more). Trinistor VS1 - KU202N or another, with a higher value of the allowable voltage.

All parts are mounted on a printed circuit board made of fiberglass foiled on one side with a thickness of 1,5 mm (Fig. 4). The VS1 trinistor and the AA size elements that make up the GB1 battery are fixed on it with brackets from a single-core mounting wire with a diameter of 0,6 ... 0,8 mm, soldered into the corresponding foil areas.

Probe for commissioning and electrical work
Rice. 4. Device circuit board

The body of the probe is made of a piece of plastic cable channel with a section of 40x25 mm. Probe X1 is made in the form of a piece of hard wire with a length of 50 ... 100 mm, pointed on one side, X2 - in the form of a flexible wire with a crocodile clip at the end. The location of parts in the probe body is shown in fig. 5, and its appearance - in Fig. 6.

Probe for commissioning and electrical work
Rice. 5. Location of parts in the probe body

Probe for commissioning and electrical work
Rice. 6. Appearance of the probe

Properly assembled from serviceable parts, the probe does not require adjustment. When the probes X1 and X2 are closed, the HL4 incandescent lamp should be lit, if necessary, it is only necessary to select a resistor R11 to reliably open the trinistor VS1.

Then check the operation of the probe at a reduced voltage of 24 V DC or AC. With direct current, the HL2 or HL3 LED should light up (depending on the polarity of the applied voltage), with alternating current, both LEDs should light up simultaneously. If the probe is working properly, then you can proceed to the test at a mains voltage of 230 V. In this case, both LEDs should light up simultaneously, as well as the neon lamp HL1. The trinistor must be closed, the HL4 lamp is off. On this, the test can be considered complete - the probe is ready for work.

Note. With a 3 V battery, the HL4 lamp (6,3 V, 0,2 A) will glow weakly. To increase the brightness, you should use a lamp for a lower voltage and the same current.

Author: Yu. Nigmatulin

See other articles Section Electrician's Handbook.

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