Low-voltage probes-voltage indicators. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Low-voltage probes-voltage indicators. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Indicators, detectors

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When testing, adjusting and repairing various electronic equipment, there is often a need to quickly check the presence of voltage and determine its polarity at different points of the device, wires, power connectors, etc. For these purposes, especially during work in the "field" conditions, it is convenient to use simple small-sized probes-indicators. On fig. 1 shows a diagram of a simple voltage and polarity probe on a two-color HL1 LED. The input voltage of the indicator is 3,5 ... 18 V, the current consumption in this case is approximately from 1 to 15 mA. In accordance with the flowing current, the brightness of the LED also changes.

Low Voltage Voltage Probes

Resistor R1 limits the current through the LED, and the diodes VD1 and VD2 are turned on so that, depending on the polarity of the controlled voltage, one of the LED crystals shines. When a positive polarity voltage is applied to the input, the current flows through the current-limiting resistor R1, the red glow crystal and the VD2 diode, so the LED glow will be red. When the polarity is reversed, it will turn green. If the input voltage is AC, the glow color is yellow.

In some cases, it is necessary not only to check the voltage and polarity of various power sources, such as batteries or galvanic cells, but also to assess their condition and load capacity. To do this, it is convenient to use a probe, which includes a load that consumes a certain current. Incandescent lamps can be used as a load. The scheme of such a probe is shown in Fig. 2. With its help, the polarity of the controlled voltage is determined, for this, the EL1 lamp should be painted with heat-resistant varnish or covered with a red light filter, and EL2 - green. The operating voltage of the probe and the current it consumes are determined by the incandescent lamps used. For a small-sized probe, miniature lamps of the CMN series are suitable.

Low Voltage Voltage Probes

To check the various nodes of the HF radio transmitting equipment in which high-frequency voltage is present, you can use a probe, the circuit of which is shown in Fig. 3. It operates in the frequency range of 1 ... 30 MHz and indicates voltage from 1,5 to 20 V. The input (XP1) is connected to the controlled circuit, and the XP2 probe is connected to the common wire. Capacitor C1 is a ballast and limits the input high-frequency current, diodes VD1, VD2 rectify the alternating voltage, resistor R1 additionally limits the current through the HL1 LED.

Low Voltage Voltage Probes

By selecting capacitor C1, you can change the sensitivity of the probe. An increase in the capacitance of this capacitor, along with an increase in sensitivity, leads to an increase in the shunting effect on the monitored circuits due to a decrease in the input resistance of the device. When working with a probe, it is recommended to first connect only the XP1 input to the circuit, without connecting the XP2 probe to the common wire. If the LED is not lit, the XP2 probe is also connected.

The basis of all probes can be a pen body with an inner diameter of at least 7 mm. Most of the elements are placed in it, they are connected with pieces of mounting wire, and the soldering points are insulated with PVC tubes. As an XP1 probe, you can use a pin from a connector or a sewing needle. To connect the indicator to the common wire of the device, a flexible stranded wire 10 ... 20 cm long is used, at the end of which you can install a crocodile clip.

The probes use MLT, S2-23 resistors, a capacitor - KM-5, KT, K10-17, KD103A diodes are replaceable by any low-power rectifiers, KD521A - by KD510A, KD522B diodes. LEDs - in a plastic case with a diameter of 3 ... 5 mm with increased brightness.

Author: V. Efremov, Essentuki, Stavropol Territory

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