Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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Many radio amateurs are engaged in the design and manufacture of power supplies for their equipment. In the process of creative searches, a dilemma inevitably arises: to install or not to install pointer electrical measuring instruments? After all, often there is not enough space on the front panel. The author of the proposed article has developed a combined measuring unit with one measuring device, which automatically selects the measurement parameter depending on the radio station operation mode.

If we turn to the range of power supplies (PSU) of well-known foreign companies, it turns out that there are devices with and without measuring instruments. For example, the YAESU FP-1030A and EURO-CB T-1250GWM models have a voltmeter and an ammeter. In simple and compact VAN SON RPS-1205 and ALAN K-35 power supplies, as well as powerful SYNC RON PS-1220 and DIAMOND GSS-3000, the developers decided to do without "frills".

Of course, measuring devices increase the size of the front panel and the overall dimensions of the power supply. At the same time, an aesthetic pointer device, becoming the compositional center of the appearance of the product, transforms the monotonous design inherent in this type of equipment. But the most valuable thing is the ability to control power parameters, which makes the operator's work more convenient and meaningful. So what to do?

A reasonable alternative is to use not two, but only one, but a combined measuring instrument - a voltammeter. For example, this is done in the DAIWA RS40-II power supply, where the V / A function is selected manually using a switch. Such a circuit solution is relatively rare, but well suited for use in radio communications.

If we analyze the nuances of working at a radio station, then the following option looks quite logical. When the transceiver is in receive (RX) mode, it is sufficient to have a voltmeter showing the voltage of the power supply. In transmit mode (TX), the current consumed by the transceiver increases several times, and an ammeter is more useful here. It is highly desirable to control the transmitter current, since, knowing the current value, one can judge the performance of the output stages and, indirectly, the state of the antenna-feeder equipment.

Taking into account the above features, a measuring unit was developed for the power supply of the CB transceiver. The device implements automatic switching of voltmeter / ammeter display modes depending on the transceiver operation mode, so the measuring unit can be called adaptive. In the RX mode, the transceiver supply voltage is controlled, and the readings are taken on a convenient and visual scale, "stretched" in the most current interval of 10 ... 15 V. In the TX mode, the current consumed by the transceiver is controlled, and readings are taken on a scale of 0,5 ...2 A.

Additional conveniences are also provided to facilitate the work of the operator: indication of the measured parameter and illumination of the instrument scale.

The scheme of the measuring unit is shown in fig. one.

In the receive mode (RX), the RA1 microammeter is turned on as a voltmeter with additional resistance Radd = R2 + R3. Precision zener diode VD1 subtracts about 9 V from the measured voltage, providing a "stretch" of the voltmeter scale. Resistors R4 and R5 have little to no effect on measurements. In this state, the key on the transistor VT1 is open, so the HL3 indicator is green. The Zener diode VD2 eliminates the slight illumination of the red crystal. When the transceiver switches to transmission mode (TX), the reed relay K1 is activated and the pointer device begins to function as an ammeter with a measuring shunt RS1. Transistor VT1 closes, and the glow of the HL3 indicator changes to red.

The HL1 and HL2 LEDs provide illumination for the pointer instrument cabinet, and also indicate the inclusion of the power supply in the network.

Most of the parts of the measuring unit are mounted on a board that is put on the input terminals of the PA1 microammeter type M42102. It has a total deflection current of 200 µA, a loop resistance of 590 ohms, and face dimensions of 80x80 mm. Other types of measuring instruments of the magnetoelectric system can be used for currents from 100 μA to 1 mA. In this case, you will need to select the elements R2, R3, RS1.

If the "stretched" scale of the voltmeter is not needed, the device can be simplified, a fragment of the circuit is shown in fig. 2.

The measuring shunt RS1 is made of manganin or constantan wire, which has a high electrical resistivity. The wire diameter is about 1 mm. On fig. 3 shows a shunt assembled from C5-5V wirewound resistors (imported analogues of SQP, KNP).

Reed relay K1 - homemade. The winding is wound on a glass container of the KEM-3 reed switch over the contact group and contains 15-20 turns of PEV-2 wire 0,51 mm.

The LEDs are installed on the scale of the pointer device. A small hole is made in the back cover of the microammeter for connecting wires. Any LEDs can be used as HL1 and HL2, but it is better to choose bright imported ones, for example, green ones. Instead of LEDs, you can install miniature baseless incandescent lamps for car radios, while the resistor R1 is not needed. Bicolor LED HL3 can be ALC331A.

The adjustment of the device begins with setting the arrow of the microammeter to the limit value of the scale at an input voltage of 15 V by adjusting the resistor R3. The number of turns and the position of the winding of the relay K1 on the reed switch are selected in such a way that the reed switch operates at a current 2 ... 3 times higher than the current consumption of the transceiver in the RX mode. Then the winding is fixed with glue. The shunt resistance is adjusted by changing the length of the wire so that the microammeter needle deviates to the limit value of the scale at a current of 2 A (the current measurement limit can be increased). If the shunt is made according to the scheme in Fig. 3, the arrow deflection is set by resistor R7.

Accurate graduation of the scales of the voltmeter and ammeter can be done using a digital multimeter (for example, M-838 from MASTECH). In the author's version, the price of divisions was 0,2 V and 0,1 A). The scale of the pointer device is individual, so you need to make it yourself using a photo method, on a computer, or carefully draw it. A possible appearance of the scale is shown in Fig. 4.

Author: A.Sokolov, Moscow

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