Voltage converter 2,4/8 V to power the security alarm. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Voltage converters, rectifiers, inverters

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The author proposes a variant of a voltage converter for powering devices designed for a 6F22 battery ("Krona") from two AAA nickel-metal hydride batteries using the example of a security alarm.

Some self-powered devices operate almost continuously, consuming most of the time a small current in standby mode and only for a short time - tens of times more. An example of such a device is an autonomous security alarm with a built-in IR motion sensor and an acoustic emitter (Fig. 1). In standby mode, the current consumed by the signaling device does not exceed a fraction of a milliamp, and when an alarm is given, it increases to 50...60 mA.

Rice. 1. Autonomous security alarm with built-in IR motion sensor and acoustic emitter

There are conflicting requirements for the power supply of this device. On the one hand, it must have a voltage of 6 ... 9 V and an output current of several tens of milliamps to ensure sufficient volume of the alarm signal, on the other hand, a large capacitance and small dimensions. This device is powered by a 6F22 ("Krona") battery.

Of course, there are batteries of galvanic cells or batteries of a similar size that meet these requirements, but they are not cheap. If you use cheap ones, they may not provide the required current, usually have a high self-discharge current and therefore require frequent replacement.

One solution to this problem is to use nickel-cadmium, nickel-metal hydride batteries or AA batteries and a boost converter to power the signaling device. Such a converter can be assembled on the basis of a specialized NCP1400ASN50T1 microcircuit, its circuit is shown in fig. 2. This microcircuit is designed to build a stabilized switching voltage converter with an output voltage of 5 V. The microcircuit works in such a way that it maintains the specified constant voltage at its OUT input (pin 2). In order to obtain a voltage approximately twice as high at the output of the converter, two rectifiers on diodes VD1 and VD2 are used, and the storage choke L1 is made with a tap.

Rice. 2. Converter circuit based on the NCP1400ASN50T1 chip

But whatever the output current, the converter itself still draws current from the power supply. To reduce it, you can periodically turn off the converter, and during a pause in operation, power the signaling device from the storage capacitor. This is exactly what was done in this device, since the microcircuit has a CE control input (pin 1). Turning the converter on and off is carried out by a field effect transistor VT1.

Immediately after the supply voltage is applied, the capacitor C3 is discharged, the transistor is closed, and a high level is applied to the CE input, which turns on the converter. The charging of the capacitor C3 begins, and when the voltage at the gate of the transistor becomes sufficient to open it, the voltage at the CE input of the microcircuit will decrease to almost zero, the converter will turn off. When the capacitor C3 discharges a little, the transistor closes and the converter turns on again.

As a result, the current consumed by the converter when the signaling device is in standby mode is of a pulsed nature, and the voltage on the capacitor varies between two values ​​Umin and Umax (Fig. 3). The current pulse amplitude is about 200 mA, the pulse duration is about 1 ms, the repetition period is about 1,5 s. Therefore, the average current consumed from the power supply in this mode does not exceed 1 mA.

Rice. 3. Graphs of voltage and current changes

Using the fact that the signaling device works normally in the supply voltage range of 7 ... 10 V, it was decided to set (using the trimmer resistor R3) the output voltage to 7,5 ... 8 V. Thus, the converter periodically turns on and off, maintaining specified voltage at the output. Therefore, the instability of the output voltage is relatively high - ±0,5 V, but this does not affect the performance of the signaling device. As the battery discharges, the on period decreases. Without a field effect transistor, the output voltage of the converter is 9 ... 9,5 V.

When the signaling device goes into alarm mode, the period of turning on the converter is sharply reduced. If the output voltage drops below 8V, the FET will turn off and the converter will run continuously. The thermal stability of the output voltage is determined primarily by the parameters of the field-effect transistor. In this case, the voltage temperature coefficient is negative, equal to several millivolts per degree Celsius.

If the signaling device is turned off using the standard switch, the converter will continue to work, but the period of its activation will increase several times and the current consumed from the power source will decrease. Therefore, in some cases, it is possible to do without installing a special switch in the power supply circuit of the converter, and for long-term storage in the off state, the batteries or galvanic cells must simply be removed from the battery compartment. But if you wish, you can install an additional switch, there is enough space in the signaling device housing for this.

Most of the elements are mounted on a printed circuit board made of fiberglass foiled on both sides with a thickness of 1,5 mm; its drawing is shown in Fig. 4. All elements are placed on one side, the second is left metallized. The converter uses fixed resistors for surface mounting of size 1206, but MLT, S2-23, tuning - SP3-19, oxide capacitors - tantalum for surface mounting are also suitable. Instead of SS12 diodes, you can use low-power pulse or detector germanium diodes or Schottky diodes, designed for a forward current of at least 60 mA. The inductor is wound on a ferrite ring with a diameter of 6 ... 9 mm from the transformer of the electronic ballast of a compact fluorescent lamp and contains nine turns of PEV-2 0,4 wire with a branch from the fourth, counting from the left according to the output circuit.

Rice. 4. Drawing of a printed circuit board and the location of elements on it

Rice. 5. Mounted board

The board with its long side (bottom in Fig. 4) is soldered to a base measuring 26x50 mm, made of fiberglass laminated on one side (Fig. 5). On the narrow sides of the base, contacts-holders of batteries or galvanic cells are soldered (Fig. 6). To do this, printed areas for soldering the holders are cut out on the base board. As a result, the converter "fits" into the overall dimensions of the 6F22 battery and is placed in the signaling device's battery compartment (Fig. 7).

Rice. 6. Mounted board

Rice. 7. Battery compartment of the signaling device

Rice. 8. The converter in the battery compartment of the DT-83X series multimeter

This converter can also be used to power the multimeters of the DT-83X series (Fig. 8), it will fit into the battery compartment. To do this, the field effect transistor and all the resistors on the board can not be installed, and the CE input (pin 1) of the microcircuit is connected to the positive terminal of the capacitor C1. The number of turns of the throttle is 10, with a tap from the middle. Since the multimeter is used infrequently, a small-sized sliding power switch should be installed in the power supply circuit of the converter, which is placed in the multimeter housing in the lower left corner (Fig. 9). The converter can also be used in other devices powered by a 6F22 battery.

Rice. 9. Small size sliding power switch

Author: N. Nechaev

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