Converting an atx unit into a laboratory power supply. Converting an ATX power supply into an adjustable one. Let's see what the power supply looks like in operation
Hello, now I’ll talk about converting the ATX power supply of the codegen 300w 200xa model into a laboratory power supply with voltage regulation from 0 to 24 Volts, and current limitation from 0.1 A to 5 Amperes. I’ll post the diagram that I came up with, maybe someone will improve or add something. The box itself looks like this, although the sticker may be blue or a different color.
Moreover, the boards of the 200xa and 300x models are almost the same. Under the board itself there is an inscription CG-13C, maybe CG-13A. Perhaps there are other models similar to this one, but with different inscriptions.
Soldering unnecessary parts
Initially the diagram looked like this:
You need to remove all unnecessary wires from the atx connector, unsolder and wind up unnecessary windings on the group stabilization choke. Under the choke on the board, where it says +12 volts, we leave that winding, we wind up the rest. Unsolder the braid from the board (main power transformer); under no circumstances bite it off. Remove the radiator along with the Schottky diodes, and after we remove everything unnecessary, it will look like this:
The final circuit after the rework will look like this:
In general, we solder all the wires and parts.
Making a shunt
We are making a shunt from which we will relieve tension. The meaning of the shunt is that the voltage drop across it tells the PWM about how current-loaded the power supply output is. For example, we got the shunt resistance to be 0.05 (Ohm), if we measure the voltage on the shunt at the moment of passing 10 A, then the voltage on it will be:
U=I*R = 10*0.05 = 0.5 (Volts)
I won’t write about the manganin shunt, because I didn’t buy it and I don’t have one, I used two tracks on the board itself, we close the tracks on the board as in the photo to get a shunt. It’s clear that it’s better to use manganin, but it works more than fine.
We install inductor L2 (if any) after the shunt
In general, they need to be calculated, but if anything happens, somewhere on the forum there was a program for calculating chokes.
We apply a common minus to PWM
You don’t have to apply it if it’s already ringing on the 7th PWM leg. It’s just that on some boards there was no general negative on pin 7 after desoldering the parts (I don’t know why, I could be mistaken that there wasn’t one :)
Solder the PWM wire to pin 16
We solder a PWM wire to pin 16, and feed this wire to pins 1 and 5 of the LM358
Between 1 PWM leg and the plus output, solder a resistor
This resistor will limit the voltage output from the power supply. This resistor and R60 form a voltage divider that will divide the output voltage and supply it to 1 leg.
The op-amp (PWM) inputs on the 1st and 2nd legs are used for the output voltage task.
The task of the output voltage of the power supply unit comes to the 2nd leg, since a maximum of 5 volts (vref) can arrive at the second leg, then the reverse voltage should also arrive at the 1st leg no more than 5 volts. For this, we need a voltage divider made of 2 resistors, R60 and the one that we will install from the power supply output to 1 leg.
How it works: let’s say a variable resistor is set to 2.5 Volts on the second leg of the PWM, then the PWM will produce such pulses (increase the output voltage from the power supply output) until 1 leg of the op-amp reaches 2.5 (volts). Let’s say if this resistor is missing, the power supply will reach the maximum voltage, because there is no feedback from the power supply output. The resistor value is 18.5 kOhm.
We install capacitors and a load resistor at the power supply output
The load resistor can be set from 470 to 600 Ohm 2 Watts. Capacitors of 500 microfarads for a voltage of 35 volts. I didn’t have capacitors with the required voltage, so I installed 2 in series at 16 volts 1000 uF. We solder the capacitors between 15-3 and 2-3 PWM legs.
Soldering the diode assembly
We install the diode assembly that was 16C20C or 12C20C, this diode assembly is designed for 16 amperes (12 amperes, respectively), and 200 volts of reverse peak voltage. The 20C40 diode assembly will not suit us - don’t think about installing it - it will burn out (checked :)).
If you have any other diode assemblies, make sure that the reverse peak voltage is at least 100 V and for the current, whichever is greater. Ordinary diodes will not work - they will burn out; these are ultra-fast diodes, just for a switching power supply.
Place a jumper for PWM power supply
Since we removed the piece of the circuit that was responsible for supplying power to the PSON PWM, we need to power the PWM from the 18 V standby power supply. Actually, we install a jumper instead of the Q6 transistor.
Solder the power supply output +
Then we cut the common minus that goes to the body. We make sure that the common negative does not touch the housing, otherwise by shorting the positive with the power supply housing, everything will burn out.
Solder the wires, common minus and +5 Volts, power supply control output
We will use this voltage to power the volt-ampere meter.
Solder the wires, common negative and +18 volts to the fan
We will use this wire through a 58 Ohm resistor to power the fan. Moreover, the fan must be turned so that it blows on the radiator.
Solder the wire from the transformer braid to the common minus
Solder 2 wires from the shunt for the LM358 op-amp
We solder the wires, as well as resistors to them. These wires will go to the LM357 op-amp through 47 Ohm resistors.
Solder the wire to the 4th leg of the PWM
With a positive +5 Volt voltage at this PWM input, there is a limitation of the control limit at the outputs C1 and C2, in this case, with an increase in the DT input, the duty cycle at C1 and C2 increases (you need to look at how the transistors at the output are connected). In a word - stop the power supply output. We will use this 4th PWM input (we will supply +5 V there) to stop the power supply output in the event of a short circuit (above 4.5 A) at the output.
Assembling a current amplification and short circuit protection circuit
Attention: this is not the full version - see the forum for details, including photos of the alteration process.
Discuss the article LABORATORY PSU WITH PROTECTION FROM A REGULAR COMPUTER
A good laboratory power supply is quite expensive and not all radio amateurs can afford it.
Nevertheless, at home you can assemble a power supply with good characteristics, which can cope well with providing power to various amateur radio designs, and can also serve as a charger for various batteries.
Such power supplies are assembled by radio amateurs, usually from , which are available and cheap everywhere.
In this article, little attention is paid to the conversion of the ATX itself, since converting a computer power supply for a radio amateur of average qualification into a laboratory one, or for some other purpose, is usually not difficult, but beginning radio amateurs have many questions about this. Basically, what parts in the power supply need to be removed, what parts should be left, what should be added in order to turn such a power supply into an adjustable one, and so on.
Especially for such radio amateurs, in this article I want to talk in detail about converting ATX computer power supplies into regulated power supplies, which can be used both as a laboratory power supply and as a charger.
For the modification, we will need a working ATX power supply, which is made on a TL494 PWM controller or its analogues.
The power supply circuits on such controllers, in principle, do not differ much from each other and are all basically similar. The power of the power supply should not be less than that which you plan to remove from the converted unit in the future.
Let's look at a typical ATX power supply circuit with a power of 250 W. For Codegen power supplies, the circuit is almost no different from this one.
The circuits of all such power supplies consist of a high-voltage and low-voltage part. In the picture of the power supply printed circuit board (below) from the track side, the high-voltage part is separated from the low-voltage part by a wide empty strip (without tracks), and is located on the right (it is smaller in size). We will not touch it, but will work only with the low-voltage part.
This is my board and using its example I will show you an option for converting an ATX power supply.
The low-voltage part of the circuit we are considering consists of a TL494 PWM controller, an operational amplifier circuit that controls the output voltages of the power supply, and if they do not match, it gives a signal to the 4th leg of the PWM controller to turn off the power supply.
Instead of an operational amplifier, transistors can be installed on the power supply board, which in principle perform the same function.
Next comes the rectifier part, which consists of various output voltages, 12 volts, +5 volts, -5 volts, +3.3 volts, of which for our purposes only a +12 volt rectifier will be needed (yellow output wires).
The remaining rectifiers and accompanying parts will need to be removed, except for the “duty” rectifier, which we will need to power the PWM controller and cooler.
The duty rectifier provides two voltages. Typically this is 5 volts and the second voltage can be around 10-20 volts (usually around 12).
We will use a second rectifier to power the PWM. A fan (cooler) is also connected to it.
If this output voltage is significantly higher than 12 volts, then the fan will need to be connected to this source through an additional resistor, as will be later in the circuits under consideration.
In the diagram below, I marked the high-voltage part with a green line, the “standby” rectifiers with a blue line, and everything else that needs to be removed with red.
So, we unsolder everything that is marked in red, and in our 12 volt rectifier we change the standard electrolytes (16 volts) to higher voltage ones, which will correspond to the future output voltage of our power supply. It will also be necessary to unsolder the 12th leg of the PWM controller and the middle part of the winding of the matching transformer - resistor R25 and diode D73 (if they are in the circuit) in the circuit, and instead of them solder a jumper into the board, which is drawn with a blue line in the diagram (you can simply close diode and resistor without soldering them). In some circuits this circuit may not exist.
Next, in the PWM harness on its first leg, we leave only one resistor, which goes to the +12 volt rectifier.
On the second and third legs of the PWM, we leave only the Master RC chain (in the diagram R48 C28).
On the fourth leg of the PWM we leave only one resistor (in the diagram it is designated as R49. Yes, in many other circuits between the 4th leg and the 13-14 legs of the PWM there is usually an electrolytic capacitor, we don’t touch it (if any) either, since it is designed for a soft start of the power supply. My board simply didn’t have it, so I installed it.
Its capacity in standard circuits is 1-10 μF.
Then we free the 13-14 legs from all connections, except for the connection with the capacitor, and also free the 15th and 16th legs of the PWM.
After all the operations performed, we should get the following.
This is what it looks like on my board (in the picture below).
Here I rewound the group stabilization choke with a 1.3-1.6 mm wire in one layer on the original core. It fit somewhere around 20 turns, but you don’t have to do this and leave the one that was there. Everything works well with him too.
I also installed another load resistor on the board, which consists of two 1.2 kOhm 3W resistors connected in parallel, the total resistance was 560 Ohms.
The native load resistor is designed for 12 volts of output voltage and has a resistance of 270 Ohms. My output voltage will be about 40 volts, so I installed such a resistor.
It must be calculated (at the maximum output voltage of the power supply at idle) for a load current of 50-60 mA. Since operating the power supply completely without load is not desirable, that’s why it is placed in the circuit.
View of the board from the parts side.
Now what will we need to add to the prepared board of our power supply in order to turn it into an regulated power supply;
First of all, in order not to burn the power transistors, we will need to solve the problem of load current stabilization and short circuit protection.
On forums for remaking similar units, I came across such an interesting thing - when experimenting with the current stabilization mode, on the forum pro-radio, forum member DWD I cited the following quote, I will quote it in full:
“I once told you that I couldn’t get the UPS to operate normally in current source mode with a low reference voltage at one of the inputs of the error amplifier of the PWM controller.
More than 50mV is normal, but less is not. In principle, 50mV is a guaranteed result, but in principle, you can get 25mV if you try. Anything less didn’t work. It does not work stably and is excited or confused by interference. This is when the signal voltage from the current sensor is positive.
But in the datasheet on the TL494 there is an option when negative voltage is removed from the current sensor.
I converted the circuit to this option and got an excellent result.
Here is a fragment of the diagram.
Actually, everything is standard, except for two points.
Firstly, is the best stability when stabilizing the load current with a negative signal from the current sensor an accident or a pattern?
The circuit works great with a reference voltage of 5mV!
With a positive signal from the current sensor, stable operation is obtained only at higher reference voltages (at least 25 mV).
With resistor values of 10 Ohm and 10 KOhm, the current stabilized at 1.5 A up to the output short circuit.
I need more current, so I installed a 30 Ohm resistor. Stabilization was achieved at a level of 12...13A at a reference voltage of 15mV.
Secondly (and most interestingly), I don’t have a current sensor as such...
Its role is played by a fragment of a track on the board 3 cm long and 1 cm wide. The track is covered with a thin layer of solder.
If you use this track at a length of 2cm as a sensor, then the current will stabilize at the level of 12-13A, and if at a length of 2.5cm, then at the level of 10A."
Since this result turned out to be better than the standard one, we will go the same way.
First, you will need to unsolder the middle terminal of the secondary winding of the transformer (flexible braid) from the negative wire, or better without soldering it (if the signet allows) - cut the printed track on the board that connects it to the negative wire.
Next, you will need to solder a current sensor (shunt) between the track cut, which will connect the middle terminal of the winding to the negative wire.
It is best to take shunts from faulty (if you find them) pointer ampere-voltmeters (tseshek), or from Chinese pointer or digital instruments. They look something like this. A piece 1.5-2.0 cm long will be sufficient.
You can, of course, try to do as I wrote above. DWD, that is, if the path from the braid to the common wire is long enough, then try to use it as a current sensor, but I didn’t do this, I came across a board of a different design, like this one, where the two wire jumpers that connected the output are indicated by a red arrow braids with a common wire, and printed tracks ran between them.
Therefore, after removing unnecessary parts from the board, I removed these jumpers and in their place soldered a current sensor from a faulty Chinese "tseshka".
Then I soldered the rewound inductor in place, installed the electrolyte and load resistor.
This is what my piece of board looks like, where I marked with a red arrow the installed current sensor (shunt) in place of the jumper wire.
Then you need to connect this shunt to the PWM using a separate wire. From the side of the braid - with the 15th PWM leg through a 10 Ohm resistor, and connect the 16th PWM leg to the common wire.
Using a 10 Ohm resistor, you can select the maximum output current of our power supply. On the diagram DWD The resistor is 30 ohms, but start with 10 ohms for now. Increasing the value of this resistor increases the maximum output current of the power supply.
As I said earlier, the output voltage of my power supply is about 40 volts. To do this, I rewound the transformer, but in principle you can not rewind it, but increase the output voltage in another way, but for me this method turned out to be more convenient.
I’ll tell you about all this a little later, but for now let’s continue and start installing the necessary additional parts on the board so that we have a working power supply or charger.
Let me remind you once again that if you did not have a capacitor on the board between the 4th and 13-14 legs of the PWM (as in my case), then it is advisable to add it to the circuit.
You will also need to install two variable resistors (3.3-47 kOhm) to adjust the output voltage (V) and current (I) and connect them to the circuit below. It is advisable to make the connection wires as short as possible.
Below I have given only part of the diagram that we need - such a diagram will be easier to understand.
In the diagram, newly installed parts are indicated in green.
Diagram of newly installed parts.
Let me give you a little explanation of the diagram;
- The topmost rectifier is the duty room.
- The values of the variable resistors are shown as 3.3 and 10 kOhm - the values are as found.
- The value of resistor R1 is indicated as 270 Ohms - it is selected according to the required current limitation. Start small and you may end up with a completely different value, for example 27 Ohms;
- I did not mark capacitor C3 as a newly installed part in the expectation that it might be present on the board;
- The orange line indicates elements that may have to be selected or added to the circuit during the process of setting up the power supply.
Next we deal with the remaining 12-volt rectifier.
Let's check what maximum voltage our power supply can produce.
To do this, we temporarily unsolder from the first leg of the PWM - a resistor that goes to the output of the rectifier (according to the diagram above at 24 kOhm), then you need to turn on the unit to the network, first connect it to the break of any network wire, and use a regular 75-95 incandescent lamp as a fuse Tue In this case, the power supply will give us the maximum voltage it is capable of.
Before connecting the power supply to the network, make sure that the electrolytic capacitors in the output rectifier are replaced with higher voltage ones!
All further switching on of the power supply should be carried out only with an incandescent lamp; it will protect the power supply from emergency situations in case of any errors. In this case, the lamp will simply light up, and the power transistors will remain intact.
Next we need to fix (limit) the maximum output voltage of our power supply.
To do this, we temporarily change the 24 kOhm resistor (according to the diagram above) from the first leg of the PWM to a tuning resistor, for example 100 kOhm, and set it to the maximum voltage we need. It is advisable to set it so that it is 10-15 percent less than the maximum voltage that our power supply is capable of delivering. Then solder a permanent resistor in place of the tuning resistor.
If you plan to use this power supply as a charger, then the standard diode assembly used in this rectifier can be left, since its reverse voltage is 40 volts and it is quite suitable for a charger.
Then the maximum output voltage of the future charger will need to be limited in the manner described above, around 15-16 volts. For a 12-volt battery charger, this is quite enough and there is no need to increase this threshold.
If you plan to use your converted power supply as an regulated power supply, where the output voltage will be more than 20 volts, then this assembly will no longer be suitable. It will need to be replaced with a higher voltage one with the appropriate load current.
I installed two assemblies on my board in parallel, 16 amperes and 200 volts each.
When designing a rectifier using such assemblies, the maximum output voltage of the future power supply can be from 16 to 30-32 volts. It all depends on the model of the power supply.
If, when checking the power supply for the maximum output voltage, the power supply produces a voltage less than planned, and someone needs more output voltage (40-50 volts for example), then instead of the diode assembly, you will need to assemble a diode bridge, unsolder the braid from its place and leave it hanging in the air, and connect the negative terminal of the diode bridge in place of the soldered braid.
Rectifier circuit with diode bridge.
With a diode bridge, the output voltage of the power supply will be twice as high.
Diodes KD213 (with any letter) are very suitable for a diode bridge, the output current with which can reach up to 10 amperes, KD2999A,B (up to 20 amperes) and KD2997A,B (up to 30 amperes). The last ones are best, of course.
They all look like this;
In this case, it will be necessary to think about attaching the diodes to the radiator and isolating them from each other.
But I took a different route - I simply rewound the transformer and did it as I said above. two diode assemblies in parallel, since there was space for this on the board. For me this path turned out to be easier.
Rewinding a transformer is not particularly difficult, and we’ll look at how to do it below.
First, we unsolder the transformer from the board and look at the board to see which pins the 12-volt windings are soldered to.
There are mainly two types. Just like in the photo.
Next you will need to disassemble the transformer. Of course, it will be easier to deal with smaller ones, but larger ones can also be dealt with.
To do this, you need to clean the core from visible varnish (glue) residues, take a small container, pour water into it, put the transformer there, put it on the stove, bring to a boil and “cook” our transformer for 20-30 minutes.
For smaller transformers this is quite enough (less is possible) and such a procedure will not harm the core and windings of the transformer at all.
Then, holding the transformer core with tweezers (you can do it right in the container), using a sharp knife we try to disconnect the ferrite jumper from the W-shaped core.
This is done quite easily, since the varnish softens from this procedure.
Then, just as carefully, we try to free the frame from the W-shaped core. This is also quite easy to do.
Then we wind up the windings. First comes half of the primary winding, mostly about 20 turns. We wind it up and remember the direction of winding. The second end of this winding does not need to be unsoldered from the point of its connection with the other half of the primary, if this does not interfere with further work with the transformer.
Then we wind up all the secondary ones. Usually there are 4 turns of both halves of 12-volt windings at once, then 3+3 turns of 5-volt windings. We wind everything up, unsolder it from the terminals and wind a new winding.
The new winding will contain 10+10 turns. We wind it with a wire with a diameter of 1.2 - 1.5 mm, or a set of thinner wires (easier to wind) of the appropriate cross-section.
We solder the beginning of the winding to one of the terminals to which the 12-volt winding was soldered, we wind 10 turns, the direction of winding does not matter, we bring the tap to the “braid” and in the same direction as we started - we wind another 10 turns and the end solder to the remaining pin.
Next, we isolate the secondary and wind the second half of the primary onto it, which we wound earlier, in the same direction as it was wound earlier.
We assemble the transformer, solder it into the board and check the operation of the power supply.
If during the process of adjusting the voltage any extraneous noise, squeaks, or crackles occur, then to get rid of them, you will need to select the RC chain circled in the orange ellipse below in the figure.
In some cases, you can completely remove the resistor and select a capacitor, but in others you can’t do it without a resistor. You can try adding a capacitor, or the same RC circuit, between 3 and 15 PWM legs.
If this does not help, then you need to install additional capacitors (circled in orange), their ratings are approximately 0.01 uF. If this doesn’t help much, then install an additional 4.7 kOhm resistor from the second leg of the PWM to the middle terminal of the voltage regulator (not shown in the diagram).
Then you will need to load the power supply output, for example, with a 60-watt car lamp, and try to regulate the current with resistor “I”.
If the current adjustment limit is small, then you need to increase the value of the resistor that comes from the shunt (10 Ohms) and try to regulate the current again.
You should not install a tuning resistor instead of this one; change its value only by installing another resistor with a higher or lower value.
It may happen that when the current increases, the incandescent lamp in the network wire circuit will light up. Then you need to reduce the current, turn off the power supply and return the resistor value to the previous value.
Also, for voltage and current regulators, it is best to try to purchase SP5-35 regulators, which come with wire and rigid leads.
This is an analogue of multi-turn resistors (only one and a half turns), the axis of which is combined with a smooth and coarse regulator. At first it is regulated “Smoothly”, then when it reaches the limit, it begins to be regulated “Roughly”.
Adjustment with such resistors is very convenient, fast and accurate, much better than with a multi-turn. But if you can’t get them, then buy ordinary multi-turn ones, such as;
Well, it seems like I told you everything that I planned to complete on remaking the computer power supply, and I hope that everything is clear and intelligible.
If anyone has any questions about the design of the power supply, ask them on the forum.
Good luck with your design!
The basis of modern business is obtaining large profits with relatively low investments. Although this path is disastrous for our own domestic developments and industry, business is business. Here, either introduce measures to prevent the penetration of cheap stuff, or make money from it. For example, if you need a cheap power supply, then you don’t need to invent and design, killing money - you just need to look at the market for common Chinese junk and try to build what is needed based on it. The market, more than ever, is flooded with old and new computer power supplies of various capacities. This power supply has everything you need - various voltages (+12 V, +5 V, +3.3 V, -12 V, -5 V), protection of these voltages from overvoltage and overcurrent. At the same time, computer power supplies of the ATX or TX type are lightweight and small in size. Of course, the power supplies are switching, but there is practically no high-frequency interference. In this case, you can go in the standard proven way and install a regular transformer with several taps and a bunch of diode bridges, and control it with a high-power variable resistor. From the point of view of reliability, transformer units are much more reliable than switching ones, because switching power supplies have several tens of times more parts than in a transformer power supply of the USSR type, and if each element is somewhat less than unity in reliability, then the overall reliability is the product of all elements and, as a result, Switching power supplies are much less reliable than transformer ones by several tens of times. It seems that if this is the case, then there is no point in fussing and we should abandon switching power supplies. But here, a more important factor than reliability, in our reality is the flexibility of production, and pulse units can quite easily be transformed and rebuilt for absolutely any equipment, depending on production requirements. The second factor is the trade in zaptsatsk. With a sufficient level of competition, the manufacturer strives to sell the goods at cost, while accurately calculating the warranty period so that the equipment breaks down the next week, after the end of the warranty, and the client would buy spare parts at inflated prices. Sometimes it comes to the point that it is easier to buy new equipment than to repair a used one from the manufacturer.
For us, it’s quite normal to screw in a trans instead of a burnt-out power supply or prop up the red gas start button in Defect ovens with a tablespoon, rather than buy a new part. Our mentality is clearly seen by the Chinese and they strive to make their goods irrepairable, but we, like in war, manage to repair and improve their unreliable equipment, and if everything is already a “pipe,” then at least remove some of the clutter and throw it into other equipment.
I needed a power supply to test electronic components with adjustable voltage up to 30 V. There was a transformer, but adjusting through a cutter is not serious, and the voltage will float at different currents, but there was an old ATX power supply from a computer. The idea was born to adapt the computer unit to a regulated power source. Having googled the topic, I found several modifications, but they all suggested radically throwing out all the protection and filters, and we would like to save the entire block in case we have to use it for its intended purpose. So I started experimenting. The goal is to create an adjustable power supply with voltage limits from 0 to 30 V without cutting out the filling.
Part 1. So-so.
The block for experiments was quite old, weak, but stuffed with many filters. The unit was covered in dust, so before starting it I opened it and cleaned it. The appearance of the details did not raise suspicions. Once everything is satisfactory, you can do a test run and measure all the voltages.
12 V - yellow
5 V - red
3.3 V - orange
5 V - white
12 V - blue
0 - black
There is a fuse at the input of the block, and the block type LC16161D is printed next to it.
The ATX type block has a connector for connecting it to the motherboard. Simply plugging the unit into a power outlet does not turn on the unit itself. The motherboard shorts two pins on the connector. If they are closed, the unit will turn on and the fan - the power indicator - will begin to rotate. The color of the wires that need to be shorted to turn on is indicated on the unit cover, but usually they are “black” and “green”. You need to insert the jumper and plug the unit into the outlet. If you remove the jumper the unit will turn off.
The TX unit is turned on by a button located on the cable coming out of the power supply.
It is clear that the unit is working and before starting the modification, you need to unsolder the fuse located at the input and solder in a socket with an incandescent light bulb instead. The more powerful the lamp, the less voltage will drop across it during tests. The lamp will protect the power supply from all overloads and breakdowns and will not allow the elements to burn out. At the same time, pulse units are practically insensitive to voltage drops in the supply network, i.e. Although the lamp will shine and consume kilowatts, there will be no drawdown from the lamp in terms of output voltages. My lamp is 220 V, 300 W.
The blocks are built on the TL494 control chip or its analogue KA7500. A microcomputer LM339 is also often used. All the harness comes here and this is where the main changes will have to be made.
The voltage is normal, the unit is working. Let's start improving the voltage regulation unit. The block is pulsed and regulation occurs by regulating the opening duration of the input transistors. By the way, I always thought that field-effect transistors oscillate the entire load, but, in fact, fast switching bipolar transistors of type 13007 are also used, which are also installed in energy-saving lamps. In the power supply circuit, you need to find a resistor between 1 leg of the TL494 microcircuit and the +12 V power bus. In this circuit it is designated R34 = 39.2 kOhm. Nearby there is a resistor R33 = 9 kOhm, which connects the +5 V bus and 1 leg of the TL494 microcircuit. Replacing resistor R33 does not lead to anything. It is necessary to replace resistor R34 with a variable resistor of 40 kOhm, more is possible, but raising the voltage on the +12 V bus only turned out to the level of +15 V, so there is no point in overestimating the resistance of the resistor. The idea here is that the higher the resistance, the higher the output voltage. At the same time, the voltage will not increase indefinitely. The voltage between the +12 V and -12 V buses varies from 5 to 28 V.
You can find the required resistor by tracing the tracks along the board, or using an ohmmeter.
We set the variable soldered resistor to the minimum resistance and be sure to connect a voltmeter. Without a voltmeter it is difficult to determine the change in voltage. We turn on the unit and the voltmeter on the +12 V bus shows a voltage of 2.5 V, while the fan does not spin, and the power supply sings a little at a high frequency, which indicates PWM operation at a relatively low frequency. We twist the variable resistor and see an increase in voltage on all buses. The fan turns on at approximately +5 V.
We measure all voltages on the buses
12 V: +2.5 ... +13.5
5 V: +1.1 ... +5.7
3.3 V: +0.8 ... 3.5
12 V: -2.1 ... -13
5 V: -0.3 ... -5.7
The voltages are normal, except for the -12 V rail, and they can be varied to obtain the required voltages. But computer units are made in such a way that the protection on the negative buses is triggered at sufficiently low currents. You can take a 12 V car light bulb and connect it between the +12 V bus and the 0 bus. As the voltage increases, the light bulb will shine more and more brightly. At the same time, the lamp turned on instead of the fuse will gradually light up. If you turn on a light bulb between the -12 V bus and the 0 bus, then at low voltage the light bulb lights up, but at a certain current consumption the unit goes into protection. The protection is triggered by a current of about 0.3 A. The current protection is made on a resistive diode divider; in order to deceive it, you need to disconnect the diode between the -5 V bus and the midpoint that connects the -12 V bus to the resistor. You can cut off two zener diodes ZD1 and ZD2. Zener diodes are used as overvoltage protection, and it is here that current protection also goes through the zener diode. At least we managed to get 8 A from the 12 V bus, but this is fraught with breakdown of the feedback microcircuit. As a result, cutting off the zener diodes is a dead end, but the diode is fine.
To test the block you need to use a variable load. The most rational is a piece of a spiral from a heater. Twisted nichrome is all you need. To check, turn on the nichrome through an ammeter between the -12 V and +12 V terminals, adjust the voltage and measure the current.
The output diodes for negative voltages are much smaller than those used for positive voltages. The load is correspondingly also lower. Moreover, if the positive channels contain assemblies of Schottky diodes, then a regular diode is soldered into the negative channels. Sometimes it is soldered to a plate - like a radiator, but this is nonsense and in order to increase the current in the -12 V channel you need to replace the diode with something stronger, but at the same time, my assemblies of Schottky diodes burned out, but ordinary diodes are fine pulled well. It should be noted that the protection does not work if the load is connected between different buses without bus 0.
The last test is short circuit protection. Let's shorten the block. The protection only works on the +12 V bus, because the zener diodes have disabled almost all protection. All other buses do not turn off the unit for a short time. As a result, an adjustable power supply was obtained from a computer unit with the replacement of one element. Fast and therefore economically feasible. During the tests, it turned out that if you quickly turn the adjustment knob, the PWM does not have time to adjust and knocks out the KA5H0165R feedback microcontroller, and the lamp lights up very brightly, then the input power bipolar transistors KSE13007 can fly out if there is a fuse instead of the lamp.
In short, everything works, but is quite unreliable. In this form, you only need to use the regulated +12 V rail and it is not interesting to slowly turn the PWM.
Part 2. More or less.
The second experiment was the ancient TX power supply. This unit has a button to turn it on - quite convenient. We begin the alteration by resoldering the resistor between +12 V and the first leg of the TL494 mikruhi. The resistor is from +12 V and 1 leg is set to variable at 40 kOhm. This makes it possible to obtain adjustable voltages. All protections remain.
Next you need to change the current limits for the negative buses. I soldered a resistor that I removed from the +12 V bus, and soldered it into the gap of the 0 and 11 bus with the leg of a TL339 mikruhi. There was already one resistor there. The current limit changed, but when connecting a load, the voltage on the -12 V bus dropped significantly as the current increased. Most likely it drains the entire negative voltage line. Then I replaced the soldered cutter with a variable resistor - to select current triggers. But it didn’t work out well - it doesn’t work clearly. I'll have to try removing this additional resistor.
The measurement of the parameters gave the following results:
|
Voltage bus, V |
No-load voltage, V |
Load voltage 30 W, V |
Current through load 30 W, A |
I started re-soldering with rectifier diodes. There are two diodes and they are quite weak.
I took the diodes from the old unit. Diode assemblies S20C40C - Schottky, designed for a current of 20 A and a voltage of 40 V, but nothing good came of it. Or there were such assemblies, but one burned out and I simply soldered two stronger diodes.
I stuck cut radiators and diodes on them. The diodes began to get very hot and shut down :), but even with stronger diodes, the voltage on the -12 V bus did not want to drop to -15 V.
After resoldering two resistors and two diodes, it was possible to twist the power supply and turn on the load. At first I used a load in the form of a light bulb, and measured voltage and current separately.
Then I stopped worrying, found a variable resistor made of nichrome, a Ts4353 multimeter - measured the voltage, and a digital one - the current. It turned out to be a good tandem. As the load increased, the voltage dropped slightly, the current increased, but I loaded only up to 6 A, and the input lamp glowed at a quarter incandescence. When the maximum voltage was reached, the lamp at the input lit up at half power, and the voltage at the load dropped somewhat.
By and large, the rework was a success. True, if you turn on between the +12 V and -12 V buses, then the protection does not work, but otherwise everything is clear. Happy remodeling everyone.
However, this alteration did not last long.
Part 3. Successful.
Another modification was the power supply with mikruhoy 339. I’m not a fan of desoldering everything and then trying to start the unit, so I did this step by step:
I checked the unit for activation and short circuit protection on the +12 V bus;
I took out the fuse for the input and replaced it with a socket with an incandescent lamp - it’s safe to turn it on so as not to burn the keys. I checked the unit for switching on and short circuit;
I removed the 39k resistor between 1 leg 494 and the +12 V bus and replaced it with a 45k variable resistor. Turned on the unit - the voltage on the +12 V bus is regulated within the range of +2.7...+12.4 V, checked for short circuit;
I removed the diode from the -12 V bus, it is located behind the resistor if you go from the wire. There was no tracking on the -5 V bus. Sometimes there is a zener diode, its essence is the same - limiting the output voltage. Soldering mikruhu 7905 puts the block into protection. I checked the unit for switching on and short circuit;
I replaced the 2.7k resistor from 1 leg 494 to ground with a 2k one, there are several of them, but it is the change in 2.7k that makes it possible to change the output voltage limit. For example, using a 2k resistor on the +12 V bus, it became possible to regulate the voltage to 20 V, respectively, increasing 2.7k to 4k, the maximum voltage became +8 V. I checked the unit for switching on and short circuit;
Replaced the output capacitors on the 12 V rails with a maximum of 35 V, and on the 5 V rails with 16 V;
I replaced the paired diode of the +12 V bus, it was tdl020-05f with a voltage of up to 20 V but a current of 5 A, I installed the sbl3040pt at 40 A, there is no need to unsolder the +5 V bus - the feedback at 494 will be broken. I checked the unit;
I measured the current through the incandescent lamp at the input - when the current consumption in the load reached 3 A, the lamp at the input glowed brightly, but the current at the load no longer grew, the voltage dropped, the current through the lamp was 0.5 A, which fit within the current of the original fuse. I removed the lamp and put back the original 2 A fuse;
I turned the blower fan over so that air was blown into the unit and the radiator was cooled more efficiently.
As a result of replacing two resistors, three capacitors and a diode, it was possible to convert the computer power supply into an adjustable laboratory power supply with an output current of more than 10 A and a voltage of 20 V. The downside is the lack of current regulation, but short-circuit protection remains. Personally, I don’t need to regulate this way - the unit already produces more than 10 A.
Let's move on to practical implementation. There is a block, though TX. But it has a power button, which is also convenient for laboratory use. The unit is capable of delivering 200 W with a declared current of 12 V - 8A and 5 V - 20 A.
It is written on the block that it cannot be opened and there is nothing inside for amateurs. So we're kind of like professionals. There is a switch on the block for 110/220 V. Of course, we will remove the switch as it is not needed, but we will leave the button - let it work.
The internals are more than modest - there is no input choke and the charge of the input condensers goes through a resistor, and not through a thermistor, as a result there is a loss of energy that heats the resistor.
We throw away the wires to the 110V switch and anything that gets in the way of separating the board from the case.
We replace the resistor with a thermistor and solder in the inductor. We remove the input fuse and solder in an incandescent light bulb instead.
We check the operation of the circuit - the input lamp lights up at a current of approximately 0.2 A. The load is a 24 V 60 W lamp. The 12 V lamp is on. Everything is fine and the short circuit test works.
We find a resistor from leg 1 494 to +12 V and raise the leg. We solder a variable resistor instead. Now there will be voltage regulation at the load.
We are looking for resistors from 1 leg 494 to the common minus. There are three of them here. All are quite high-resistance, I soldered out the lowest resistance resistor at 10k and soldered it at 2k instead. This increased the regulation limit to 20 V. However, this is not yet visible during the test; overvoltage protection is triggered.
We find a diode on the -12 V bus, located after the resistor and raise its leg. This will disable the surge protection. Now everything should be fine.
Now we change the output capacitor on the +12 V bus to the limit of 25 V. And plus 8 A is a stretch for a small rectifier diode, so we change this element to something more powerful. And of course we turn it on and check it. The current and voltage in the presence of a lamp at the input may not increase significantly if the load is connected. Now, if the load is turned off, the voltage is regulated to +20 V.
If everything suits you, replace the lamp with a fuse. And we give the block a load.
To visually assess voltage and current, I used a digital indicator from Aliexpress. There was also such a moment - the voltage on the +12V bus started at 2.5V and this was not very pleasant. But on the +5V bus from 0.4V. So I combined the buses using a switch. The indicator itself has 5 wires for connection: 3 for measuring voltage and 2 for current. The indicator is powered by a voltage of 4.5V. The standby power supply is just 5V and the tl494 mikruha is powered by it.
I’m very glad that I was able to remake the computer power supply. Happy remodeling everyone.
LABORATORY POWER SUPPLY FROM COMPUTER ATX
Every year, it becomes more and more difficult to get a good transformer for a power supply. So that the voltage and current are required. Recently I needed to assemble an adapter for one device, so it turns out that the prices for ordinary transformers in radio stores are in the range of 5-15 euros! Therefore, when it was necessary to make a good laboratory power supply, with voltage and protection current adjustments, the choice fell on a computer one as the basis of the design. Moreover, its price is now not much more than the price of a conventional transformer.
For our purposes, absolutely any computer power supply will be suitable. At least 250 watts, at least 500. The current that it will provide is enough for an amateur radio power supply.
The modification is minimal and can be repeated even by novice radio amateurs. The main thing is to remember that the ATX switching computer power supply has many elements on the board that are under 220 V mains voltage, so be extremely careful when testing and configuring!The changes affected mainly the output part of the ATX power supply.
For ease of operation, this laboratory power supply can be supplied with current and voltage. This can be done either on a microcontroller or on a specialized chip.
All main and additional parts of the power supply are mounted inside the ATX power supply case. There is enough space there for them, and for a digital voltammeter, and for all the necessary sockets and regulators.
The last advantage is also very important, because enclosures are often a big problem. Personally, I have a lot of devices in my desk drawer that never got their own box.
The body of the resulting power supply can be covered with decorative black self-adhesive film or simply painted. We make the front panel with all the inscriptions and designations in Photoshop, print it on photo paper and paste it onto the body.
Many people assemble various radio-electronic structures, and their use sometimes requires a powerful power source. Today I’ll tell you how with an output power of 250 watts, and the ability to adjust the voltage from 8 to 16 volts at the output, from an ATX unit model FA-5-2.
The advantage of this power supply is output power protection (that is, against short circuit) and voltage protection.
Reworking the ATX block will consist of several stages
1. First, we unsolder the wires, leaving only gray, black, yellow. By the way, to turn on this unit you need to short the gray wire to ground, not the green one (as in most ATX units).
2. We unsolder from the circuit the parts that are in the +3.3v, -5v, -12v circuits (we don’t touch +5 volts yet). What to remove is shown in red, and what to redo is shown in blue in the diagram:
3. Next, we unsolder (remove) the +5 volt circuit, replace the diode assembly in the 12V circuit with S30D40C (taken from the 5V circuit).
We install a tuning resistor and a variable resistor with a built-in switch as shown in the diagram:
That is, like this:
Now we turn on the 220V network and connect the gray wire to ground, having previously placed the trimming resistor in the middle position, and the variable in the position at which there will be the least resistance on it. The output voltage should be about 8 volts; increasing the resistance of the variable resistor, the voltage will increase. But don’t rush to raise the voltage, since we don’t have voltage protection yet.
4. We provide power and voltage protection. Add two trim resistors:
5. Indicator panel. Add a couple of transistors, several resistors and three LEDs:
The green LED lights up when connected to the network, yellow - when there is voltage at the output terminals, red - when the protection is triggered.
You can also build in a voltammeter.
Setting voltage protection in the power supply
Setting up the voltage protection is done as follows: we twist the resistor R4 to the side where the ground is connected, set R3 to maximum (higher resistance), then by rotating R2 we achieve the voltage we need - 16 volts, but set it 0.2 volts more - 16.2 volts, slowly turn R4 before the protection is triggered, turn off the block, slightly reduce the resistance R2, turn on the block and increase the resistance R2 until the output reaches 16 volts. If during the last operation the protection was triggered, then you went overboard with the R4 turn and will have to repeat everything again. After setting up the protection, the laboratory unit is completely ready for use.
Over the past month I have already made three such blocks, each cost me about 500 rubles (this is together with a voltammeter, which I assembled separately for 150 rubles). And I sold one power supply unit as a charger for a car battery for 2100 rubles, so that’s already a plus :)
Ponomarev Artyom (stalker68) was with you, see you again on the pages of Technoreview!