Conversion of computer power supplies with PWM controllers such as dr-b2002, dr-b2003, sg6105 into laboratory power supplies. Replacing diode assemblies with more powerful ones

Chip ULN2003 (ULN2003a) is essentially a set of powerful composite keys for use in inductive load circuits. Can be used to control large loads, including electromagnetic relays, motors direct current, solenoid valves, in various control circuits and others.

Chip ULN2003 - description

Brief description of ULN2003a. The ULN2003a microcircuit is a Darlington transistor assembly with high-power output switches, which has protective diodes at the outputs, which are designed to protect the control electrical circuits from the reverse voltage surge from the inductive load.

Each channel (Darlington pair) in ULN2003 is rated for 500mA load and can handle a maximum current of 600mA. Inputs and outputs are located opposite each other in the microcircuit case, which greatly facilitates wiring printed circuit board.

ULN2003 belongs to the ULN200X family of microcircuits. Different versions of this IC are designed for specific logic. In particular, the ULN2003 microcircuit is designed to work with TTL logic (5V) and CMOS logic devices. ULN2003 is widely used in control circuits of a wide range of loads, as relay drivers, display drivers, line drivers, etc. ULN2003 is also used in stepper motor drivers.

Block diagram of ULN2003

Schematic diagram

Specifications

Nominal collector current of one key - 0.5A;
Maximum output voltage up to 50 V;
Protective diodes at the outputs;
The input is adapted to all kinds of logic;
Possibility of using for relay control.

Analogue ULN2003

Below is a list of what can replace ULN2003 (ULN2003a):

Foreign analogue of ULN2003 - L203, MC1413, SG2003, TD62003.
The domestic analogue of ULN2003a is a microcircuit.

Microcircuit ULN2003 - connection diagram

The ULN2003 is often used to control a stepper motor. Below is the wiring diagram for the ULN2003a and the stepper motor.

Introduction

Great advantage computer unit power supply lies in the fact that it works stably when the mains voltage changes from 180 to 250 V, and some copies work even with a larger voltage variation. It is possible to get a useful load current of 15-17 A from a 200 W unit, and in a pulsed (short-term mode of increased load) - up to 22 A. and below, most often made on microcircuits 2003, AT2005Z, SG6105, KA3511, LPG-899, DR-B2002, IW1688. Such devices contain fewer discrete elements on the board, and have a lower cost than those built on the basis of the popular PWM - TL494 microcircuits. In this article, we will look at several approaches for repairing the aforementioned power supplies and give some practical advice.

Blocks and diagrams

The computer power supply can be used not only for its intended purpose, but also as a source for a wide range of electronic structures for the home, requiring for their work constant voltage 5 and 12 V. With a minor alteration described below, this is not at all difficult to do. And you can buy a PSU PC separately both in a store and used on any radio market (if there are not enough own "bins") for a symbolic price.

In this way, the computer's power supply compares favorably with the prospect of using a radio master in a home laboratory from all other industrial options. For example, we will take JNC units of models LC-B250ATX and LC-B350ATX, as well as InWin IP-P300AQ2, IP-P350AQ2, IP-P400AQ2, IP-P350GJ20, which use the 2003 IFF LFS 0237E chip in their design. Some others have BAZ7822041H or 2003 BAY05370332H. All these microcircuits are structurally different from each other in the purpose of the pins and the "stuffing", but the principle of operation is the same for them. So the 2003 IFF LFS 0237E microcircuit (hereinafter we will call it 2003) is a PWM (pulse width modulator of signals) in a DIP-16 package. Until recently, most budget computer power supplies manufactured by Chinese firms were based on the Texas Instruments TL494 PWM controller chip (http://www.ti.com) or its counterparts from other manufacturers, such as Motorola, Fairchild, Samsung and others. The same microcircuit has a domestic analogue of KR1114EU4 and KR1114EU3 (the pinout of the conclusions in the domestic version is different). Let's start with the methods of diagnosing and testing problems.

How to change the input voltage

The signal, the level of which is proportional to the load power of the converter, is taken from the midpoint of the primary winding of the isolation transformer T3, then through the diode D11 and the resistor R35 it is fed to the correction circuit R42R43R65C33, after which it is fed to the PR pin of the microcircuit. Therefore, in this scheme, it is difficult to establish the priority of protection for any one voltage. Here the scheme would have to be drastically changed, which is unprofitable in terms of time.

In other computer power supply circuits, for example, in LPK-2-4 (300 W), the voltage from the cathode of a double Schottky diode of the S30D40C type, an output voltage rectifier of +5 V, goes to the UVac input of the U2 microcircuit and is used to control the input supply alternating voltage BP. Adjustable output voltage can be useful for a home laboratory. For example, for power supply from a computer power supply unit of electronic devices for a car, where the voltage is on-board network(with the engine running) 12.5-14 V. The higher the voltage level, the greater the useful power of the electronic device. This is especially important for radio stations. For example, consider the adaptation of a popular radio station (transceiver) to our LC-B250ATX power supply unit - increasing the voltage on the 12 V bus to 13.5-13.8 V.

We solder a trimmer resistor, for example, SP5-28V (preferably with the index "B" in the designation - a sign of linearity of the characteristic) with a resistance of 18-22 kΩ between pin 6 of the U2 microcircuit and the +12 V bus. At the +12 V output, we install a car light bulb 5- 12 W as an equivalent load (you can also connect a 5-10 Ohm fixed resistor with a dissipated power of 5 W or more). After the considered minor revision of the power supply unit, the fan can not be connected and the board itself can not be inserted into the case. We start the power supply unit, connect a voltmeter to the +12 V bus and control the voltage. Rotating the engine variable resistor set the output voltage to 13.8 V.

Turn off the power and measure the resulting trimmer resistance with an ohmmeter. Now, between the +12 V bus and pin 6 of the U2 microcircuit, we solder a constant resistor of the corresponding resistance. In the same way, you can adjust the voltage at the +5 V output. The limiting resistor itself is connected to pin 4 of the 2003 IFF LFS 0237E microcircuit.

The principle of operation of the circuit 2003

The supply voltage Vcc (pin 1) to the U2 microcircuit comes from the standby voltage source + 5V_SB. The negative input of the IN error amplifier of the microcircuit (pin 4) receives the sum of the output voltages of the power supply +3.3 V, +5 V and +12 V. The adder is made, respectively, on resistors R57, R60, R62. The controlled zener diode of the U2 microcircuit is used in the optocoupler feedback circuit in the standby voltage source + 5V_SB, the second zener diode is used in the + 3.3V output voltage stabilization circuit. The control circuit of the output half-bridge converter of the power supply unit is made according to push-pull scheme on transistors Q1, Q2 (designation on the printed circuit board) type E13009 and transformer T3 type EL33-ASH according to the standard scheme used in computer units.

Interchangeable transistors - MJE13005, MJE13007, Motorola MJE13009 are produced by many foreign manufacturers, therefore, instead of the MJE abbreviation, the symbols ST, PHE, KSE, HA, MJF and others can be present in the transistor marking. A separate winding of the standby transformer T2, type EE-19N, is used to power the circuit. The more power the T3 transformer has (the thicker the wire is used in the windings), the greater the output current of the power supply itself. In some printed circuit boards that I had to repair, "swing" transistors were named 2SC945 and Н945Р, 2SC3447, 2SC3451, 2SC3457, 2SC3460 (61), 2SC3866, 2SC4706, 2SC4744, BUT11A, BUT12A, BUT18A, BU13005, MJ the board was listed as Q5 and Q6. And at the same time, there were only 3 transistors on the board! The very same microcircuit 2003 IFF LFS 0237E was designated as U2, and at the same time there is not a single designation U1 or U3 on the board. However, let's leave this oddity in the designation of elements on printed circuit boards on the conscience of the Chinese manufacturer. The designations themselves are not essential. The main difference between the considered power supplies of the LC-B250ATX type is the presence on the board of one microcircuit of the 2003 IFF LFS 0237E type and appearance boards.

The microcircuit uses a controlled zener diode (pins 10, 11), similar to the TL431. It is used to stabilize the 3.3 V power supply circuit. Note that in my practice of repairing power supplies, the above circuit is the weakest point in a computer PSU. However, before changing the 2003 microcircuit, I recommend that you first check the circuit itself.

Diagnostics of ATX power supplies on a 2003 chip

If the power supply does not start, then you must first remove the housing cover and check the oxide capacitors and other elements on the printed circuit board by external inspection. Oxide (electrolytic) capacitors clearly have to be replaced if their bodies are swollen and if they have a resistance of less than 100 kΩ. This is determined by "dialing" an ohmmeter, for example, the M830 model in the appropriate measurement mode. One of the most common failures of a power supply unit based on a 2003 microcircuit is the lack of a stable start. The launch is carried out by the Power button on the front panel of the system unit, while the button contacts are closed, and pin 9 of the U2 microcircuit (2003 and similar) is connected to the "case" by a common wire.

In a "braid", these are usually green and black wires. In order to quickly restore the device's operability, it is enough to disconnect pin 9 of the U2 chip from the printed circuit board. Now the power supply unit should turn on stably by pressing the key on the back panel of the system unit. This method is good in that it allows you to continue without repair, which is not always financially beneficial, to use an outdated computer power supply unit, or when the unit is used for other purposes, for example, to power electronic structures in a home radio amateur laboratory.

If you hold down the reset button before turning on the power and release it after a few seconds, the system will simulate an increase in the delay of the Power Good signal. So you can check the reasons for the failure of data loss in the CMOS (after all, the battery is not always to blame). If data, such as time, is intermittently lost, then the shutdown delay should be checked. To do this, "reset" is pressed before the power is turned off and held for a few more seconds, simulating the acceleration of the removal of the Power Good signal. If the data is saved during such a shutdown, it is a long delay during shutdown.

Increase in power

The printed circuit board contains two high-voltage electrolytic capacitors with a capacity of 220 μF. To improve filtering, attenuate impulse noise and, as a result, to ensure the stability of the computer power supply unit to maximum loads, these capacitors are replaced with analogs of higher capacity, for example, 680 μF for an operating voltage of 350 V. negates the filtering of the supply voltage. The voltage on the plates of the oxide capacitor in power supply devices is about 200 V, and the capacitance is in the range of 200-400 μF. Chinese manufacturers (VITO, Feron and others) install, as a rule, the cheapest film capacitors, not worrying much about the temperature regime or the reliability of the device. In this case, the oxide capacitor is used in the power supply device as a high-voltage power filter, therefore it must be high-temperature. Despite the operating voltage indicated on such a capacitor of 250-400 V (with a margin, as it should be), it still "hand over" due to its poor quality.

For replacement, I recommend oxide capacitors from KX, CapXon, namely HCY CD11GH and ASH-ELB043 - these are high-voltage oxide capacitors specially designed for use in electronic devices nutrition. Even if an external examination did not allow us to find faulty capacitors, the next step is still to solder the capacitors on the +12 V bus and instead of them we install analogs of a larger capacity: 4700 μF for an operating voltage of 25 V. to be replaced is shown in Figure 4. We carefully remove the fan and install it vice versa - so that it blows inward and not outward. Such modernization improves the cooling of radioelements and, as a result, increases the reliability of the device during long-term operation. A drop of machine or household oil in the mechanical parts of the fan (between the impeller and the shaft of the electric motor) will not hurt. In my experience, it can be said that the noise of the blower during operation is significantly reduced.

Replacing diode assemblies with more powerful ones

On the printed circuit board of the power supply, the diode assemblies are mounted on the radiators. In the center there is an assembly UF1002G (for 12 V power supply), on the right of this radiator there is a diode assembly D92-02, which provides power to –5 V. If such a voltage is not needed in a home laboratory, this type assembly can be irretrievably evaporated. In general, the D92-02 is designed for a current up to 20 A and a voltage of 200 V (in a pulsed short-time mode, several times higher), so it is quite suitable for installation instead of the UF1002G (current up to 10 A).

Fuji D92-02 diode assembly can be replaced, for example, by S16C40C, S15D40C or S30D40C. All of them, in this case, are suitable for replacement. Diodes with a Schottky barrier have less voltage drop and, accordingly, heating.

The peculiarity of the replacement is that the "standard" diode assembly at the output (12 V bus) UF1002G has a completely plastic composite case, therefore it is attached to a common radiator or a current-conducting plate using thermal paste. And the Fuji D92-02 diode assembly (and similar ones) has a metal plate in the case, which requires special care when installing it on a radiator, that is, through a mandatory insulating gasket and a dielectric washer for a screw. The reason for the failure of the UF1002G diode assemblies is voltage surges on the diodes with an amplitude that increases when the power supply is operating under load. At the slightest excess of the permissible reverse voltage, Schottky diodes receive an irreversible breakdown, therefore, the recommended replacement for more powerful diode assemblies in the case of the prospective use of a power supply unit with a powerful load is fully justified. Finally, there is one tip that will allow you to test the functionality of the protective mechanism. We will short-circuit a thin wire, for example, MGTF-0.8, the +12 V bus to the body (common wire). So the tension should completely disappear. To restore it, turn off the power supply unit for a couple of minutes to discharge high-voltage capacitors, remove the shunt (jumper), remove the equivalent load and turn on the power supply unit again; it will work normally. Converted in this way, computer power supplies operate for years in a 24 hour mode at full load.

Power output

Suppose you need to use the power supply for domestic purposes and you need to remove two terminals from the block. I did this using two (of the same length) pieces of unnecessary mains power wire of the computer PSU and connected all three pre-soldered cores in each conductor to the terminal block. To reduce the power loss in the conductors going from the PSU to the load, another electrical cable with a copper (less loss) multicore cable is also suitable - for example, PVSN 2x2.5, where 2.5 is the cross-section of one conductor. You can also not lead the wires to the terminal block, but connect the 12 V output in the PC power supply case to the unused connector of the PC monitor network cable.
Pin assignment of the microcircuit 2003
PSon 2 - Input of the PS_ON signal that controls the operation of the power supply unit: PSon = 0, the power supply unit is on, all output voltages are present; PSon = 1, power supply unit is off, only standby voltage + 5V_SB is present
V33-3 - Voltage input +3.3 V
V5-4 - Voltage input +5 V
V12-6 - Voltage input +12 V
OP1 / OP2-8 / 7 - Control outputs for a push-pull half-bridge power supply converter
PG-9 - Testing. Output with open collector signal PG (Power Good): PG = 0, one or more output voltages are abnormal; PG = 1, PSU output voltages are within specified limits
Vref1-11 - Controlled zener diode control electrode
Fb1-10 - Cathode controlled zener diode
GND-12 - Common wire
COMP-13 - Error amplifier output and negative input of the PWM comparator
IN-14 - Negative input of the error amplifier
SS-15 - Positive input of the error amplifier, connected to the internal source Uref = 2.5 V. The output is used to organize a "soft start" of the converter
Ri-16 - Input for connecting an external 75 kOhm resistor
Vcc-1 - Supply voltage, connected to the standby source + 5V_SB
PR-5 - Input for organizing power supply protection

Charger from a computer power supply with your own hands

Different situations require power supplies of different voltage and power. Therefore, many people buy or make one that is enough for all occasions.

And the easiest way is to take the computer as a basis. This laboratory power supply with characteristics 0-22 V 20 A redesigned with a little tweak from computer ATX on PWM 2003. For alteration I used JNC mod. LC-B250ATX. The idea is not new and there are many similar solutions on the Internet, some were studied, but the final turned out to be its own. I am very pleased with the result. Now I'm waiting for a package from China with combined voltage and current indicators, and, accordingly, I will replace it. Then it will be possible to call my development LBP - charger for car batteries.

Scheme regulated unit power supply:

First of all, I removed all the wires of the output voltages +12, -12, +5, -5 and 3.3 V. I removed all but +12 V diodes, capacitors, load resistors.

Replaced input high-voltage electrolytes 220 x 200 by 470 x 200. If there is, then it is better to put a larger capacity. Sometimes the manufacturer saves on the input filter for power supply - accordingly, I recommend re-soldering if not available.

Output choke + 12V rewound. New - 50 turns with a wire with a diameter of 1 mm, removing the old windings. The capacitor was replaced with 4,700 microfarads x 35 V.

Since the unit has a standby power supply with voltages of 5 and 17 volts, I used them to power the 2003rd and through the voltage test unit.

I applied a direct voltage of +5 volts to pin 4 from the "duty room" (that is, I connected it to pin 1). Using a resistor 1.5 and 3 kΩ voltage divider from 5 volts of the standby power, I made 3.2 and applied it to input 3 and to the right terminal of resistor R56, which then goes to pin 11 of the microcircuit.

Having installed the 7812 microcircuit at the 17 volt output from the duty room (capacitor C15), I received 12 volts and connected it to a 1 Kom resistor (without a number in the diagram), which is connected with the left end to pin 6 of the microcircuit. Also, through a 33 Ohm resistor, I fed the cooling fan, which I simply turned it over so that it blew inside. The resistor is needed in order to reduce the speed and noise of the fan.

The entire chain of resistors and diodes of negative voltages (R63, 64, 35, 411, 42, 43, C20, D11, 24, 27) was dropped from the board, pin 5 of the microcircuit was short-circuited to ground.

Added adjustment voltage and output voltage indicator from a Chinese online store. It is only necessary to power the latter from the duty room +5 V, and not from the measured voltage (it starts to work from +3 V). Power supply tests

The tests were carried out simultaneous connection of several car lamps (55 + 60 + 60) W.

This is about 15 Amperes at 14 V. I worked for 15 minutes without problems. Some sources recommend isolating the common 12 V output wire from the case, but then a whistle appears. Using the car radio as a power source, I did not notice any interference either on the radio or in other modes, and 4 * 40 W pulls perfectly. Best regards, Andrey Petrovsky.

Tell in:

The article presents a simple design of a PWM regulator, with which you can easily convert a computer power supply assembled on a controller other than the popular tl494, in particular, dr-b2002, dr-b2003, sg6105 and others, into a laboratory one with an adjustable output voltage and limiting the current in the load. Also here I will share the experience of reworking computer power supplies and describe the proven ways to increase their maximum output voltage.

But all good things come to an end sometime and recently more and more computer power supplies began to come across in which other PWM controllers were installed, in particular, dr-b2002, dr-b2003, sg6105. The question arose: how can these PSUs be used for the manufacture of laboratory IPs? The search for circuits and communication with radio amateurs did not allow progress in this direction, although it was possible to find a brief description and a circuit for switching on such PWM controllers in the article "PWM controllers sg6105 and dr-b2002 in computer power supplies." From the description it became clear that these controllers much more difficult tl494 and trying to control them from the outside to regulate the output voltage is hardly possible. Therefore, it was decided to abandon this idea. However, when studying the circuits of the "new" power supply units, it was noted that the construction of the control circuit for a push-pull half-bridge converter was carried out similarly to the "old" power supply unit - on two transistors and an isolation transformer.

An attempt was made to install the tl494 with its standard strapping instead of the dr-b2002 microcircuit, connecting the collectors of the tl494 output transistors to the transistor bases of the power supply converter control circuit. As a strapping tl494 to ensure regulation of the output voltage, the aforementioned M. Shumilov's circuit was repeatedly tested. This inclusion of the PWM controller allows you to disable all interlocks and protection schemes available in the power supply, besides, this scheme is very simple.

An attempt to replace the PWM controller was crowned with success - the power supply unit started working, adjusting the output voltage and current limiting also worked, as in the converted “old” power supply units.

Description of the device diagram

Construction and details

The PWM regulator block is assembled on a printed circuit board from one-sided foil-coated fiberglass with a size of 40x45 mm. The printed circuit board drawing and the layout of the elements are shown in the figure. The drawing is shown from the component installation side.

The board is designed for the installation of output components. There are no special requirements for them. The vt1 transistor can be replaced with any other direct conduction bipolar transistor of similar parameters. The board provides for the installation of trimming resistors r5 of different standard sizes.

Installation and commissioning

The board is fastened in a convenient place with one screw closer to the installation site of the PWM controller. The author found it convenient to attach the board to one of the power supply heatsinks. The pwm1, pwm2 outputs are soldered directly into the corresponding holes of the previously installed PWM controller - the leads of which go to the bases of the converter control transistors (pins 7 and 8 of the dr-b2002 microcircuit). The vcc output is connected to the point at which there is an output voltage of the standby power circuit, the value of which can be in the range of 13 ... 24V.

Adjustment of the MT output voltage is carried out by potentiometer r5, the minimum output voltage depends on the value of the resistor r7. The r8 resistor can be used to limit the maximum output voltage. The value of the maximum output current is regulated by the selection of the value of the resistor r3 - the lower its resistance, the greater the maximum output current of the power supply unit.

The procedure for converting a computer power supply unit into a laboratory IP

The work on alteration of the power supply unit is associated with work in circuits with high voltage, therefore, it is strongly recommended to connect the power supply unit to the network through an isolation transformer with a capacity of at least 100W. In addition, to prevent the failure of key transistors in the process of setting up the IP, it should be connected to the network through a "safety" incandescent lamp for 220V with a power of 100W. It can be soldered to the PSU instead of the mains fuse.

Before proceeding with the alteration of a computer power supply, it is advisable to make sure that it is working properly. Before switching on, 12V car bulbs with a power of up to 25W should be connected to the + 5V and + 12V output circuits. Then connect the power supply unit to the network and connect the ps-on pin (usually green) to the common wire. If the power supply unit is working properly, the "safety" lamp will flash briefly, the power supply unit will start working and the lamps in the + 5V, + 12V load will light up. If, after switching on, the "safety" lamp lights up at full heat, a breakdown of power transistors, rectifier bridge diodes, etc. is possible.

Next, you should find on the power supply board the point at which there is the output voltage of the standby power circuit. Its value can be in the range of 13 ... 24V. From this point in the future we will take power for the PWM controller unit and the cooling fan.

Then you should unsolder the standard PWM controller and connect the PWM regulator unit to the power supply board according to the diagram (Fig. 1). The p_in input is connected to the 12-volt power supply output. Now you need to check the operation of the regulator. To do this, connect a load in the form of a car light bulb to the p_out output, bring the r5 resistor slider to the left (to the minimum resistance position) and connect the power supply unit to the network (again through a “safety” lamp). If the load lamp lights up, make sure that the adjustment circuit is working. To do this, you need to carefully turn the slider of the resistor r5 to the right, while it is advisable to control the output voltage with a voltmeter so as not to burn the load lamp. If the output voltage is regulated, then the PWM regulator unit is working and you can continue to upgrade the power supply unit.

We solder all the load wires of the power supply unit, leaving one wire in the +12 V circuits and a common one for connecting the PWM controller unit. We solder: diodes (diode assemblies) in circuits +3.3 V, +5 V; rectifier diodes -5 V, -12 V; all filter capacitors. Electrolytic capacitors the filter of the +12 V circuit should be replaced with capacitors of the same capacity, but with an allowable voltage of 25 V or more, depending on the expected maximum output voltage of the manufactured laboratory power supply. Next, install the load resistor shown in the diagram in Fig. 1 as r2 required to ensure stable operation of the MT without external load. The load power should be about 1W. The resistance of the resistor r2 can be calculated based on the maximum output voltage of the power supply. In the simplest case, a 2-watt 200-300 ohm resistor is suitable.

Next, you can remove the piping elements of the old PWM controller and other radio components from the unused output circuits of the power supply unit. In order not to accidentally drop out something "useful", it is recommended to unsolder the parts not completely, but one by one, and only after making sure that the MT is working, remove the part completely. Regarding the filter choke l1, the author usually does not do anything with it and uses the standard +12 V circuit winding.This is due to the fact that, for safety reasons, the maximum output current of the laboratory power supply is usually limited to a level not exceeding the rating for the +12 V power supply circuit. ...

After cleaning the installation, it is recommended to increase the capacitance of the filter capacitor C1 of the standby power supply by replacing it with a capacitor with a nominal value of 50 V / 100 μF. In addition, if the vd1 diode installed in the circuit is low-power (in a glass case), it is recommended to replace it with a more powerful one, soldered from the rectifier of the -5 V or -12 V circuit. You should also select the resistance of the resistor r1 for comfortable operation of the cooling fan M1.

The experience of reworking computer power supplies showed that using various control schemes for a PWM controller, the maximum output voltage of the power supply will be in the range of 21 ... 22 V. This is more than enough for the manufacture of chargers for car batteries, however, it is still not enough for a laboratory power supply. To obtain an increased output voltage, many radio amateurs suggest using a bridge rectifying circuit of the output voltage, but this is due to the installation of additional diodes, the cost of which is quite high. I consider this method irrational and use another way to increase the output voltage of the power supply unit - modernization power transformer.

There are two main ways to upgrade a power transformer IP. The first method is convenient in that its implementation does not require disassembling the transformer. It is based on the fact that usually the secondary winding is wound in several wires and it is possible to "stratify" it. The secondary windings of the power transformer are schematically shown in Fig. a). This is the most common pattern. Usually, a 5-volt winding has 3 turns, wound in 3-4 wires (windings "3.4" - "common" and "common" - "5.6"), and a 12-volt winding - additionally 4 turns in one wire (windings "1" - "3.4" and "5.6" - "2").

To do this, the transformer is desoldered, the taps of the 5-volt winding are carefully unsoldered and the "pigtail" of the common wire is unwound. The task is to disconnect the parallel connected 5-volt windings and turn on all or part of them in series, as shown in the diagram in Fig. b).

It is not difficult to isolate the windings, but it is quite difficult to phase them correctly. For this purpose, the author uses a low-frequency sine signal generator and an oscilloscope or AC millivoltmeter. By connecting the output of the generator, tuned to a frequency of 30 ... 35 kHz, to the primary winding of the transformer, the voltage on the secondary windings is monitored using an oscilloscope or millivoltmeter. By combining the connection of 5-volt windings, they achieve an increase in the output voltage compared to the original by the required amount. In this way, you can achieve an increase in the output voltage of the PSU up to 30 ... 40 V.

The second way to upgrade a power transformer is to rewind it. This is the only way to obtain an output voltage of more than 40 V. The most difficult task here is to disconnect the ferrite core. The author has adopted a method of boiling a transformer in water for 30-40 minutes. But before you digest the transformer, you should think carefully about the method of separating the core, given the fact that after digestion it will be very hot, and besides, hot ferrite becomes very fragile. To do this, it is proposed to cut out two wedge-shaped strips from the tin, which can then be inserted into the gap between the core and the frame, and with their help separate the halves of the core. In case of breaking or chipping off parts of the ferrite core, you should not be particularly upset, since it can be successfully glued with cyacrylane (the so-called "superglue").

After freeing the coil of the transformer, it is necessary to wind up the secondary winding. Have pulse transformers there is one unpleasant feature - the primary winding is wound in two layers. First, the first part of the primary winding is wound on the frame, then the screen, then all the secondary windings, again the screen and the second part of the primary winding. Therefore, you need to carefully wind the second part of the primary winding, while remembering its connection and winding direction. Then remove the screen, made in the form of a layer of copper foil with a soldered wire leading to the terminal of the transformer, which must first be unsoldered. Finally, wind up the secondary windings to the next screen. Now, be sure to dry the coil well with a jet of hot air to evaporate the water that has penetrated into the winding during digestion.

The number of turns of the secondary winding will depend on the required maximum output voltage of the MT at the rate of approximately 0.33 turns / V (that is, 1 turn - 3 V). For example, the author wound 2x18 turns of PEV-0.8 wire and received the maximum output voltage of the power supply unit of about 53 V. The wire cross-section will depend on the requirement for the maximum output current of the power supply unit, as well as on the dimensions of the transformer frame.

The secondary winding is wound in 2 wires. The end of one wire is immediately sealed to the first terminal of the frame, and the second is left with a margin of 5 cm to form a "pigtail" of the zero terminal. Having finished the winding, the end of the second wire is sealed to the second terminal of the frame and a "pigtail" is formed in such a way that the number of turns of both half-windings is necessarily the same.

Now you need to restore the screen, wind up the previously wound second part of the primary winding of the transformer, observing the original connection and the direction of winding, and assemble the magnetic core of the transformer. If the wiring of the secondary winding is soldered correctly (to the terminals of the 12-volt winding), then you can solder the transformer into the power supply board and check its performance.

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The materials of this article were published in the journal Radioamator - 2013, No. 11

The article presents a simple design of a PWM regulator, with which you can easily convert a computer power supply assembled on a controller other than the popular TL494, in particular DR-B2002, DR-B2003, SG6105 and others, into a laboratory one with an adjustable output voltage and limiting the current in the load. Also here I will share the experience of reworking computer power supplies and describe the proven ways to increase their maximum output voltage.

In the amateur radio literature, there are many schemes for converting outdated computer power supplies (PSUs) into chargers and laboratory power supplies (IP). But they all relate to those PSUs in which the control unit is built on the basis of a TL494 PWM controller microcircuit, or its analogs DBL494, KIA494, KA7500, KR114EU4. We have reworked more than a dozen of these power supplies. Chargers made according to the scheme described by M. Shumilov in the article "Computer power supply - charger", (Radio - 2009, No. 1) with the addition of a pointer measuring instrument to measure the output voltage and charging current... On the basis of the same scheme, the first laboratory power supplies were manufactured until the “Universal board for control of laboratory power supplies” (Radio Yearbook - 2011, No. 5, p. 53) came into view. Much more functional power supplies could be made using this scheme. A digital ammeter was developed specifically for this regulator circuit, described in the article "A simple built-in ammeter on the PIC16F676".

But all good things come to an end someday, and recently more and more computer power supplies began to come across in which other PWM controllers were installed, in particular, DR-B2002, DR-B2003, SG6105. The question arose: how can these PSUs be used for the manufacture of laboratory IPs? The search for circuits and communication with radio amateurs did not allow progress in this direction, although it was possible to find a brief description and connection diagram of such PWM controllers in the article "PWM controllers SG6105 and DR-B2002 in computer power supplies". From the description it became clear that these controllers are much more complicated than the TL494 and it is hardly possible to try to control them from the outside to regulate the output voltage. Therefore, it was decided to abandon this idea. However, when studying the circuits of the "new" power supply units, it was noted that the construction of the control circuit for the push-pull half-bridge converter was carried out similarly to the "old" power supply unit - on two transistors and an isolation transformer.

An attempt was made to install the TL494 instead of the DR-B2002 microcircuit with its standard strapping, connecting the collectors of the TL494 output transistors to the transistor bases of the power supply converter control circuit. To ensure regulation of the output voltage, the above-mentioned M. Shumilov's circuit was repeatedly selected as the TL494 strapping. This inclusion of the PWM controller allows you to disable all interlocks and protection schemes available in the power supply, besides, this scheme is very simple.

An attempt to replace the PWM controller was crowned with success - the power supply unit started working, the output voltage regulation and current limiting also worked, as in the converted “old” power supply units.

Description of the device diagram

Construction and details

The board is designed for the installation of output components. There are no special requirements for them. Transistor VT1 can be replaced with any other bipolar direct conduction transistor of similar parameters. The board provides for the installation of trimming resistors R5 of different standard sizes.

Installation and commissioning

The board is fastened in a convenient place with one screw closer to the installation site of the PWM controller. The author found it convenient to attach the board to one of the power supply heatsinks. The outputs PWM1, PWM2 are soldered directly into the corresponding holes of the previously installed PWM controller - the leads of which go to the bases of the converter control transistors (pins 7 and 8 of the DR-B2002 microcircuit). The Vcc output is connected to the point at which there is an output voltage of the standby power circuit, the value of which can be in the range of 13 ... 24V.

The power supply output voltage is regulated by the potentiometer R5, the minimum output voltage depends on the value of the resistor R7. Resistor R8 can be used to limit the maximum output voltage. The value of the maximum output current is regulated by the selection of the value of the resistor R3 - the lower its resistance, the greater the maximum output current of the power supply unit.

The procedure for converting a computer power supply unit into a laboratory IP

The work on alteration of the power supply unit is associated with work in high voltage circuits, therefore it is strongly recommended to connect the power supply unit to the network through an isolation transformer with a capacity of at least 100W. In addition, to prevent the failure of key transistors in the process of setting up the IP, it should be connected to the network through a "safety" incandescent lamp for 220V with a power of 100W. It can be soldered to the PSU instead of the mains fuse.

Before proceeding with the alteration of a computer power supply, it is advisable to make sure that it is working properly. Before switching on, 12V car bulbs with a power of up to 25W should be connected to the + 5V and + 12V output circuits. Then connect the power supply unit to the network and connect the PS-ON pin (usually green) to the common wire. If the power supply unit is working properly, the "safety" lamp will briefly flash, the power supply unit will start working and the lamps in the + 5V, + 12V load will light up. If, after switching on, the "safety" lamp lights up at full heat, a breakdown of power transistors, rectifier bridge diodes, etc. is possible.

Then you should unsolder the standard PWM controller and connect the PWM regulator unit to the power supply board according to the diagram (Fig. 1). The P_IN input is connected to the 12-volt power supply output. Now you need to check the operation of the regulator. To do this, connect a load in the form of a car light bulb to the P_OUT output, bring the motor of resistor R5 to the left (to the position of minimum resistance) and connect the power supply unit to the network (again through a “safety” lamp). If the load lamp lights up, make sure that the adjustment circuit is working properly. To do this, you need to carefully turn the slider of the resistor R5 to the right, while it is advisable to control the output voltage with a voltmeter so as not to burn the load lamp. If the output voltage is regulated, then the PWM regulator unit is working and you can continue to upgrade the power supply unit.

We solder all the load wires of the power supply unit, leaving one wire in the +12 V circuits and a common one for connecting the PWM controller unit. We solder: diodes (diode assemblies) in circuits +3.3 V, +5 V; rectifier diodes -5 V, -12 V; all filter capacitors. The electrolytic capacitors of the +12 V circuit filter should be replaced with capacitors of the same capacity, but with an allowable voltage of 25 V or more, depending on the expected maximum output voltage of the manufactured laboratory power supply. Next, install the load resistor shown in the diagram in Fig. 1 as R2 required to ensure stable operation of the power supply without external load. The load power should be about 1W. The resistance of the resistor R2 can be calculated based on the maximum output voltage of the power supply. In the simplest case, a 2-watt 200-300 ohm resistor is suitable.

Next, you can remove the piping elements of the old PWM controller and other radio components from the unused output circuits of the power supply unit. In order not to accidentally drop out something "useful", it is recommended to unsolder the parts not completely, but one by one, and only after making sure that the MT is working, remove the part completely. Regarding the L1 filter choke, the author usually does not do anything with it and uses the standard + 12V circuit winding.This is due to the fact that, for safety reasons, the maximum output current of the laboratory power supply is usually limited to a level not exceeding the rating for the +12 V power supply circuit. ...

After cleaning the installation, it is recommended to increase the capacitance of the filter capacitor C1 of the standby power supply by replacing it with a capacitor with a nominal value of 50 V / 100 μF. In addition, if the VD1 diode installed in the circuit is low-power (in a glass case), it is recommended to replace it with a more powerful one, soldered from the -5 V or -12 V circuit rectifier. You should also select the resistance of the resistor R1 for comfortable operation of the M1 cooling fan.

The experience of reworking computer power supplies showed that using various control schemes for a PWM controller, the maximum output voltage of the power supply will be in the range of 21 ... 22 V. This is more than enough for the manufacture of chargers for car batteries, however, it is still not enough for a laboratory power supply. To obtain an increased output voltage, many radio amateurs suggest using a bridge rectifying circuit for the output voltage, but this is due to the installation of additional diodes, the cost of which is quite high. I think this method is irrational and I use another way to increase the output voltage of the power supply - modernization of the power transformer.

After freeing the coil of the transformer, it is necessary to wind up the secondary winding. Pulse transformers have one unpleasant feature - the primary winding is wound in two layers. First, the first part of the primary winding is wound on the frame, then the screen, then all the secondary windings, again the screen and the second part of the primary winding. Therefore, you need to carefully wind the second part of the primary winding, while remembering its connection and winding direction. Then remove the screen, made in the form of a layer of copper foil with a soldered wire leading to the terminal of the transformer, which must first be unsoldered. Finally, wind up the secondary windings to the next screen. Now, be sure to dry the coil well with a jet of hot air to evaporate the water that has penetrated into the winding during digestion.