Mains voltage indicator on the LM3914N-1 chip. Encyclopedia of radio electronics and electrical engineering

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In the mains voltage indicator, it is convenient to use a ruler of conventional LEDs located on a straight line or on an arc of a circle, imitating the scale of a dial gauge. Reading the readings of such an indicator is almost as convenient as a dial indicator. The use of LEDs of different glow colors attracts attention in the event of emergency situations. The readings of such an indicator can be monitored in poor lighting conditions and from a considerable distance.

The scheme of the proposed indicator is shown in fig. 1. It is made on the LM3914N-1 chip, which is a DC voltage to ten-position code converter. The outputs of the microcircuit allow direct, without current-limiting resistors, connection with the cathodes of the LEDs, the anodes of which are connected to the plus of the power source. If necessary, the microcircuit can also control vacuum-luminescent or LCD indicators.

Rice. 1 (click to enlarge)

It can work in two modes: "continuous scale" (the number of LEDs on is proportional to the input voltage) and "floating point" (only one LED is on, the number of which is proportional to the input voltage). The proposed device uses a more economical second mode (for this, pin 9 of the LM3914N-1 chip is left free).

The DC voltage supplied to the input of the microcircuit is formed from the AC network using a half-wave rectifier from diodes VD6, VD7. It is reduced to the required level using an adjustable resistive voltage divider R3R4. The high-voltage (150 V) VD4 zener diode eliminates excess voltage by "stretching" the scale of the device. The VD5 zener diode limits short-term voltage surges that are always possible in the network to a value that is safe for the input of the microcircuit.

The capacitance of the smoothing capacitor C5 is chosen such that the amplitude of the rectified voltage ripples is sufficient to ensure that, at intermediate values ​​of the mains voltage, not one, but two adjacent LEDs light up. This increases the accuracy of "by eye" stress estimation.

Please note that in the "floating point" mode, the HL1 LED does not go out when other LEDs are turned on, but only shines with reduced brightness, allowing you to see the "beginning" of the scale. It goes out completely only at a voltage below the corresponding glow with full brightness.

Resistors R7-R9 are designed to equalize the brightness of the glow of different types of LEDs. If this is not necessary, resistors can be discarded by replacing them with jumpers. You can also install such resistors in the circuit of other LEDs.

The supply voltage of the microcircuit and LEDs was obtained using a rectifier on diodes VD1, VD2 with quenching capacitors C1, C2. It is limited to the desired value (12 V) by the VD3 zener diode. Resistor R1 reduces the charging current of capacitors C1, C2 when the device is connected to the network. Resistor R2 discharges these capacitors after being disconnected from the mains.

The indicator was mounted on a board made of 90x70 mm insulating sheet material. Her photograph is shown in Fig. 2. The parts are placed in such a way that all connections can be made using their leads and a few jumpers from the mounting wire. Surface mounting reduces the likelihood of breakdown along the surface of the printed circuit board between the thin edges of conductors with a large potential difference. In industrial devices, this problem is solved not only by increasing the gaps between the conductors, but also by air gaps in the board dielectric specially located on the path of possible surface breakdowns.

Fig. 2

Resistor R1 is desirable to use a wire or special imported in a matte gray case. MLT resistors and the like are unsuitable here. their conductive layer can burn out to a break after several inclusions of the device in the network.

Trimmer resistor R4 is desirable to use multi-turn, such as SP5-22. Trimmer resistors SPZ-38 and others in an open version are not suitable for this device due to low reliability and stability. To improve the smoothness of adjustment and its stability, you can use a trimming resistor of a smaller value than indicated in the diagram by connecting a selected constant resistor in series with it.

Capacitors C1, C2 - film K73-17, K73-24, K73-39 for a constant voltage of at least 630 V. Imported analogues of these capacitors are usually less reliable. Oxide capacitors - K50-35 or imported. Ceramic capacitor C4 - for surface mounting. It is soldered directly to the power pins of the DD1 chip.

Diodes 1N4007 can be replaced with 1N4006 KD243Zh, KD247D, KD257D. R2K zener diode - on R2M or any other low-power one with a stabilization voltage of 140 ... 155 V. Such zener diodes are widely used in modern kinescope TVs, and their acquisition usually does not cause problems. The zener diode 1N4738A can be replaced with KS 182Ts, KS 182Ts1, 2S175Ts, 2S175K1, KS175Ts. The transistor of the KT315 KT3102 series is also suitable - the output of its emitter is connected to the positive terminal of the capacitor C5, the base terminal is connected to the negative terminal, and the collector terminal is left free. The D815D zener diode will replace two 1N5341 zener diodes connected in series, an analogue of the LM3914N-1 microcircuit - LM3914V, made in a surface-mounted package. Chips LM3915, LM3916 are also suitable. The LEDs of the types indicated in the diagram, if necessary, can be replaced by any others that are suitable in terms of color and brightness of the glow, as well as the size of the case. they should not be placed too closely, this will make it difficult to interpret the indicator readings.

It is convenient to adjust and check the indicator using an adjustable autotransformer (LATR). By setting the voltage to exactly 220 V, the adjusted resistor R4 ensures that only the HL5 LED is turned on at full brightness (as already mentioned, the HL1 LED shines “half-heartedly”). A slight deviation of the voltage from the nominal should lead to the inclusion of the neighboring LEDs HL4 or HL6 with a small brightness. Further, by changing the voltage supplied to the indicator, its values ​​​​corresponding to the middle of the glow zones with the maximum brightness of each of the LEDs are noted. it is these values ​​that should be written at the LEDs of the finished device, those indicated on the diagram are indicative.

It should be borne in mind that cheap digital multimeters of the 830-838 series measure alternating voltage, the value of which is about 220 V with an absolute error of up to ± 10 V. Therefore, it is advisable to use a more accurate device as a reference voltmeter when calibrating the indicator. You can expand or narrow the range of voltage values ​​that the indicator shows by selecting the VD4 zener diode, respectively, with a lower or higher stabilization voltage.

If you connect pins 9 and 3 of the LM3914N-1 microcircuit, the indicator will work in the "continuous scale" mode, in which all LEDs from HL1 to the corresponding measured voltage turn on simultaneously. Since the current consumed by the device in this case will increase significantly, it is necessary to double the capacitance of the capacitors C1 and C2, and supply the VD3 zener diode with a heat sink with an area of ​​​​about 50 cm. The value of the resistor R5 should be increased to 18 kOhm and repeat the graduation of the LED scale.

When working with the indicator, you need to remember that its elements are under mains voltage, and take the necessary care and safety measures.

Author: Butov A.

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