AC power indicators. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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When operating sound reproducing equipment, it is always useful to have an indicator of the maximum power supplied to the speakers. This is especially true if the maximum output power of your UMZCH exceeds the maximum allowable for speakers. In this case, prolonged operation of the dynamic heads of the system under overload conditions can cause irreversible damage to them. In addition, with such operation of the equipment, the distortion of the audio signal reproduced by the speakers increases significantly.

Some amplifier models allow you to control the output power using LED or fluorescent indicators mounted on the UMZCH case. If the equipment is not equipped with such a device, you can make it yourself according to the descriptions in [1-3].

I offer a few more such developments. They are designed to work with speakers (loudspeakers) with a resistance of 4 ohms and allow you to indicate two power thresholds - 25 and 50 watts. However, they are easy to adapt to work with systems that have other resistances and power. The indicators are powered only by the AF voltage supplied to the system from the UMZCH, and consume little energy.

The simplest indicator (Fig. 1) consists of a half-wave diode rectifier VD1, a smoothing capacitor C1 and two threshold devices. They are identical, each of them includes a field-effect transistor current generator, a zener diode and an LED.

When the UMZCH is operating, the alternating voltage supplied to the indicator input is rectified, the capacitor C1 is charged. When the input voltage reaches 10 V, the VD3 zener diode will open, the HL1 green LED will flash and indicate that a 25 W signal has arrived at the speaker. The current through the zener diode and the LED limits the current generator on the transistor VT2.

If the input voltage rises to 14 V, the VD2 zener diode opens, the red HL2 LED starts flashing, indicating that the maximum power of 50 W has been reached.

The indicator parts are mounted on a board (Fig. 2) made of one-sided foil fiberglass. Zener diodes need to be selected according to the stabilization voltage: for VD2 it should be 12 V, for VD3 - 8 V.

This is of course not the best solution. The second drawback of the device is the fuzzy response thresholds, since the volt-ampere characteristic of the zener diodes is not steep enough.

When adjusting the indicator, the response voltage of the threshold devices is more accurately set by selecting the resistor R1.

In the second indicator (Fig. 3), the zener diodes VD2, VD3 are included in the control electrode circuits of low-power trinistors VS1 and VS2, respectively. And already the trinistors light up the LEDs. Thanks to this solution, it was possible to achieve more stable operation of threshold devices. But the need for the selection of zener diodes remained.

The current generator on the transistor VT1 feeds both indication circuits - in this embodiment, such a simplification turned out to be acceptable.

The details of this indicator are also mounted on a printed circuit board (Fig. 4) made of one-sided foil material.

If the LEDs do not go out during the operation of the indicator, you will have to install a capacitor C2 of a smaller capacity.

The third version of the indicator is somewhat more complicated (Fig. 5), but it is free from the above disadvantages. It contains a bridge rectifier on diodes VD1 -VD4, two D-flip-flops (DD1.1, DD1.2), key stages on transistors VT1, VT2, a current generator on transistor VT1, a zener diode VD5, which limits the supply voltage of the microcircuit and key stages with LEDs at the level of 5 ... 6 V. This measure made it possible to achieve accurate operation of triggers [4].

The thresholds for the operation of transistor switches, and hence the ignition of LEDs, are set by trimming resistors R2, R3. This option is more convenient, since it allows you to adjust the indicator to speakers with almost any resistance and any power. True, when operating the indicator with high-power AC (more than 50 W), the field-effect transistor in the current generator must be replaced with a bipolar one (Fig. 6). since the drain-source voltage may exceed the maximum allowable for a given transistor.

A drawing of a printed circuit board for mounting indicator parts (with a field effect transistor) is shown in fig. 7.

About the details of indicators. LEDs - any green and red glow colors, series AL307, AL 102. Diodes - series D220, D223, KD521, KD522 or others, designed for a rectified current of at least 50 mA and a reverse voltage of at least 100 V. Trinistors - any of the KU101 series , preferably with a minimum opening voltage on the control electrode. The field effect transistor KP302BM can be replaced by KP302 with letter indices A, B, G; KP307 with indices G, D or any other with an initial drain current of 20 ... 30 mA. Bipolar transistors - any of the series indicated on the diagrams.

The board of any indicator is placed inside the speaker, and the LEDs are fixed in the holes drilled in its front wall of the case.

It is convenient to adjust the indicators using a network step-down transformer, on the secondary winding of which the voltage is 15 ... 20 V [3]. Parallel to the secondary winding, a variable resistor with a resistance of 1 kOhm is included, from the engine of which and one of the extreme terminals, voltage is removed at the indicator input. An AC voltmeter is connected in parallel with the input. Having set the desired alternating voltage at the gathering, set the threshold for turning on the corresponding LED.

The required input voltage is calculated using the well-known formula: U = √РR, where U - input voltage, V; P is the indicated output power of the amplifier, W; R - AC resistance, Ohm.

Literature

1. Lukyanov D. Loudspeaker overload indicator. - Radio, 1984, No. 7, p. 27.
2. Nechaev I. LED signal level indicator. - Radio, 1988, No. 12, p. 52.
3. Parfenov A. LED indicator of AC power. - Radio, 1992, No. 2-3, p. 45,46.
4. Potachin I. Peak power indicator. - Radio, 1996, No. 2, p. 16.

Author: I.Potachin, Fokino, Bryansk region

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