To power electrical appliances, it is necessary to ensure the nominal values of the power supply parameters stated in their documentation. Of course, most modern electrical appliances operate on 220 Volt AC power, but it happens that you need to provide power to devices for other countries where the voltage is different or to power something from the car’s on-board network. In this article we will look at how to increase DC and AC voltage and what is needed for this.
There are two ways to increase the alternating voltage - use a transformer or an autotransformer. The main difference between them is that when using a transformer there is galvanic isolation between the primary and secondary circuits, while when using an autotransformer there is no galvanic isolation.
Interesting! Galvanic isolation is the absence of electrical contact between the primary (input) circuit and the secondary (output) circuit.
Let's look at frequently asked questions. If you find yourself outside the borders of our vast homeland and the electrical networks there differ from our 220 V, for example, 110 V, then to raise the voltage from 110 to 220 Volts you need to use a transformer, for example, such as is shown in the figure below:
It should be said that such transformers can be used “in any direction.” That is, if the technical documentation of your transformer says “the voltage of the primary winding is 220V, the secondary is 110V,” this does not mean that it cannot be connected to 110V. Transformers are reversible, and if the same 110V is applied to the secondary winding, 220V or another increased value will appear on the primary winding, proportional to the transformation ratio.
The next problem that many people face is that this is especially common in private homes and garages. The problem is related to the poor condition and overload of power lines. To solve this problem, you can use LATR (laboratory autotransformer). Most modern models can both lower and smoothly increase network parameters.
Its diagram is shown on the front panel, and we will not dwell on explanations of the principle of operation. LATRs are sold in different capacities, the one in the figure is approximately 250-500 VA (volt-amperes). In practice, there are models up to several kilowatts. This method is suitable for supplying nominal 220 Volts to a specific electrical appliance.
If you need to cheaply increase the voltage throughout the house, your choice is a relay stabilizer. They are also sold in different capacities and the range is suitable for most typical applications (3-15 kW). The device is also based on an autotransformer. We talked about this in the article to which we referred.
Everyone knows that transformers do not work on direct current, then how can the voltage be increased in such cases? In most cases, the constant is increased using a field-effect or bipolar transistor and a PWM controller. In other words, it is called a transformerless voltage converter. If these three main elements are connected as shown in the figure below and a PWM signal is applied to the base of the transistor, then its output voltage will increase Ku times.
Ku=1/(1-D)
We will also consider typical situations.
Let's say you want to backlight your keyboard using a small piece of LED strip. The power of a smartphone charger (5-15 W) is quite enough for this, but the problem is that its output voltage is 5 Volts, and common types of LED strips operate on 12 V.
Then how to increase the voltage on the charger? The easiest way to boost is with a device such as a “dc-dc boost converter” or “pulse boost DC-DC converter.”
Such devices allow you to increase the voltage from 5 to 12 Volts, and are sold both with a fixed value and adjustable, which in most cases will allow you to increase from 12 to 24 and even up to 36 Volts. But keep in mind that the output current is limited by the weakest element of the circuit, in the situation under discussion - the current on the charger.
When using the specified board, the output current will be less than the input current by as many times as the output voltage has increased, without taking into account the efficiency of the converter (it is around 80-95%).
Such devices are built on the basis of MT3608, LM2577, XL6009 microcircuits. With their help, you can make a device for checking the regulator relay not on the car’s generator, but on the desktop, adjusting the values from 12 to 14 Volts. Below you see a video test of such a device.
Interesting! DIY enthusiasts often ask the question “how to increase the voltage from 3.7 V to 5 V in order to make a Power bank on lithium batteries with your own hands?” The answer is simple - use the FP6291 converter board.
On such boards, the purpose of the contact pads for connection is indicated using silk-screen printing, so you do not need a diagram.
Another situation that often arises is the need to connect a 220V device to a car battery, and it happens that outside the city you really need to get 220V. If you don’t have a gasoline generator, use a car battery and an inverter to increase the voltage from 12 to 220 Volts. A 1 kW model can be purchased for $35 - this is an inexpensive and proven way to connect a 220V drill, grinder, boiler or refrigerator to a 12V battery.
If you are a truck driver, the above inverter will not be suitable for you, due to the fact that your on-board network is most likely 24 Volts. If you need to increase the voltage from 24V to 220V, then pay attention to this when purchasing an inverter.
Although it is worth noting that there are universal converters that can operate on both 12 and 24 volts.
In cases where you need to obtain a high voltage, for example, increase it from 220 to 1000V, you can use a special multiplier. Its typical diagram is shown below. It consists of diodes and capacitors. You will get a direct current output, keep this in mind. This is the Latour-Delon-Grenacher doubler:
And this is what the circuit of an asymmetrical multiplier (Cockroft-Walton) looks like.
With its help, you can increase the voltage by the required number of times. This device is built in cascades, the number of which determines how many volts you get at the output. The following video describes how the multiplier works.
In addition to these circuits, there are many others; below are quadrupler circuits, 6- and 8-fold multipliers, which are used to increase the voltage:
In conclusion, I would like to remind you about safety precautions. When connecting transformers, autotransformers, as well as working with inverters and multipliers, be careful. Do not touch live parts with bare hands. Connections should be made with no power supplied to the device, and should not be used in damp areas where water or splashes may occur. Also, do not exceed the current of the transformer, converter or power supply declared by the manufacturer if you do not want it to burn out. We hope the tips provided will help you increase the voltage to the desired value! If you have any questions, ask them in the comments below the article!
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I started this experiment with frequency due to the lack of power in the power supply.
When the computer was purchased, its power was quite sufficient for this configuration:
AMD Duron 750Mhz / RAM DIMM 128 mb / PC Partner KT133 / HDD Samsung 20Gb / S3 Trio 3D/2X 8Mb AGP
For example, two diagrams: 
Frequency f for this circuit it turned out to be 57 kHz.
And for this frequency f equal to 40 kHz.
The frequency can be changed by replacing the capacitor C or/and resistor R to a different denomination.
It would be correct to install a capacitor with a smaller capacitance, and replace the resistor with a series-connected constant resistor and a variable type SP5 with flexible leads.
Then, decreasing its resistance, measure the voltage until the voltage reaches 5.0 volts. Then solder a constant resistor in place of the variable one, rounding the value up.
I took a more dangerous path - I sharply changed the frequency by soldering in a capacitor of smaller capacity.
I have had:
R 1 =12kOm
C 1 =1.5nF
According to the formula we get
f=61.1 kHz
After replacing the capacitor
R 2 =12kOm
C 2 =1.0nF
f
=91.6 kHz
According to the formula:
the frequency increased by 50% and the power increased accordingly.
If we do not change R, then the formula simplifies:
Or if we don’t change C, then the formula is:
Trace the capacitor and resistor connected to pins 5 and 6 of the microcircuit. and replace the capacitor with a capacitor with a smaller capacity.
After overclocking the power supply, the voltage became exactly 5.00 (the multimeter can sometimes show 5.01, which is most likely an error), almost without reacting to the tasks being performed - with a heavy load on the +12 volt bus (simultaneous operation of two CDs and two screws) - the voltage on the bus is + 5V may briefly drop 4.98.
The key transistors began to warm up more strongly. Those. If before the radiator was slightly warm, now it is very warm, but not hot. The radiator with rectifier half-bridges did not get hotter. The transformer also does not heat up. From 09/18/2004 to the present day (01/15/05) there are no questions to the power supply unit. Currently the following configuration:
Rennie's comments: The fact that you increased the frequency, you increased the number of sawtooth pulses over a certain period of time, and as a result, the frequency with which power instabilities are monitored increased, since power instabilities are monitored more often, the pulses for closing and opening of transistors in a half-bridge switch occur at double frequency . Your transistors have characteristics, specifically their speed: By increasing the frequency, you have thereby reduced the size of the dead zone. Since you say that the transistors do not heat up, it means that they are in that frequency range, which means that everything seems to be fine here. But, there are also pitfalls. Do you have an electrical circuit diagram in front of you? I’ll explain it to you now using the diagram. There in the circuit, look where the key transistors are, diodes are connected to the collector and emitter. They serve to dissolve the residual charge in the transistors and transfer the charge to the other arm (to the capacitor). Now, if these comrades have a low switching speed, through currents are possible - this is a direct breakdown of your transistors. Perhaps this will cause them to heat up. Now further, this is not the case, the point is that after the direct current that passed through the diode. It has inertia and when a reverse current appears: for some time the value of its resistance is not restored and therefore they are characterized not by the frequency of operation, but by the recovery time of the parameters. If this time is longer than possible, then you will experience partial through currents, which is why surges in both voltage and current are possible. In the secondary it’s not so scary, but in the power department it’s just fucked up: to put it mildly. So let's continue. In the secondary circuit, these switchings are not desirable, namely: There, Schottky diodes are used for stabilization, so at 12 volts they are supported with a voltage of -5 volts (approx. I have silicon ones at 12 volts), so at 12 volts that If only they (Schottky diodes) could be used with a voltage of -5 volts. (Due to the low reverse voltage, it is impossible to simply put Schottky diodes on the 12 volt bus, so they are distorted this way). But silicon diodes have more losses than Schottky diodes and the reaction is less, unless they are one of the fast-recovery diodes. So, if the frequency is high, then the Schottky diodes have almost the same effect as in the power section + the inertia of the winding at -5 volts relative to +12 volts makes it impossible to use Schottky diodes, so an increase in frequency can eventually lead to failure of them. I'm considering the general case. So let's move on. Next is another joke, finally connected directly with the feedback circuit. When you create negative feedback, you have such a thing as the resonant frequency of this feedback loop. If you reach resonance, then your entire scheme will be screwed. Sorry for the rude expression. Because this PWM chip controls everything and requires its operation in mode. And finally, a “dark horse” ;) Do you understand what I mean? It's a transformer, so this bitch also has a resonant frequency. So this crap is not a standardized part, the transformer winding product is manufactured individually in each case - for this simple reason you do not know the characteristics of it. What if you introduce your frequency into resonance? You burn your trance and you can safely throw away the power supply. Externally, two absolutely identical transformers can have completely different parameters. Well, the fact is that by choosing the wrong frequency you could easily burn out the power supply. Under all other conditions, how can you still increase the power of the power supply? We increase the power of the power supply. First of all, we need to understand what power is. The formula is extremely simple - current to voltage. The voltage in the power section is 310 volts constant. So, we cannot influence the voltage in any way. We have only one trans. We can only increase the current. The amount of current is dictated to us by two things - transistors in the half-bridge and buffer capacitors. The conductors are larger, the transistors are more powerful, so you need to increase the capacitance rating and change the transistors to ones that have a higher current in the collector-emitter circuit or just a collector current, if you don’t mind, you can plug in 1000 uF there and not strain yourself with calculations. So in this circuit we did everything we could, here, in principle, nothing more can be done, except perhaps taking into account the voltage and current of the base of these new transistors. If the transformer is small, this will not help. You also need to regulate such crap as the voltage and current at which your transistors will open and close. Now it seems like everything is here. Let's go to the secondary circuit. Now we have a lot of current at the output windings....... We need to slightly correct our filtering, stabilization and rectification circuits. For this, we take, depending on the implementation of our power supply, and change the diode assemblies first of all, so that we can ensure the flow of our current. In principle, everything else can be left as is. That's all, it seems, well, at the moment there should be a margin of safety. The point here is that the technique is impulsive - this is its bad side. Here almost everything is built on the frequency response and phase response, on t reaction.: that’s all
How to make a full-fledged power supply yourself with an adjustable voltage range of 2.5-24 volts is very simple; anyone can repeat it without any amateur radio experience.
We will make it from an old computer power supply, TX or ATX, it doesn’t matter, fortunately, over the years of the PC Era, every home has already accumulated a sufficient amount of old computer hardware and a power supply unit is probably also there, so the cost of homemade products will be insignificant, and for some masters it will be zero rubles .
I got this AT block for modification.
Look what is written on the case.
There are many options for modifying a standard computer power supply, but they are all based on a change in the wiring of the IC chip - TL494CN (its analogues DBL494, KA7500, IR3M02, A494, MV3759, M1114EU, MPC494C, etc.).
Let's look at several options execution of computer power supply circuits, perhaps one of them will be yours and dealing with the wiring will become much easier.
Scheme No. 1.
Let's get to work.
First you need to disassemble the power supply housing, unscrew the four bolts, remove the cover and look inside.
In my case, a KA7500 chip was found on the board, which means we can begin to study the wiring and the location of unnecessary parts that need to be removed.
Let's disconnect the power and fan, solder or cut out the output wires so that they don't interfere with our understanding of the circuit, leave only the necessary ones, one yellow (+12v), black (common) and green* (start ON) if there is one.
This is done because our modified unit will produce not +12 volts, but up to +24 volts, and without replacement, the capacitors will simply explode during the first test at 24v, after a few minutes of operation. When selecting a new electrolyte, it is not advisable to reduce the capacity; increasing it is always recommended.
The most important part of the job.
We will remove all unnecessary parts in the IC494 harness and solder other nominal parts so that the result is a harness like this (Fig. No. 1).
We will only need these legs of the microcircuit No. 1, 2, 3, 4, 15 and 16, do not pay attention to the rest.
Explanation of symbols.
Some resistors that are already soldered into the wiring diagram can be suitable without replacing them, for example, we need to put a resistor at R=2.7k connected to the “common”, but there is already R=3k connected to the “common”, this suits us quite well and we leave it there unchanged (example in Fig. No. 2, green resistors do not change).
Thus, we review and redo all the circuits on the six legs of the microcircuit.
This was the most difficult point in the rework.
We make voltage and current regulators.
Voltage and current control.
To control we need a voltmeter (0-30v) and an ammeter (0-6A).
IMPORTANT- inside the device there is a Current resistor (Current sensor), which we need according to the diagram (Fig. No. 1), therefore, if you use an ammeter, then you do not need to install an additional Current resistor; you need to install it without an ammeter. Usually a homemade RC is made, a wire D = 0.5-0.6 mm is wound around a 2-watt MLT resistance, turn to turn for the entire length, solder the ends to the resistance terminals, that's all.
Everyone will make the body of the device for themselves.
You can leave it completely metal by cutting holes for regulators and control devices. I used laminate cutoffs, they are easier to drill and cut.
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 to suit 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. We 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 defenses 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 the 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. Good luck with your remodels.
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 for 110/220 V on the block. 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 glows 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.
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 bus + 5V 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. Good luck with the change everyone.
The converter is made on transistor VT1 and transformer T1. The mains voltage is supplied through the mains filter (SF) to the mains rectifier (RM), where it is rectified, filtered by the filter capacitor SF and through the winding W1 of transformer T1 is supplied to the collector of transistor VT1. When a rectangular pulse is applied to the base circuit of the transistor, the transistor opens and an increasing current Ik flows through it. The same current will flow through the winding W1 of transformer T1, which will lead to an increase in the magnetic flux in the transformer core, while a self-induction emf is induced in the secondary winding W2 of the transformer. Ultimately, a positive voltage will appear at the output of the diode VD. Moreover, if we increase the duration of the pulse applied to the base of transistor VT1, the voltage in the secondary circuit will increase, because more energy will be released, and if we decrease the duration, the voltage will decrease accordingly. Thus, by changing the pulse duration in the base circuit of the transistor, we can change the output voltages of the secondary winding T1, and therefore stabilize the output voltages of the power supply.
The only thing that is needed for this is a circuit that will generate trigger pulses and control their duration (latitude). A PWM controller is used as such a circuit. PWM is pulse width modulation. The PWM controller includes a master pulse generator (which determines the operating frequency of the converter), protection and control circuits, and a logic circuit that controls the pulse duration.
To stabilize the output voltages of the UPS, the PWM controller circuit “must know” the magnitude of the output voltages. For these purposes, a tracking circuit (or feedback circuit) is used, made on optocoupler U1 and resistor R2. An increase in voltage in the secondary circuit of transformer T1 will lead to an increase in the intensity of the LED radiation, and therefore a decrease in the junction resistance of the phototransistor (part of the optocoupler U1). Which in turn will lead to an increase in the voltage drop across resistor R2, which is connected in series with the phototransistor and a decrease in the voltage at pin 1 of the PWM controller. A decrease in voltage causes the logic circuit included in the PWM controller to increase the pulse duration until the voltage at the 1st pin corresponds to the specified parameters. When the voltage decreases, the process is reversed.
The UPS uses 2 principles for implementing tracking circuits - “direct” and “indirect”. The method described above is called “direct”, since the feedback voltage is removed directly from the secondary rectifier. With “indirect” tracking, the feedback voltage is removed from the additional winding of the pulse transformer:
A decrease or increase in the voltage on winding W2 will lead to a change in voltage on winding W3, which is also applied through resistor R2 to pin 1 of the PWM controller.
I think we’ve sorted out the tracking circuit, now let’s consider a situation such as a short circuit (short circuit) in the UPS load. In this case, all the energy supplied to the secondary circuit of the UPS will be lost and the output voltage will be almost zero. Accordingly, the PWM controller circuit will try to increase the pulse duration in order to raise the level of this voltage to the appropriate value. As a result, transistor VT1 will remain open longer and longer, and the current flowing through it will increase. Ultimately, this will lead to the failure of this transistor. The UPS provides protection for the converter transistor against current overloads in such emergency situations. It is based on a resistor Rprotect, connected in series to the circuit through which the collector current Ik flows. An increase in the current Ik flowing through transistor VT1 will lead to an increase in the voltage drop across this resistor, and, consequently, the voltage supplied to pin 2 of the PWM controller will also decrease. When this voltage drops to a certain level, which corresponds to the maximum permissible current of the transistor, the logic circuit of the PWM controller will stop generating pulses at pin 3 and the power supply will go into protection mode or, in other words, turn off.
In conclusion of the topic, I would like to describe in more detail the advantages of the UPS. As already mentioned, the frequency of the pulse converter is quite high, and therefore, the overall dimensions of the pulse transformer are reduced, which means, as paradoxical as it may sound, the cost of a UPS is less than a traditional power supply, since there is less metal consumption for the magnetic core and copper for the windings, not even despite the fact that the number of parts in the UPS is increasing. Another advantage of the UPS is the small capacitance of the secondary rectifier filter capacitor compared to a conventional power supply. Reducing the capacitance was made possible by increasing the frequency. And finally, the efficiency of a switching power supply reaches 85%. This is due to the fact that the UPS consumes power from the electrical network only when the converter transistor is open; when it is closed, energy is transferred to the load due to the discharge of the secondary circuit filter capacitor.
The disadvantages include the complication of the UPS circuit and the increase in pulse noise emitted by the UPS itself. The increase in interference is due to the fact that the converter transistor operates in switch mode. In this mode, the transistor is a source of pulse noise that occurs during transient processes of the transistor. This is a disadvantage of any transistor operating in switching mode. But if the transistor operates with low voltages (for example, transistor logic with a voltage of 5 volts), this is not a problem; in our case, the voltage applied to the collector of the transistor is approximately 315 volts. To combat this interference, the UPS uses more complex network filter circuits than a conventional power supply.
