For any electronic circuits required power sources. And if one device can work directly from the network, then others need different voltages: for digital microcircuits, as a rule, + 5V (for TTL logic) or + 7..9V (for CMOS technologies).
By the way, what is it: TTL and CMOS can be read
Different toys usually require +5 ... 12V. to power the LEDs +3 .. + 5V, for amplifiers in general it is diverse ..
In general, one way or another, the question arises of making a power source, and not just a source, but such that it meets the relevant requirements: the necessary voltage and current at the output, the presence of protection, and so on.
We have dedicated a separate category to food sources, which is called Power supplies (materials in the category), here we will consider the simplest option transformerless power supply for simple products that can be made in just a couple of minutes. Here is his diagram:
Of course, the power of such a source is small and it can be used only for the simplest schemes, but the most important thing is that it is stabilized.
It is the "+", microcircuits for negative voltage are marked 79XX.
In the diagram of the above, the output voltage is + 5V (according to the type of KENENKI used), but if necessary, it can also be changed by installing another chip.
Only in this case it will be necessary to pay attention to the zener diode at the input: it must be chosen so that the voltage at the input and output of the RCC has a minimum difference of 2V.
Well, that’s not all: even using a chip with a standard output voltage, you can still change the output voltage a little if necessary (for example, get 7.5V or 6.5). To do this, you need to add an additional circuit from diodes or zener diodes to the microcircuit, and you can read how to do this.
Even such a simple power source can be “a bit more power-fed”, that is, to achieve a higher current in the load. But then the introduction of additional ballast resistors at the input will be required. So, for example, here is a diagram of a transformerless power supply with an output voltage of + 12V
When we are dealing with devices that operate on a low voltage power source, we usually have several options for powering them. In addition to simple but expensive and bulky transformers, you can use transformerless power supply.
For example, you can get 5 volts out of 220 volts using a quenching resistor or using the reactance of a capacitor. However, this solution is only suitable for devices that have a very low current consumption. If we need more current, for example, to power the LED circuit, then here we will face a performance limit.
If any device consumes a large current and it is fundamentally necessary to power it from a 220 volt network, that is, one original solution. It consists in using only part of the sinusoid for nutrition during its growth and fall, i.e. at the moment when the mains voltage is equal to or less than the required value.
Description of the operation of the transformerless power supply
The peculiarity of the circuit is to control the moment of opening the MOSFET transistor - VT2 (IRF830). If the current value of the input mains voltage is lower than the stabilization voltage of the Zener diode VD5 minus the voltage drop across the resistor R3, then the transistor VT1 will be closed. Due to this, a positive voltage goes through the resistor R4 to the transistor VT2, as a result of which it is in the open state.
Current is flowing through transistor VT2 and the current value of the mains voltage is charging capacitor C2. Of course, the voltage in the network drops to zero, so it is necessary to include a VD7 diode in the circuit, which prevents the capacitor from being discharged back into the power supply circuit.
When the input voltage of the network exceeds the threshold, the current passing through the zener diode VD5 leads to the opening of the transistor VT1. The transistor shunts the gate of the transistor VT2 with its collector, as a result, VT2 closes. Thus, the capacitor C2 is charged only with the necessary voltage.
Powerful VT2 transistor opens only at low voltage, so that its total dissipating power in the circuit is very small. Of course, the stability of the power supply depends on the control voltage of the zener diode, therefore, for example, if we want to power the circuit with a microcontroller, then the output must be supplemented with a small one.
Resistor R1 protects the circuit and reduces the power surge on first use. The Zener diode VD6 limits the maximum voltage at the control electrode of the transistor VT2 in the region of 15 volts. Naturally, when switching the transistor VT2, electromagnetic interference occurs. To avoid transmission of interference to the mains, a simple LC filter consisting of L1 and C1 components is used in the input circuit.
Articles we began to get acquainted with the art of healing computer power supplies. Let us continue this exciting business and look carefully at their high-voltage part.
Checking the high voltage part of the power supply
After examining the board and restoring the rations, you should check the fuse with a multimeter (in the resistance measurement mode).
I hope you have well understood and remembered the safety precautions, outlined earlier!
If it burns out, this usually indicates a malfunction in the high-voltage part.
Most often, the malfunction of the fuse is visible (if glass) visually: inside it is “dirty” (“dirt” is an evaporated lead thread).
Sometimes a glass tube shatters into pieces.
In this case, it is necessary to check (by the same tester) the operability of high-voltage diodes, power key transistors and power transistor of the standby voltage source. Power transistors of the high-voltage part are usually located on a common radiator.
With a fuse blown, the collector-emitter terminals often “ring” shortly, and you can verify this without having to solder the transistor. With field-effect transistors, the situation is somewhat more complicated.
How to check field and bipolar transistors, you can read and.
The high-voltage part is located in that part of the board where the high-voltage capacitors are located (they are larger in volume than the low-voltage ones). These capacitors indicate their capacitance (330 - 820 μF) and operating voltage (200 - 400 V).
You may not be surprised that the operating voltage can be 200 V. In most circuits, these capacitors are connected in series, so that their total operating voltage will be 400 V. But there are also circuits with one capacitor per operating voltage of 400 V (or even more) .
It often happens that, together with power cells, electrolytic capacitors fail - both low-voltage and high-voltage (high-voltage - less often).
In most cases, this is clearly visible - the capacitors swell, their top cover bursts.
In the most severe cases, electrolyte flows from them. It bursts not just like that, but in places where its thickness is less.
This is done specifically to get by with a little blood. They didn’t do this before, and during the explosion, the capacitor scattered its insides far around. And with a monolithic aluminum sheath, it was possible to get it in the forehead too.
All such capacitors must be replaced with similar ones. Traces of electrolyte on the board should be carefully removed.
Power Unit Electrolytic Capacitors and ESR
We remind you that special low-voltage capacitors with low ESR (equivalent series resistance, EPS) are used in power supplies.
Similar are installed on computer motherboards.
You can recognize them by marking.
For example, a CapXon low ESR capacitor is labeled “LZ”. An “ordinary” capacitor does not have LZ letters. Each company produces a large number of different types of capacitors. The exact ESR value of a specific type of capacitor can be found on the manufacturer's website.
Manufacturers of power supplies often save on capacitors, putting ordinary ones with higher EPS (and they are cheaper). Sometimes they even write “Low ESR” on the capacitor banks.
This is a hoax, and it’s better to replace such capacitors right away.
In the most difficult mode, the filter capacitors work on the buses +3.3 V, +5 V, +12 V, since large currents circulate through them.
There are also "vile" cases when, over time, capacitors of small capacity in the source of standby voltage dry. At the same time, their capacity decreases, and the ESR grows.
Or the capacity drops slightly, and the ESR grows strongly. However, there may not be any external changes in shape, since their dimensions and capacity are small.
This can lead to a change in the voltage value of the standby source. If it is less than normal, the main inverter of the power supply will not turn on at all.
If it is larger, the computer will crash and “freeze”, as part of the components of the motherboard is under exactly this voltage.
Capacity can be measured.
However, most testers can only measure capacities up to 20 uF, which is clearly not enough.
Note that it is impossible to measure the ESR with a regular tester.
Need a special ESR meter!
For large capacitors, ESR can be in the tenths or hundredths of an Ohm, and for small capacitors it can be in the tenths or units of Ohms.
If it is larger, such a capacitor must be replaced.
If there is no such meter, the “suspicious” capacitor must be replaced with a new (or obviously working) one.
Hence the moral - do not leave the source of standby voltage in the power supply turned on. The less time it will work, the longer the capacitors in it will dry.
After finishing work, you must either remove the voltage with the filter switch, or remove the plug of the power cable from the power outlet.
In conclusion, let's say a few more words
About the elements of the high-voltage part of the power supply
In low-cost, low-power (up to 400 W) power bipolar transistors 13007 or 13009 with collector currents of 8 and 12 A, respectively, and a voltage between the emitter and collector of 400 V. are often used as key ones.
A power field-effect transistor 2N60 with a drain current of 2A and a drain-source voltage of 600 V can be used in the standby voltage source.
However, field-effect transistors can be used as key, and bipolar in the source of the standby mode.
In the absence of the necessary transistors, they can be replaced by analogs.
Analogs of bipolar transistors must have an operating voltage between the emitter and the collector and the collector current is not lower than that of the replaced ones.
Analogs of field-effect transistors must have an operating voltage of drain-source and drain current not lower than that of a replaceable one, and the resistance of the open channel “drain-source” not higherthan the replacement.
An attentive reader may ask: “Why should this channel resistance be no higher? After all, the greater the value of the parameters, the better, as it were? ”
I answer - with the same operating current on the channel with a higher resistance, in accordance with the Joule-Lenz law, a larger power will be dissipated. And, therefore, it (i.e., the entire transistor) will be heated more strongly.
Extra heat is useless to us!
We have a power supply, not a heating radiator!
On this, friends, we will end today. We still have to get acquainted with the treatment of the low-voltage part, which we will do in the next article.
See you on the blog!
Microcontroller devices require a constant stabilized voltage of 3.3-5 volts for their operation. Typically, this voltage is obtained from an alternating mains voltage using a transformer power source, and in the simplest case, it is the following circuit.
Step-down transformer, diode bridge, smoothing capacitor and linear / pulse stabilizer. Additionally, such a source may include a fuse, filter circuits, a soft start circuit, an overload protection circuit, etc.
This power source (with the appropriate choice of components) allows you to receive large currents and has galvanic isolation from the AC network, which is important for safe operation of the device. However, such a source can have large dimensions, thanks to the transformer and filtering capacitors.
In some devices on microcontrollers, galvanic isolation is not required from the network. For example, if the device is a sealed unit with which the end user does not contact in any way. In this case, if the circuit consumes a relatively low current (tens of milliamps), it can be powered from a 220 V network using a transformerless power source.
In this article we will consider the principle of operation of such a power source, the sequence of its calculation and a practical example of use.
Principle of operation of a transformerless power supply
Resistor R1 discharges capacitor C1 when the circuit is disconnected from the network. This is necessary so that the power source does not shock you when you touch the input contacts.
When the power source is connected to the network, the discharged capacitor C1 is, roughly speaking, a conductor, and a huge current flows through the zener diode VD1 for a short time, which can disable it. Resistor R2 limits the inrush current when the device is turned on.
"Inrush current" at the initial moment of switching on the circuit. The mains voltage is drawn in blue, the current consumed by the power source in red. For clarity, the current graph is increased several times.
If you connect the circuit to the network at the time the voltage crosses zero, there will be no inrush. But what is the likelihood that you will succeed?
Any capacitor resists the flow of alternating current. (For DC, the capacitor is an open.) The magnitude of this resistance depends on the frequency of the input voltage and capacitance of the capacitor and can be calculated by the formula. Capacitor C1 plays the role of ballast resistance, on which most of the input voltage of the network will fall.
You may have a reasonable question: why can not you put a regular resistor instead of C1? It is possible, but power will dissipate on it, as a result of which it will warm up. This does not happen with the capacitor - the active power released on it for one period of the mains voltage is zero. In the calculations, we will touch on this point.
So, part of the input voltage drops on capacitor C1. (The voltage drop across the resistor R2 can not be taken into account, since it has a small resistance.) The remaining voltage will be applied to the zener diode VD1.
In a positive half-cycle, the input voltage will be limited by the zener diode at the level of its rated stabilization voltage. In the negative half-cycle, the input voltage will be applied to the zener diode in the forward direction and the zener diode will have a voltage of approximately minus 0.7 Volts.
Naturally, such a pulsating voltage is not suitable for powering the microcontroller, so after the zener diode there is a chain of a semiconductor diode VD2 and an electrolytic capacitor C2. When the voltage at the zener diode is positive, current flows through the VD2 diode. At this moment, capacitor C2 is charged and the load is powered. When the voltage on the zener diode drops, the diode VD2 is locked and the capacitor C2 gives off the stored energy to the load.
The voltage across capacitor C2 will fluctuate (ripple). In the positive half-period of the mains voltage, it will increase to Ust minus the voltage on VD2, in the negative half-period it will fall due to the discharge to the load. The amplitude of voltage fluctuations on C2 will depend on its capacitance and the current consumed by the load. The larger the capacitance of the capacitor C2 and the smaller the load current, the smaller these ripples will be.
If the load current and ripple are small, then after the capacitor C2 it is already possible to put the load, but for devices on microcontrollers it is better to still use a circuit with a stabilizer. If we correctly calculate the ratings of all components, then at the output of the stabilizer we get a constant voltage.
The circuit can be improved by adding a diode bridge to it. Then the power supply will use both half-periods of the input voltage - both positive and negative. This will make it possible to obtain better ripple parameters with a smaller capacitor C2. The diode between the zener diode and the capacitor can be excluded from this circuit.
To be continued...
Many hams do not consider power supplies without transformers. But despite this, they are used quite actively. In particular, in security devices, in radio control circuits of the chandelier, loads and in many other devices. In this video tutorial, consider the simple design of such a 5 volt rectifier, 40-50 mA. However, you can change the circuit and get almost any voltage.
Transformerless sources are also used as chargers and are used in powering LED lamps and in Chinese lanterns.
For hams there is everything in this Chinese store.
Circuit analysis.
Consider a simple transformerless circuit. The voltage from the network 220 volts through the limiting resistor, which simultaneously acts as a fuse, goes to the quenching capacitor. The output is also mains voltage, but the current is repeatedly reduced.
Transformerless rectifier circuit
Next, to a half-wave diode rectifier, at its output we get a constant current, which is stabilized by means of a stabilizer VD5 and smoothed by a capacitor. In our case, the capacitor is 25 V, 100 μF, electrolytic. Another small capacitor is installed in parallel with the power supply.
Then it goes to a linear voltage regulator. In this case, the linear stabilizer 7808 was used. There is a small typo in the circuit, the output voltage is actually approximately 8 V. Why is there a linear stabilizer, zener diode in the circuit? In most cases, it is not allowed to supply voltage regulators above 30 V to linear voltage stabilizers. Therefore, a zener diode is needed in the circuit. The output current rating is determined to a greater extent by the capacity of the quenching capacitor. In this embodiment, it has a capacity of 0.33 μF, with a rated voltage of 400 V. A discharge resistor with a resistance of 1 MΩ is installed in parallel with the capacitor. The value of all resistors can be 0, 25 or 0, 5 watts. This resistor so that after turning off the circuit from the network, the capacitor does not hold the residual voltage, that is, is discharged.
The diode bridge can be assembled from four rectifiers per 1 A. The reverse voltage of the diodes must be at least 400 V. You can also use ready-made diode assemblies of the type KTs405. In the directory you need to look at the permissible reverse voltage through the diode bridge. A zener diode is preferably 1 watts. The stabilization voltage of this zener diode should be from 6 to 30 V, not more. The current at the output of the circuit depends on the rating of this capacitor. At a capacitance of 1 μF, the current will be in the region of 70 mA. You should not increase the capacitance of the capacitor more than 0.5 μF, since a fairly large current, of course, burns the zener diode. This scheme is good in that it is small-sized, you can collect from improvised means. But the disadvantage is that it does not have galvanic isolation from the network. If you intend to use it, then be sure to use it in a closed case so as not to touch the high-voltage parts of the circuit. And, of course, you should not associate great hopes with this circuit, since the output current of the circuit is small. That is, enough to power low-power devices with a current of up to 50 mA. In particular, the power supply of LEDs and the construction of LED lamps and nightlights. The first start must be done in series with a light bulb.
In this embodiment, there is a 300 ohm resistor, which in case of failure. We already do not have this resistor on the board, so we added a light bulb that will light up a bit during the operation of our circuit. In order to check the output voltage, we will use the most ordinary multimeter, a constant meter 20 V. We connect the circuit to a 220 V network. Since we have a protective light, it will save the situation if there are any problems in the circuit. Exercise extreme caution during operation with high voltage, since still 220 V is supplied to the circuit.
Conclusion
The output is 4.94, that is, almost 5 V. At a current of not more than 40-50 mA. Great for low power LEDs. You can power the LED rulers from this circuit, only at the same time replace the stabilizer with a 12-volt one, for example, 7812. In principle, you can get any voltage within a reasonable range at the output. That's all. Do not forget to subscribe to the channel and leave your feedback about further videos.
Attention! When the power supply is assembled, it is important to place the assembly in a plastic case or carefully insulate all contacts and wires to prevent accidental contact with them, since the circuit is connected to a 220 volt network and this increases the likelihood of electric shock! Use caution and TB!