Phase indicator LED. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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During electrical work, a phase indicator is often required. Previously, gas-discharge indicator lamps were used in such devices; today, instead of them, high-brightness LEDs can be used, which glow noticeably at a current of several tens of microamperes. Capacitive coupling of the indicator with the user's hand can provide galvanic isolation.

Fig. 1

The scheme of the proposed phase indicator is shown in fig. 1. The phase voltage is supplied to the diode bridge VD1 through the current-limiting resistor R1 and the constructive capacitor CR. The rectified voltage is applied to the HL1 LED, and it glows, thereby indicating that the wire under test is indeed a phase wire. The indicator is mounted in a plastic pen case of a suitable size. Capacitor Sk is formed by a piece of aluminum foil rolled into a cylinder and glued to the inner surface of the case and by the user's hand. The dielectric gasket of the capacitor is the housing wall.

Fig. 2

A variant of the indicator design is shown in fig. 2. A pin (probe) 2 is inserted into the tip 1 of the pen body - a metal rod with a diameter of 1,5 ... 2 and a length of 20 ... 25 mm, to which a current-limiting resistor 4 (R1) is soldered. The rod is fixed in the tip with epoxy glue 3. Instead of a button (or inside it), an LED 8 is installed in the cap of the housing 9, to the terminals of which a diode bridge 7 is soldered.

One of the free terminals of the diode bridge is connected with a thin insulated wire to the resistor R1, and the other - with the same wire with a rectangular piece of foil 6, the width of which should be approximately three times the inner diameter of the case 5, and the length - 10-15 mm shorter than the length its inner cylindrical part. To ensure reliable contact, the end of the wire is stripped over a length of 30.40 mm, wrapped several times with the edge of the foil and tightly clamped with pliers. Then the foil is rolled into a cylinder and glued to the inner surface of the body.

When selecting a housing, you should choose one with a larger diameter and thinner walls - this will provide a larger capacity of the structural capacitor. To increase its capacity, the indicator case should be held in the hand as tightly as possible, the brightness of the LED will depend on this.

Fig. 3

The current flowing through the capacitor Sk in this design is very small (only a few microamperes), so not every LED will glow noticeably. To make the indication more noticeable without increasing the current through the device, a relaxation oscillator based on a DB3 symmetrical dinistor or similar can be introduced into it (Fig. 3). In this case, when the probe touches the phase wire, the capacitor C1 is first charged, and when the voltage across it reaches about 35 V, the dinistor opens and a current pulse flows through the LED, causing a flash of light that is clearly visible. The flash frequency depends on the capacitance of the capacitors Sk and C1: with an increase in the capacitance of the first of them, it increases, and the second - decreases. Generator parts are mounted directly on the terminals of the diode bridge.

Fig. 4

A further increase in the brightness of the light signal is possible by increasing the current through the LED. To do this, the capacitor Sk is replaced by resistors R1, R3 (Fig. 4) and the contact E1 electrically connected to the first of them (preferably made of metal with a stainless coating) is installed on the outer surface of the indicator housing. In this case, the foil is not needed, the relaxation generator on the VS1 dynistor can be left or excluded (i.e., connect the LED directly to the terminals of the diode bridge). The appearance of the indicator is shown in fig. 5.

Fig. 5

The device uses resistors MLT, S2-23, capacitors - ceramic K10-17v. A DB3 symmetrical dynis-tor can be found in a failed compact fluorescent lamp (CFL), 1N4007 diodes can also be removed from it to assemble a rectifier bridge instead of that indicated in the diagram. LED - any increased brightness of the glow in a housing with a diameter of 3.5 mm. It should be selected according to the brightness of the glow at low current. To do this, the existing LEDs are connected in turn to a 12 V power source through a 100 kΩ resistor and an instance with maximum brightness is selected.

Author: I. Alexandrov

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