Charger for multimeter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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The use of a nickel-metal hydride battery with a converter to power the multimeter [1] allows you to significantly save on rather expensive batteries. However, the battery still needs to be recharged from time to time. Many devices have been developed for charging batteries, but most of them are quite complex due to their versatility. In addition, some require constant monitoring, since their operation does not rule out overcharging the battery, leading to overheating and reduced service life.

In many cases, it is quite possible to get by with a simple set-top box powered by a mobile phone charger (charger). As a rule, the charger is a fairly powerful and small-sized, and in most models even a stabilized power supply, equipped with protection for the current consumed by the load. Most of the time, the memory usually lies idle, and it makes sense to find additional uses for it.

The proposed attachment is a voltage stabilizer and is assembled on two transistors. At first, the charging current of a discharged battery is constant, and then, as it is being charged, it decreases according to a law close to exponential [2], and when the battery is fully charged, it is limited to a safe level. The prefix is ​​designed to be powered by a charger for a FLY phone with a stabilized output voltage of 5 V. Of course, chargers of other phones will also work. The attachment diagram is shown in fig. 1.

Fig. 1

A control element is assembled on the transistor VT2, a control element is assembled on the transistor VT1. The stabilization voltage is determined by the sum of the voltage drop across the diode VD1 and at the emitter junction of the transistor VT1, which eliminates the need for a resistive divider at the output of the set-top box. With the elements indicated in the diagram, the output voltage is approximately 1,25 ... 1,3 V. Within a small range, it can be changed using other types of diodes. In addition, the output voltage is affected by the current through the resistor R2.

Resistor R3 serves to limit the charging current. The use of a resistor is due to its higher reliability compared to a transistor. In addition, in the event of a resistor failure, the battery is practically disconnected from the memory. With the resistance of the resistor R3 indicated in the diagram, the output current of the set-top box is limited to approximately 100 mA.

The set-top box works like this: when power is applied, if the battery is discharged, the transistor VT1 is closed. Resistor R2 determines the base current of transistor VT2, which is in saturation, and the output current of the attachment is determined by the resistance of resistor R3. As the battery charges, the voltage at the base of transistor VT1 increases and it begins to open. In this case, the transistor VT2 first comes out of saturation, and then gradually closes, providing an "exponential" output characteristic of the set-top box.

When the battery is fully charged, the transistor VT2 is closed, the current of the resistor R2 flows through the open transistor VT1 and the diode VD1. The latter circumstance imposes some restrictions on the operation of set-top boxes with different memory devices. The fact is that many memory devices of especially cheap models can have a voltage spread from 4,6 to 9 V, that is, almost twice. In this case, the output voltage of the set-top box can vary from 1,2 to 1,5 V, which, of course, is unacceptable. The charging current can also change significantly. In this case, the resistor R2 must be replaced by a current generator (approximately 3 ... 5 mA), for example, on a field effect transistor. The rest of the elements do not require special explanations: the resistor R1 and the HL1 LED are used to control the supply voltage (many memory devices do not have it), the resistor R4 and the RA1 microammeter - to control the current and the degree of battery charge.

The prefix uses MLT resistors, except for the R3 resistor - it is imported with a power of 2 watts. Instead of KT315I (VT1), you can use any transistors of the KT315, KT3102 series, and instead of KT630A (VT2) - any KT630 ​​series and powerful KT815, KT817. The current meter uses the M88501 recording level indicator with a total deviation current of 300 μA from the tape recorder. The scale of the microammeter is calibrated by selecting the resistor R4. The final division of the scale corresponds to a current of 100 mA. The XS1 connector can be anything, the XP1 connector will have to be chosen similar to the phone or memory connector. All parts of the attachment are mounted on a printed circuit board made of fiberglass laminated on one side, the drawing of which is shown in Fig. 2. The board is made by cutting conductors with a scalpel or cutter. It is housed in a case glued from polystyrene 3 mm thick, its appearance is shown in fig. 3.

Fig. 2

Fig. 3

The set-top box is set up in the following order: power is supplied to the set-top box input and the voltage at its output is checked. It should be about 1,3 V. Of course, the HL1 LED should be on. If the voltage is very different from the specified one, you can try to pick up diodes of other series instead of KD510A or pick up resistor R2. Then the output of the set-top box is closed with an ammeter to a current of 1 A. If the charging current is too high, you can increase the resistance of the resistor R3. Then, by selecting the resistor R4, the pointer of the microammeter RA1 is set to the final division and the scale is calibrated.

It should be noted that the scale of the M88501 microammeter used is non-linear, so the measurement error can reach 10...12%. Since the microammeter is used, rather, as an indicator of battery charging, it is possible to completely abandon the numerical graduation, replacing it with a color one: the area between zero and the first scale divisions (Fig. 3) is painted over with green, between the 70 and 100 mA marks - with red, the rest of the scale - yellow. It should be noted that such devices were produced with a variety of scales, including in the form of colored sectors or a gradually expanding strip. In such cases, it is convenient to use an existing scale, simply by rewriting the numbers on it or by painting over already finished sections.

The prefix has been in operation for more than a year together with the converter [1] and has never caused any complaints.

Note. The voltage of 1,25 ... 1,3 V indicated in the article is not enough to fully charge the nickel-metal hydride battery. To fully charge such a battery, a voltage of 1,38 ... 1,45 V is required (depending on the specific instance). To do this, the KD510A (VD1) diode can be replaced with two or three Schottky diodes, for example 1N5817, or with a resistor, choosing its resistance.

Literature

1. Gerasimov E. Converter for powering a digital multimeter. - Radio, 2014, No. 9, p. 20, 21.
2. Dorofeev M. Charger option. - Radio, 1993, No. 2, p. 12, 13.

Author: E. Gerasimov

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