Power supply 220/13,8 volts 10 amps. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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The proposed power supply (Fig. 1) is designed to work with a powerful low-voltage load, for example, with VHF FM radio stations with an output power of about 50 W ("Alinco DR-130"). Its advantages are a low voltage drop across the rectifier diodes and the control transistor [1] and the presence of short circuit protection [2, 3].

(click to enlarge)

Mains voltage through the closed contacts of the switch SA1. fuse FU1 and mains filter C5-L1-L2-C6 is supplied to the winding I of the power transformer T1. From the secondary winding II T1, which has a tap from the middle, the positive voltage half-waves through the rectifier diodes VD2 and VD3 are fed to the smoothing filter capacitor C9.

A linear stabilizer with a regulating element on a field-effect transistor (FET) VT2 is connected to the filter. To control this transistor, a voltage of 2,5 ... .3 V is required, so there is no need for a separate rectifier to power the control circuits of the FET, as, for example, in [4]. To increase the stabilization coefficient in the stabilizer, an "adjustable zener diode" is used - a DA1 TL431 microcircuit (domestic analogue - KR142EN19). Transistor VT1 - matching, zener diode VD1 stabilizes the voltage in its base circuit. The output voltage of the stabilizer can be calculated using the approximate formula

The stabilizer works as follows. Let's say that when the load is connected, the output voltage decreases. Then the voltage at the midpoint of the divider R5-R6 decreases, the DA1 chip (as a parallel stabilizer) consumes less current, and the voltage drop decreases on its load (resistor R2). This resistor is in the emitter circuit of the transistor VT2 and, since the voltage at its base is stabilized by the zener diode VD1. the transistor opens more strongly, providing an increase in the voltage at the gate of the regulating transistor VT2. The latter opens more and compensates for the voltage drop at the output of the stabilizer. Thus, the stabilization of the output voltage is ensured. The output voltage is set by resistor R6. Zener diode VD6. connected between the source and gate VT2. serves to protect the FET from exceeding the permissible gate-source voltage and is an indispensable element in stabilizers with an input voltage of 15 V and higher.

This power supply is a variant of the device described in [3]. The same stabilizer with protection is used here, but the two-stage start-up of the PSU and the overvoltage protection circuit are excluded. A meter for the output voltage and load current was added to the power supply unit on the RA1 pointer device (the head of the M2001 microammeter with a total deviation current of 100 μA), an additional resistor R7, a shunt RS1, an interference suppression capacitor C12 and a switch SA2 ("Voltage / current"). Since the temperature regime of the PT in this PSU is light, the PT of the IRF2505 type was used in the TO-220 package, which has a higher thermal resistance than the IRF2505S [3].

The TN-60 transformer is found in two modifications: powered only from a 220 V network and with a combination of primary windings that allow the transformer to be connected to a network with voltages of 110.127. 220 and 237 V. The connection of the windings T1 in Fig. 1 is shown for a voltage of 237 V. This is done in order to reduce the no-load current T1, reduce the stray field and heat the transformer, and increase efficiency. In networks with low voltage (relative to 220 V), terminals 2 and 4 of the primary windings are connected to each other. Instead of the TN-60 transformer, TN-61 can be used.

To reduce the "drawdown" of the voltage under load, a rectifier circuit with a midpoint using Shot-ki diodes was used. the inclusion of the T1 windings is optimized in order to evenly distribute the load on them. The installation of the power supply circuits of the power supply unit is made with a wire with a core cross section of at least 1 mm2. The Schottky diodes are installed without gaskets on a small common radiator from an old computer monitor (aluminum plate), which, using the available pins, is soldered into the panel, on which a set of C9 capacitors (4 pieces of 10000 uFx25 V) is placed. The RS1 shunt for measuring the load current is a "positive" wire connecting the bus on the printed circuit board from the C9 pins to the load connection terminal.

Structurally, the PSU is made very simply (Fig. 2).

Its rear wall is a radiator, the front wall (panel) is a piece of duralumin of the same length and width, 4 tAtA thick. The walls are fastened together with 4 studs 07 mm made of steel. They have end holes with M4 thread. A shelf made of 4 mm thick duralumin according to the size of the transformer is screwed to the lower pins (4 M2 screws). A plate of one-sided foil fiberglass 1,5 mm thick is attached in the same way. on which capacitors C9 and a radiator with diodes VD2, VD3 are mounted. On the front panel there are two pairs of output terminals (parallel), measuring head PA1. output voltage regulator R6, current/voltage switch SA2. fuse holder FU1 and power switch SA1. The case for the PSU (U-bracket) can be bent from mild steel or assembled from separate panels. The radiator for the PT (123x123x20 mm) was used ready-made, from the power supply unit of the old Kama-R VHF radio station. The length of the fastening pins is 260 mm. but can be shortened up to 200 mm with tighter mounting. Plate dimensions: duralumin under T1 - 117,5x90x2 mm, fiberglass - 117.5x80x1,5 mm.

Line filter coils L1. L2 are wound with a flat two-wire power cord on a ferrite core (400НН.. .600НН) from the magnetic antenna of the radio receiver (before filling). Rod length - 160...180 mm, diameter - 8...10 mm. Capacitors of the K73-17 type are soldered to the coil terminals, designed for an operating voltage of at least 500 V. The assembled filter is wrapped in a non-hygroscopic material, for example, electrical cardboard, on top of which a solid tinplate screen is made. The seams of the screen are soldered, the leads pass through the insulating sleeves.

A stabilizer is good for everyone, but what happens if the load current exceeds the limit value for the regulating transistor, for example, due to a short circuit in the load? Obeying the described algorithm of work. VT2 will fully open, overheat, and quickly fail. For protection, you can apply a circuit on an optocoupler [2]. In a slightly modified form, this protection is shown in Fig. 1.

The parametric stabilizer on the VD4 zener diode provides a reference voltage of -6,2 V, voltage surges and noise are blocked by the capacitor SU. The output voltage of the stabilizer is compared with the reference voltage through the LED chain of the optocoupler VU1-VD5-R10. The output voltage of the stabilizer is higher than the reference voltage, therefore, it biases the junction of the diode VD5. locking him up. No current flows through the LED. When the output terminals of the stabilizer are shorted on the right output R10 according to the diagram, the negative voltage disappears, the reference one opens the diode VD5. The optocoupler LED lights up and the phototriac of the optocoupler fires. which closes the gate and source VT2. The control transistor closes, i.e. the output current of the stabilizer is limited. To bring it into operation after the protection is triggered, the PSU is turned off using SA1, the short circuit is eliminated and turned on again. In this case, the protection circuit returns to standby mode.

The use of such stabilizers with a low voltage drop across the FET makes it unnecessary to protect the powered equipment from overvoltage resulting from the breakdown of the control transistor. In this case, the output voltage increases by only 0.5 ... 1 V, which is usually included in the tolerance standards for most equipment.

Most of the PSU elements (circled in Fig. 1 by a dotted line) are placed on a printed circuit board 52x55 mm in size. the drawing of which is shown in Fig. 3, and the location of the parts on the board - in Fig. 4. The board is made of double-sided foil fiberglass with a thickness of 1 ... 1.5 mm. The foil on the bottom side of the board is connected to the negative output bus of the stabilizer ("grounded" in Fig. 1) with a separate wire. Free conclusions of the VU1 optocoupler can not be soldered anywhere. Holes are marked on the board at the soldering points, but mounting can be carried out from above, from the side of the printed conductors, without drilling holes. In this case, the drawing of the board corresponds to Fig.4. A drawing of the board, on which the heat sink with diodes and filter capacitors are located, is shown in Fig. 5.

Before assembling the PSU, be sure to check the ratings of all parts and their serviceability. Connections inside the PSU are made with thick wires of minimal length. In parallel with all oxide capacitors, ceramic capacitors with a capacity of 0.1 ... 0.22 μF are soldered directly to their terminals.

The current meter can be calibrated by connecting an adjustable load to the PSU output terminals in series with an ammeter for a current of 2 ... 5 A. Having set the current, for example, 2 A, by the ammeter, we select such a length of wire (shunt), twisting a loop from it so that the arrow deflects RA1 was 20 divisions (with a scale of 100).

We transfer SA2 to another position, connect a control voltmeter to the PSU output, by selecting the resistance R7 (instead of it, you can turn on a tuning resistor with a resistance of at least 220 kOhm), we achieve the coincidence of the readings of PA1 with the readings of the voltmeter.

When working with radio transmitting equipment, interference with stabilizer parts, input and output wires should be excluded. To do this, at the output terminals of the PSU, you should turn on a filter similar to a network one (Fig. 1), with the only difference that the coils must be wound on a ferrite ring or ferrite tube used in old monitors and foreign-made TVs, and contain only 2-3 a turn of insulated wire of large cross section, and capacitors can be taken with a lower operating voltage.

Literature

1. V. Nechaev. Powerful voltage regulator module on a field-effect transistor. - Radio. 2005. No. 2, p.30
2. Stabilizer with very low voltage drop. - cqham.nj/uldstab.htm
3. V. Besedin. We defend ourselves ... - Radiomir, 2008. No. 3. C.12
4. Precision heat stabilizer. - klausmobile.narod.ru/appnoIes/an_11_fetreg_r.htm

Author: V.Besedin, Tyumen

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