Stabilized power supplies for the player. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Stabilized power supplies for the player. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Power Supplies

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There are more and more household devices powered by 3 V (two miniature batteries or accumulators). Most cassette players, voice recorders and pocket radios are designed for autonomous operation, but in stationary conditions, in order not to discharge the batteries, it is more convenient to power them from a 220 V network through an appropriate adapter. This requires a stabilized source with a voltage of 2,4 ... 2,9 V, providing a current in the load up to 0,3 ... 0,5 A.

On sale there are many power supplies made in the form of a power plug. Some of them do not have a stabilizer and are not suitable for this application due to the eavesdropping (in pauses) network background. Good adapters in a plastic case contain a step-down transformer, a rectifier, filter capacitors and a stabilizer on the KR142EN12 chip. A typical diagram of such a power supply is shown in fig. 2.21.

Stabilized power supplies for the player
Rice. 2.21. Typical Regulated Power Supply Diagram

With a small current consumption by the load (up to 100 mA), this source works quite normally, but when it increases, the case starts to heat up very much, which is explained by the fact that for the said microcircuit to work in stabilization mode, a voltage of at least 3,8 V must drop across it (between entry and exit). At a current of 0,5 A, the power allocated to DA1 in the form of heat will be at least 1,9 W. Usually this value is even higher due to the fact that the voltage from the secondary winding of the transformer is never chosen as the minimum allowable, since instability of the mains voltage is possible (its decrease to 180 V).

The small dimensions of the adapter case do not allow for good heat dissipation for the stabilizer chip. And, despite the fact that the heating of the elements of such a power supply is acceptable, it is no secret to anyone that with increasing temperature, the reliability and service life of the source decreases.

And if you forget such an adapter plugged into the network, and even in 30-degree heat, it’s not far from a fire (not all plug-in power supplies have thermal fuses).

Most audio players do not have a reliable auto-stop at the end of the tape, in which case the current consumption increases dramatically, which, if you stay in this state for a long time, for example, if you fall asleep with the player turned on, can damage it. So, for example, the model of an audio player with a built-in receiver WALKMAN WM-FX271 (Sony) consumes currents in the following modes: radio reception of VHF stations - 50 mA; playback from a cassette - 90 mA; braked motor (at the end of the tape in the cassette) - 320 mA.

Considering all of the above, I decided to make a low-voltage power supply that will remain cold under any operating conditions, besides having a protective shutdown in case of a short circuit at the output or in case of excess current in the load of the set level, for example, when the cassette is finished.

Such a scheme is shown in Fig. 2.22.

Rice. 2.22. Schematic diagram (click to enlarge)

To reduce heat dissipation on the stabilizer, an integrated stabilizer chip was used that operates with a small voltage drop (1,1 V) between the input and output of the KR142EN22 (imported analogues from the "LOW DROP" CD, LD1085CT series are also suitable).

The operation of the PSU is controlled using two buttons SB1, SB2. A short press on SB1 supplies power to the primary winding of the transformer. At the same time, due to the current through the resistor R7 and the LED of the optocoupler switch VS1, it works, and the SB1 button circuit is blocked. The glow of the HL1 LED will remind you that the source is connected to the network. The circuit will remain in this state until we press the second button SB2 or the transistor VT1 opens. For the transistor to operate, it is necessary that the voltage across R3 exceeds 0,6 V, which occurs when the output current is higher than the value set by this resistor.

With proper installation, the circuit starts working immediately and requires only setting the output voltage to 2,8 ... 2,9 V with resistor R6, and using R3 - the protection trip current.

It is more convenient to adjust the protection operation current with a variable resistor of about 50 ohms (equivalent load) connected to the output of the stabilizer, connected in series with an ammeter. With this resistor, we set the current in the load circuit at which the protection should work, and by adjusting R3 we achieve turning off the power source (the HL1 LED goes out).

During installation, the following parts were used: tuned resistors R3 - SP5-16VA-0.5 W; R6 - SPZ-19a-0,5 W; fixed resistors MLT and S2-23; capacitors: C1 - K50-29V for 16 V; C2 and C4 - K50-35 at 6,3 V; C3 - K10-17.

Diodes VD1 ... VD4 fit any small-sized ones that have a permissible forward current of at least 1 A. It is better to use an LED from a series. KIPD.

The transistor can be used with a similar conductivity of any type, but with a large gain. The VS1 optocoupler can be replaced with 5P19T1.

I took a unified transformer from the TP121-2-6 V series and then modified it: it has a convenient design (easy access to remove extra turns from the secondary winding - this can be done without disassembling the iron). Its load characteristic is shown in Fig. 2.23 (the dashed line shows the characteristic after removing about 40 turns from the secondary winding).

Stabilized power supplies for the player
Rice. 2.23. The dependence of the voltage in the secondary winding of the transformer. TP121-2 from load current

The design of the transformer provides for its installation directly on the board. The power supply circuit board is shown in fig. 2.24. It contains all the details, with the exception of the HL1 LED. Integrated stabilizer chip DA1 is attached to a small heatsink.

Stabilized power supplies for the player
Rice. 2.24. PCB topology and element layout

Author: Shelestov I.P.

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