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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING A low-power converter for powering a 9 volt load from a 3,7 volt Li-ion battery. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells Some modern low-power devices consume a very small current (a few milliamps), but for their power they require too exotic a source - a 9 V battery, which, moreover, lasts a maximum of 30 ... 100 hours of device operation. It looks especially strange now, when Li-ion batteries from various mobile gadgets are almost cheaper than the batteries themselves - batteries. Therefore, it is natural that a real radio amateur will try to adapt the batteries to power his device, and will not periodically look for "antique" batteries. If we consider a conventional (and popular) multimeter as a low-power load. M830, powered by an element of the "Korund" type, then to create a voltage of 9 V, at least 2-3 batteries connected in series are needed, which does not suit us, they simply will not fit inside the device case. Therefore, the only way out is to use one battery and a boost converter. Element base selection The simplest solution is to use a 555 type timer (or its 7555 CMOS version) in a pulse converter (capacitive converters are not suitable, we have too much difference between input and output voltages). An additional "plus" of this microcircuit, it has an open-collector output, moreover, a sufficiently high-voltage one capable of withstanding voltages up to +18 V at any operating supply voltage. Thanks to this, it is possible to assemble a converter from literally a dozen cheap and common parts (Fig. 1.6).
Pin 3 of the chip is a normal two-state output, it is used in this circuit to maintain generation. Pin 7 is an open-collector output that can withstand increased voltage, so it can be connected directly to the coil, without a transistor follower. The reference voltage input (pin 5) is used to regulate the output voltage. Principle of operation of the device Immediately after the supply voltage is applied, the capacitor C3 is discharged, the current through the zener diode VD1 does not flow, the voltage at the REF input of the microcircuit is 2/3 of the supply voltage, and the duty cycle of the output pulses is 2 (that is, the pulse duration is equal to the pause duration), the capacitor C3 charges at maximum speed . Diode VD2 is needed so that the discharged capacitor C3 does not affect the circuit (does not reduce the voltage at pin 5), resistor R2 "just in case", for protection. As this capacitor charges, the zener diode VD1 begins to open slightly, and the voltage at pin 5 of the microcircuit rises. From this, the pulse duration decreases, the pause duration increases, until dynamic equilibrium occurs and the output voltage stabilizes at a certain level. The value of the output voltage depends only on the stabilization voltage of the zener diode VD1 and can be up to 15 ... 18 V at a higher voltage, the microcircuit may fail. About details Coil L1 is wound on a ferrite ring. K7x5x2 (outer diameter - 7 mm, inner - 5 mm, thickness - 2 mm), approximately 50 ... 100 turns with a wire with a diameter of 0,1 mm. You can take a larger ring, then the number of turns can be reduced, or you can take an industrial choke with an inductance of hundreds of microhenries (µH). The 555 microcircuit can be replaced with the domestic analogue K1006VI1 or with the CMOS version 7555 - it has less current consumption (the battery will "last" a little longer) and a wider operating voltage range, but it has a weaker output (if the multimeter requires more than 10 mA, it may not to give such a current, especially at such a low supply voltage) and she, like all CMOS structures, "does not like" the increased voltage at its output. Device Features The device starts working immediately after assembly, the whole setting consists in setting the output voltage by selecting the zener diode VD1, while a 3 kΩ resistor (load simulator) must be connected to the output in parallel with the capacitor C3,1 (load simulator), but not a multimeter! It is forbidden to turn on the converter with an unsoldered zener diode, then the output voltage will be unlimited and the circuit can "kill" itself. You can also increase the operating frequency by reducing the resistance of the resistor R1 or capacitor C1 (if it operates at an audio frequency, a high-frequency squeak is heard). If the length of the wires from the battery is less than 10 ... 20 cm, a filtering power supply capacitor is optional, or you can put a capacitor with a capacity of 1 uF or more between pins 8 and 0,1 of the microcircuit. Identified weaknesses Firstly, the device contains two oscillators (one master oscillator of the ADC chip - analog-to-digital converter of the device, the second generator of the converter) operating at the same frequencies, that is, they will affect each other (frequency beat) and the measurement accuracy will seriously deteriorate. Secondly, the frequency of the converter generator is constantly changing depending on the load current and battery voltage (because there is a resistor in the POS - positive feedback circuit, and not a current generator), so it becomes impossible to predict and correct its influence. Specifically for a multimeter, one common oscillator for the ADC and a converter with a fixed operating frequency would be ideal. The second version of the converter The circuit of such a converter is slightly more complicated and is shown in Fig. 1.7.
A generator is assembled on the DD1.1 element, through the capacitor C2 it clocks the converter, and through C5 - the ADC chip. Most inexpensive multimeters are based on the ICL7106 double-integration ADC or its analogues (40 pins, 3,5 characters on the display), to clock this microcircuit, you just need to remove the capacitor between pins 38 and 40 (unsolder its leg from pin 38 and solder to the pin 11DD1.1). Thanks to feedback through a resistor between pins 39 and 40, the microcircuit can be clocked even with very weak signals with an amplitude of a fraction of a volt, so 3-volt signals from the DD1.1 output are quite enough for its normal operation. By the way, in this way it is possible to increase the measurement speed by 5 ... 10 times - simply by increasing the clock frequency. The measurement accuracy practically does not suffer from this; it worsens by a maximum of 3 ... 5 units of the least significant digit. It is not necessary to stabilize the operating frequency for such an ADC, so a conventional RC oscillator is quite sufficient for normal measurement accuracy. On the elements DD1.2 and DD1.3, a waiting multivibrator is assembled, the pulse duration of which, using the transistor VT2, can vary from almost 0 to 50%. In the initial state, at its output (pin 6) there is a "logical unit" (high voltage level), and the capacitor C3 is charged through the diode VD1. After the arrival of a triggering negative pulse, the multivibrator "tip", a "logical zero" (low voltage level) appears at its output, blocking the multivibrator through pin 2 of DD1.2 and opening the transistor VT1 through the inverter on DD1.4 In this state, the circuit will be until then until the capacitor C3 is discharged - after which the "zero" at pin 5 of DD1.3 will "tip" the multivibrator back into standby state (by this time C2 will have time to charge and there will also be "1" at pin 1.1 of DD1), transistor VT1 will close , and the coil L1 will be discharged to the capacitor C4. After the arrival of the next pulse, all of the above processes will repeat again. Thus, the amount of energy stored in the coil L1 depends only on the discharge time of the capacitor C3, that is, on how strongly the transistor VT2 is open, which helps it to discharge. The higher the output voltage, the stronger the transistor opens; thus, the output voltage is stabilized at a certain level, depending on the stabilization voltage of the zener diode VD3. To charge the battery, a simple converter is used on an adjustable linear stabilizer DA1. You only have to charge the battery, even with frequent use of the multimeter, only a couple of times a year, so it makes no sense to put a more complex and expensive switching regulator here. The stabilizer is set to an output voltage of 4,4 ... 4,7 V, which is lowered by 5 V by the VD0,5.0,7 diode to standard values for a charged lithium-ion battery (3,9 ... 4,1 V). This diode is needed so that the battery is not discharged through DA1 offline. To charge the battery, you need to apply a voltage of 1 ... 6 V to the XS12 input and forget about it for 3 ... 10 hours. With a high input voltage (more than 9 V), the DA1 chip gets very hot, so you need to either provide a heat sink or lower the input voltage. As DA1, you can use 5-volt stabilizers KR142EN5A, EN5V, 7805 - but then, to dampen the "excess" voltage, VD5 must be made up of two diodes connected in series. Transistors in this circuit can be used in almost any npn structures, KT315B are here only because the author has accumulated too many of them. KT3102, 9014, VS547, VS817, etc. will work normally. Diodes KD521 can be replaced with KD522 or 1N4148, VD1 and VD2 should be high-frequency ideal BAV70 or BAW56. VD5 any diode (not Schottky) of medium power (KD226, 1N4001). The VD4 diode is optional, it’s just that the author had too low-voltage zener diodes and the output voltage did not reach the minimum 8,5 V, and each additional diode in direct connection adds 0,7 V to the output voltage. The coil is the same as for the previous circuit (100. ..200 µH). The scheme for finalizing the multimeter switch is shown in fig. 1.8.
The positive terminal of the battery is connected to the central track-ring of the multimeter, but we connect this ring to the "+" of the battery. The next ring is the second contact of the switch, and it is connected to the multimeter circuit elements in 3-4 tracks. These tracks on the opposite side of the board must be broken and connected together, as well as with the +9 V output of the converter. The ring is connected to the +3 V converter power bus. Thus, the multimeter is connected to the output of the converter, and with the multimeter switch we turn the power of the converter on and off. We have to go to such difficulties due to the fact that the converter consumes some current (3 ... 5 mA) even with the load turned off, and the battery will be discharged by such a current in about a week. Here we turn off the power of the converter itself, and the battery will last for several months. A device correctly assembled from serviceable parts does not need to be configured, sometimes you only need to adjust the voltage with resistors R7, R8 (charger) and a zener diode VD3 (converter).
The board has the dimensions of a standard battery and is installed in the appropriate compartment. The battery is placed under the switch, usually there is enough space, you must first wrap it with several layers of electrical tape or at least tape. To connect the charger connector in the multimeter case, you need to drill a hole. The pinout for different XS1 connectors is sometimes different, so you may need to modify the board a little. So that the battery and the converter board do not "dangle" inside the multimeter, they need to be pressed with something inside the case. Authors: Koshkarov A.P., Koldunov A.S.
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