Circuit from 12 volts to 3 volts. Power unit. Transformer power supply on KT808

Repairing an amplifier reproduced from a foreign-made player is often difficult due to the use of a low-voltage microcircuit in it, the analogue of which is very difficult to find. Therefore, you have to make a new design using transistors or microcircuits of domestic production, but in this case the radio amateur experiences certain difficulties in choosing the desired circuit with a low value power supply voltage. For example, when repeating the circuits described in, it is necessary to use 53 radio components in the microcircuit version or 72 radio components in the transistor version. It is better to use a simplified scheme. This circuit has obvious advantages - one active element (K157UD2 microcircuit), a small number of parts used, and fairly good characteristics. But there is one significant and seemingly insurmountable drawback for a low-voltage player: the high supply voltage of the microcircuit (in this amplifier 9V). There is a way out of this situation - to use a converter of the player’s primary power supply voltage, usually 3 V, to a secondary, higher voltage, from which the amplifier is powered. In this version, the design will require only 10 elements for the converter and 21 for the amplifier.

The developed version of the power converter for the player's playback amplifier (the commutator motor is powered directly from the current source) has the following technical characteristics:

Output voltage, V, with an output current of 15 mA and an input voltage of 2-3 V.................... 7 - 10

Secondary voltage ripple factor, %, no more.................................................... ..........0.001

Conversion frequency, kHz................................................... ........................................................ .........100...200

Efficiency, %, not less................................................... ........................................................ ................................... 55

Dimensions, mm................................................... ........................................................ ................................14x10x10

The voltage converter is built according to the circuit of a push-pull generator (Fig. 1), which made it possible to obtain a fairly high efficiency. The role of switches is performed by transistors VT1 and VT2, which alternately open and close like transistors of a symmetrical multivibrator. The phasing of their operation is carried out by correspondingly switching on the collector and base windings of transformer T1. The voltage divider R2R1 ensures the start of the converter. When the supply voltage is turned on, the voltage drop across resistor R2 (about 0.7 V) is applied to the bases of the transistors and opens them. Due to the scatter in the parameters of the transistors, the collector currents (and the currents in the collector windings of transformer T1) cannot be exactly the same, and an increase in the current in one of the generator arms leads to the appearance of positive feedback to the base of this transistor and, as a consequence, an avalanche-like increase in the current until it saturates . When the rate of rise of current in the collector winding decreases, the back EMF creates a positive connection to the base of the transistor of the other arm, the collector current in the first arm decreases and increases like an avalanche in the collector circuit and the winding of the other transistor. Thus, a time-varying magnetic flux is induced in the magnetic core of the transformer, which will create an EMF in the secondary winding (pins 7-8). The diode bridge VD1 - VD4 converts alternating voltage into pulsating voltage, and its smoothing is carried out by elements of the power circuit of the playback amplifier. In the converter device, capacitor C1 increases the reliability of the self-excitation process.

The design uses the most common KT315 transistors, and you can take transistors with any letter index and parameter h 21E >50. However, you should not choose transistors with too large h 21E, since this will reduce the efficiency of the device. The use of other transistors (except KT373G) is undesirable, since the saturation voltage of the collector-emitter junction of the recommended transistors is only 0.4 V, and they are small in size. Any small-sized resistors and capacitor. The transformer is made on a ring magnetic core K7X4X2 made of ferrite grades 600NN, 400NN. The collector winding is wound in two wires (0.2 mm in diameter) and contains 11 turns, and the base winding (also in two wires with a diameter of 0.13 mm) has 17 turns. The secondary (output) winding contains 51 turns of wire with a diameter of 0.13 mm. Winding is done in bulk using PEV or PEL wire. Instead of KD522B diodes, small-sized germanium diodes can be used, with a corresponding change in the number of turns of the transformer. This will even lead to an increase in converter efficiency by 10-15%. If the converter uses a full-wave rectification circuit with output from the middle point of the secondary winding, this will reduce the number of diodes by two and further increase the efficiency, since one rectifying diode will be connected in series with the load (amplifier) instead of two. In this case, it is necessary to recalculate the converter.

The converter can be mounted in any way; its parts can be placed on the same board with the amplifier parts or designed as a separate block. In the author's design, the second option was used (Fig. 2). The parts of the converter are glued together into a three-dimensional structure consisting of three layers. The first layer is capacitor C1 and resistors R1, R2. The second is a transformer and a diode bridge, soldered from VD1-VD4. The third is transistors VT1, VT2, soldered together by the emitter terminals. Before installing transistors, to reduce the size of the block, they should be ground off from the sides to a length of 7 mm. The transformer leads are soldered directly to the parts leads. The remaining connections are made with thin conductors. After this, you should solder the input and output conductors and check the operation of the unit. If you use serviceable elements and correctly perform the installation, the structure will work immediately. If this does not happen, then you need to check the correct connection of the transformer windings. After this, the entire structure should be filled with epoxy resin. A fully manufactured and tested unit is placed in a box made of thin paper; holes for the leads are first made in it and the volume is filled with compound.

How to get a non-standard voltage that does not fit into the standard range?

Standard voltage is the voltage that is very commonly used in your electronic gadgets. This voltage is 1.5 Volts, 3 Volts, 5 Volts, 9 Volts, 12 Volts, 24 Volts, etc. For example, your antediluvian MP3 player contained one 1.5 Volt battery. The TV remote control already uses two 1.5 Volt batteries connected in series, which means 3 Volts. In the USB connector, the outermost contacts have a potential of 5 Volts. Probably everyone had a Dandy in their childhood? To power Dandy, it was necessary to supply it with a voltage of 9 volts. Well, 12 Volts are used in almost all cars. 24 Volt is already used mainly in industry. Also, for this, relatively speaking, standard series, various consumers of this voltage are “sharpened”: light bulbs, record players, etc.

But, alas, our world is not ideal. Sometimes you just really need to get a voltage that is not from the standard range. For example, 9.6 Volts. Well, neither this way nor that... Yes, the power supply helps us out here. But again, if you use a ready-made power supply, then you will have to carry it along with the electronic trinket. How to solve this issue? So, I will give you three options:

Option #1

Make a voltage regulator in the electronic trinket circuit according to this scheme (in more detail):

Option No. 2

Build a stable source of non-standard voltage using three-terminal voltage stabilizers. Schemes to the studio!

What do we see as a result? We see a voltage stabilizer and a zener diode connected to the middle terminal of the stabilizer. XX are the last two digits written on the stabilizer. There may be numbers 05, 09, 12, 15, 18, 24. There may already be even more than 24. I don’t know, I won’t lie. These last two digits tell us the voltage that the stabilizer will produce according to the classic connection scheme:

Here, the 7805 stabilizer gives us 5 Volts at the output according to this scheme. 7812 will produce 12 Volts, 7815 - 15 Volts. You can read more about stabilizers.

U Zener diode – this is the stabilization voltage on the zener diode. If we take a zener diode with a stabilization voltage of 3 Volts and a voltage regulator 7805, then the output will be 8 Volts. 8 Volts is already a non-standard voltage range ;-). It turns out that by choosing the right stabilizer and the right zener diode, you can easily get a very stable voltage from a non-standard range of voltages ;-).

Let's look at all this with an example. Since I simply measure the voltage at the terminals of the stabilizer, I do not use capacitors. If I were powering the load, then I would also use capacitors. Our guinea pig is the 7805 stabilizer. We supply 9 Volts from the bulldozer to the input of this stabilizer:

Therefore, the output will be 5 Volts, after all, the stabilizer is 7805.

Now we take a zener diode for U stabilization = 2.4 Volts and insert it according to this circuit, it is possible without capacitors, after all, we are just measuring the voltage.

Oops, 7.3 Volts! 5+2.4 Volts. Works! Since my zener diodes are not high-precision (precision), the voltage of the zener diode may differ slightly from the nameplate (voltage declared by the manufacturer). Well, I think it's no problem. 0.1 Volt will not make a difference for us. As I already said, in this way you can select any value out of the ordinary.

Option #3

There is also another similar method, but here diodes are used. Maybe you know that the voltage drop across the forward junction of a silicon diode is 0.6-0.7 Volts, and that of a germanium diode is 0.3-0.4 Volts? It is this property of the diode that we will use ;-).

So, let's get the diagram into the studio!

We assemble this structure according to the diagram. The unstabilized input DC voltage also remained 9 Volts. Stabilizer 7805.

So what's the outcome?

Almost 5.7 Volts;-), which was what needed to be proven.

If two diodes are connected in series, then the voltage will drop across each of them, therefore, it will be summed up:

Each silicon diode drops 0.7 Volts, which means 0.7 + 0.7 = 1.4 Volts. Same with germanium. You can connect three or four diodes, then you need to sum the voltages on each. In practice, more than three diodes are not used. Diodes can be installed even at low power, since in this case the current through them will still be small.

How to get a non-standard voltage that does not fit into the standard range?