Nine-volt power supply Kron, 9 volts 100 milliamps. Scheme, description

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Nine-volt Kron power supply, 9 volts 100 milliamps. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Power Supplies

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Even 15-20 years ago, 9-volt Krona batteries were widely used to power portable receivers, remote controls and other portable electronics. Now this equipment is most often powered by three-volt sources (two "finger" elements), and "Krona" are used only in electrical measuring instruments, range finders, radioactivity indicators, portable metal detectors and other measuring instruments. Unfortunately, the industry does not currently produce 9V power adapters to power these devices. In any case, I have not met such adapters. Yes, and the devices themselves with a nine-volt supply do not have sockets for connecting an external source. Therefore, for mains power supply, for example, a multimeter, a small-sized source is required with dimensions comparable to those of the Krona.

The LNK501 chip is a switching power supply generator and is specially designed for building small-sized switching power supplies of low power. It is available in 8-pin DIP package (LNK501P) and 8-pin SMD package (LNK501G). Both options allow you to assemble a miniature source. By the way, the cases are actually 7-pin, since the 6th pin is missing (a pass), but the pins are counted as if there is a 6th pin.

The LNK501 chip contains a pulse-width controller with a MOSFET output. The controller circuit, together with the MOSFET, is a circuit connected in series with the load. The load is the primary winding of the pulse transformer T1. The drain of the output transistor and the power supply circuit of the generator circuit are connected to pin 5 to pins 7 connected together. 1, 2, 3, 4 - the source of the output transistor. Pin 8 is used to control the generator. The generation frequency is fixed, equal to 42 kHz. The pulse filling frequency depends on the current through pin 8. The dependence of the pulse width on the current is inverse. The microcircuit can operate within the supply DC voltage (coming from the primary rectifier) from 90 to 700 V.

Schematic diagram of the "network Krona" is shown in the figure. This source produces a stable constant voltage of 9 V at a current of 100 mA, that is, it can replace a typical Krona even with a significant current margin.

Nine-volt Kron power supply, 9 volts 100 milliamps

AC voltage from the mains is supplied to the rectifier bridge on diodes VD1-VD4. Resistor R1 serves to limit the inrush current for charging C1 and C2 when the power is turned on. The rectified voltage is smoothed by the C1-L1-C2 circuit and then fed to pin 5 A1.

The load of the output transistor A1 is the winding 1 of the transformer T1. When the output transistor A1 is open, an increasing current flows through the winding 1 T1 and the magnetic circuit accumulates energy. In this case, the diodes VD5 and VD6 are closed, since they are under reverse voltage. After closing the output transistor, the voltage in the windings changes polarity. Diodes VD5 and VD6 open, transferring voltage to the load.

The rectifier on VD5-R3-C5 is used to obtain information about the secondary voltage by the microcircuit. The magnitude of the voltage on the secondary winding is determined by the circuit by the magnitude of the rectified voltage of the primary winding. During the period of the closed state of the transistor A1, the half-wave voltage on the primary winding T1 of the capacitor C5 is charged up to 50 ... 60 V. This voltage serves as a measuring voltage, according to which the SHI A1 circuit calculates the required pulse width. measuring through the circuit R2-C3 is fed to pin 8 A1. Resistor R2 together with the internal resistance of pin 8 A1 forms a voltage divider. You can adjust the output voltage by selecting the resistance R2.

Thus, stabilization of the output voltage at C4 is achieved. But. the change in the feedback current obtained by rectifying the voltage from the primary winding in light load mode does not depend much on the real voltage on the rectifier of the secondary winding. As a result, at a nominal output voltage of 9 V at idle (and at low current consumption), the voltage almost doubles. and rapidly decreases in the current range from zero to 20...30 mA. With a further increase in the load current, the decrease in voltage is no longer so noticeable, although it also takes place, since at a current of 100 mA it will already be below 9 V.

These changes will be very significant when powering portable devices with LCD indicators that consume minimal currents. Therefore, in order to ensure the stability of the final output voltage, a set of measures has been taken in the circuit. Firstly, the output of the secondary rectifier is loaded with the HL1 LED, which prevents the power supply from idling. The presence of this LED enters the power supply into a relatively stable mode with a voltage at the rectifier output of 11 ... 13 V. Secondly, after the rectifier, the integral stabilizer A2 is turned on, which maintains the already obtained output voltage at a stable level of 9 V.

By the way, this source can also be converted to another output voltage, for example, to 5, using the appropriate stabilizer in place A2, or to make an adjustable output voltage using an integral stabilizer with output voltage adjustment in place A2.

Transformer T1 is wound on a frame with an EF12.6 core from EPCOS. Primary winding - 130 turns of wire PEV 0,09. Then a layer of fluoroplast film (insulation from the MGTF wire is used as it) Secondary winding - 25 turns of PEV 0,25 wire. The frame of the transformer is very small, so the winding must be wound tightly turn to turn, but do not overtighten the wire so as not to break the insulation.

Inductor L1 - ready-made small-sized inductance 100-500 μH.

The bridge rectifier diodes VD1-VD4 can be replaced by others with a maximum reverse voltage of at least 500 V and a current of at least 0,3 A, for example, 1N4007, or you can use a rectifier bridge like DB105, DB106, DB107 (this is even preferable from the point of view of minimization) .

The 1N4937 diode can be replaced with a KD127A, KD247G or other silicon diode with a reverse recovery time of no more than 250 ns, with a reverse voltage of at least 600 V.

Diode 1N5819 can be replaced with KD106 KD247A KD247E or another with a reverse recovery time of no more than 500 ns and a reverse voltage of at least 40 V.

The scheme of the secondary stabilizer can be solved differently. At low load currents, you can use a parametric stabilizer on a zener diode and a resistor, or you can make a single-transistor parametric stabilizer according to a typical circuit.

The housing of the power supply unit is a housing from a spent battery of the "Krona" type. It is necessary to remove all the contents, thoroughly clean the body of oxides, and cover it from the inside with a good layer of insulation, which can be used as an epoxy varnish. The contact socket is previously removed and used when mounting the block. In the middle of this socket between the contacts, you can make a small hole through which the LED will be visible.

The installation of the power supply unit is made in a volumetric way "in the air", tightly. but so that the network circuits are not dangerously close to the secondary ones. During the installation process, adhere to the geometric dimensions of the "Krona", so that the resulting "lump" fits freely in its body. Then the "lump" is checked in operation and adjusted, if necessary. After that, it is placed in a Krona case and washed down with epoxy or some kind of insulating sealant. After the filling has completely hardened, the block is ready for use.

The block is installed in the battery compartment of the device instead of the "Krona". You will need to cut a groove in the battery compartment cover to bring the power cord out.

Author: Mokhov A.A.

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