12 volt power supply with short circuit protection. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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This power supply is easy to repeat, reliably protected from accidental short circuits, has a smooth adjustment of the output voltage from "zero", transistor collectors are attached directly to the radiator or case (chassis ground).

The block consists of a step-down transformer, a rectifier, a comparing device on an operational amplifier, which controls a composite transistor and a protection unit with its current consumption (Fig. 1).

Fig. 1.

The step-down transformer should be checked for its power output. To do this, the primary winding is connected through a fuse to a 220 volt network, having previously insulated all open wiring sections. The alternating voltage on the secondary winding should not exceed 20 volts, otherwise, after the rectifier, the direct voltage on the electrolytic capacitor will exceed 30 volts, the limit for the operational amplifier microcircuit. A voltmeter is connected in parallel to the terminals of the secondary winding of the transformer and briefly short-circuited with a powerful 20 ohm resistor. The current through the resistor will be approximately 1 amp. Usually this is enough, but "a matter of taste." If the voltmeter readings have changed slightly and such power is satisfactory, the test is completed.

In the rectifier, it is better to use the KTs-402 or KTs-405 microassembly with any letter index. Then the constant voltage at the output will be more "beautiful" due to the same parameters of the bridge diodes. When high block currents are required, the rectifier bridge is assembled from separate powerful diodes.

The comparator (see Fig. 1) consists of an operational amplifier DA1 and a measuring bridge formed by resistors R5-R7 and a zener diode VD2. A change in voltage at the output of the power supply leads to an imbalance in the measuring bridge. The operational amplifier amplifies the unbalance voltage by changing the voltage across the load resistance R4, but since this load is constant, the current passing through the microcircuit changes. This current is perfectly suitable for driving a regulating transistor, since a transistor, in general, is a current element. The idea of ​​a non-standard inclusion of an operational amplifier is taken from [1]. Any operational amplifier can be used in the comparing device, especially if the unit is used as an unregulated voltage regulator in any device. The voltage at the output of the block will be equal to twice the stabilization voltage of the applied zener diode (this ratio can be changed by resistors R5 and R6). If you need to stabilize the voltage of more than 30 volts, then you need to install a VD3 zener diode (shown in dotted line), which will extinguish the excess voltage at the op-amp. In this case, the resistance of the resistor R7 must be designed for the rated operating current of the Zener diode VD2. An open-loop op amp may be energized, requiring the insertion of capacitor C4.

Not all op amps are suitable for the adjustable block option (see Figure 2). It is necessary to ensure that when the output voltage is reduced to "zero" by the potentiometer R7, the stabilization process does not fail. Otherwise, the full voltage from the rectifier will appear at the output of the unit.

Fig. 2.

The protection unit consists of a shunt and a trinistor 2U107A. The current passing through the shunt creates a proportional voltage drop across it. As soon as the voltage reaches a certain level, the trinistor will open and unbalance the balancing bridge R5-R8 (Fig. 2). Then the composite transistor VT1-VT2 will close and the current through the block load will stop. To return the protection to its original state, the SB1 button is used. You should not use a toggle switch or a switch here: you can forget to turn on the protection. If you want to get the maximum current, you can simply keep the button pressed. A piece of manganin wire was used as a shunt. The cross section and length of the wire are selected experimentally depending on the required current and the protection threshold. Trinistor 2U107A turned out to be the most successful choice in terms of sensitivity, speed and reliability of operation. Other trinistors did not give the desired result.

A composite transistor can be assembled from any transistors, subject to the general rules, for example: VT1-KT808A, VT2-KT815A. Trimmer resistance R3 (Fig. 1) is used to adjust the composite transistor for maximum current output. To do this, you should short-circuit the output of the power supply with a load resistance (for example, 12 ohms) and set R3 to a smaller deviation of the output voltage.

Based on the above, a bipolar laboratory power supply was assembled (see Fig. 3 and photo 1-3). The upper stabilizer according to the scheme is convenient to use without protection. Together with the lower stabilizer, you can get voltage up to 25 volts, plus overload protection. Transistor VT1 must be isolated from the radiator with a mica gasket.

Rice. 3 (click to enlarge)

The details of the power supply are assembled on a printed circuit board measuring 80x110 mm. The body of the block is made of one-sided foil fiberglass with a size of 235x100x160 mm. The body parts are fastened together with tin. The top cover of the case is reinforced with triangular gussets. The front and rear walls are fastened to the pallet with rectangles. Holes are drilled in them and M3 nuts are soldered from the inside to secure the cover.

Fig. 4.

The false panel is attached to the front panel with a screw and nut through a hole drilled in the middle. LEDs are displayed on the false panel: red - lights up when the protection is triggered, green - indicates that the unit is connected to the network. Holes are cut out for the voltmeter and milliammeter. The milliammeter is adjusted by a shunt for the full deviation of the arrow and the protection operation at a current of 300 milliamps. Such protection works instantly and saved more than one device.

Fig. 5.

On the rear panel there are radiators with transistors VT1 and VT3, a fuse, output voltage terminals, a toggle switch for turning on the power supply to the network, a toggle switch for switching a voltmeter, and a "Reset protection" button.

Literature

1. Magazine "Radio", 1986 No. 9, p. 48.

Author: M. Faizullin (UA9WNH/9), faizul@rambler.ru; Publication: cxem.net

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