Thyristors are used to control power. They are used in light controllers or in controlling engine speed. During the repair process, it is not difficult to identify the malfunction of such a radio component using a multimeter. All thyristors are tested the same way. Knowing how to test the BTB16-700BW, it will be possible to determine the performance of other elements of the thyristor family.
Purpose and device
A thyristor is an electronic device built on a semiconductor single crystal with several p-n junctions. Such a device is characterized by two stable operating modes: closed, when there is no conductivity, and open - the device is in a state of high conductivity. The thyristor can be considered as an electronic key. Depending on its state, the electrical signal may or may not be sent further to the circuit.
The thyristor family includes several types of devices that differ in the type of conductivity, for example, triac, dinistor, thyristor. A triac is used to operate in an AC circuit because it can conduct current in any direction. Such a device in its design has three terminals, therefore in English literature it is called TRIAC (triode for alternating current), which translates as alternating current triode.
Two outputs of the device are called controlled, and one is called control. The triac does not have an anode and cathode. In electrical circuits, the electronic key is connected in series with the load. For it to transition from a closed state to an open state, a signal of a certain amplitude must be received at the control terminal of the device, and current can flow unhindered in both directions.
A special feature of the triac is that to maintain one or another of its states, the constant presence of voltage at the switching electrode is not required, and only a short pulse is enough to change the conductivity. But at the same time, there is a condition that a current of a certain magnitude, called the holding current, must flow through the controlled terminals.
On diagrams and in technical literature, a triac is signed with the letters VS with a number indicating its serial number. It is depicted in the form of triangles standing parallel to each other with oppositely directed vertices. From the base of one of the geometric figures, a platform is drawn, designated by the Latin letter G (gate). The other two pins are labeled T1 and T2, indicating the power pins. In some circuits, controlled electrodes may be designated by the letter A.
An interesting fact is that this semiconductor device was invented at the Mordovian Scientific Research Electrotechnical Institute in 1963.
Principle of operation
Although the element is a fairly reliable device, it happens that it fails. But in order to correctly test a triac, you need to be able not only to use measuring instruments, but also to understand the essence of its work.
A semiconductor element can be imagined as two thyristors, which are connected relative to each other in counter-parallel. Like a diode, a triac has p-n junctions, but it has more of them. The structure of the device consists of five alternating layers. Depending on the applied voltage to the control terminal, processes of recombination and movement of the main charge carriers or, conversely, expansion of band gaps begin in barrier junctions.
The triac goes into a state where charges can freely pass through it, when two conditions are met:
- a current of the required amplitude (unlocking) is supplied to its control output;
- the potential difference between the controlled electrodes corresponds to a certain value determined by the device parameters.
In other cases, the semiconductor key will be locked. When using the device in circuits with an alternating signal at the terminals, the polarity of the voltage will constantly change, so the operating mode of the device is divided into four quadrants. Each quadrant has its own conditions for unlocking or locking.
If the potential difference between the power terminals is equal to V A1-A2 >0, and relative to the control electrode at input A1 there is a negative voltage, then the triac corresponds to the second quadrant with its certain values of the unlocking (Igt) and holding (In) currents.
Due to their peculiarity of operation, this type of thyristor was originally used as a control element on production machines, allowing current to be supplied smoothly to them. These were quite large devices that required massive cooling. But The evolution of the device has led to a reduction in size and improving the technical manufacturing process. This made it possible to use triacs in conjunction with compressor equipment, electric heating systems, power tools and charging units.
Device characteristics
Like any electronic semiconductor device, a triac is characterized by a number of technical parameters. They make it possible to use it in one or another equipment. The correct operation of the device is determined by the compliance of the declared characteristics with the real ones, and the essence of the measurements comes down to obtaining the values of these parameters.
Complex measurement of characteristics requires specialized instruments that are not available for domestic use, therefore, to test a triac, radio amateurs most often use a tester and special circuits. For example, the popular triac BTB16-700BW is characterized by the following technical parameters:
In addition to these parameters, secondary characteristics are often indicated, for example, operating temperature range (from –40 °C to +125 °C), housing type (TO-220 AB).
Element testing
There are several ways to test a triac for functionality. For the simplest, you only need a multimeter, and for more complex measurements, an autonomous power source or test circuit.
With the help of a tester, testing occurs using knowledge based on the operating principle of a triac. Diagnostics with a multimeter will not be able to determine all the characteristics of the element, but it will be quite sufficient for initial performance testing.
A simple test can be done using a light bulb and a battery. To do this, one terminal of the battery is connected to the control and operating terminals of the triac, and the second to the base of the light bulb. The element's output is connected to the central contact of the illuminator. In this case, the transition must be open, then the light will light up.
If, even before voltage is applied to the control terminal, the lighting device lights up, then this indicates that the triac is faulty and its transitions are broken. Such an element can no longer be checked, since it is faulty.
Checking with a tester
A device of any type of action is suitable for carrying out tests, but it is necessary that the value of the current it produces is sufficient to switch the element. Therefore, it would be more preferable to use an analog device. For example, to test the BTB12-800CW tester, you will need to provide a current of about 30 mA, and for the BTB16-700BW this figure should be 15 mA.
You will also need to pay attention to the condition of the battery in the tester. In a digital device, the battery replacement icon should not be displayed on the screen, and in an analog device, when the probes are short-circuited to each other, the arrow should point to zero.
The essence of the measurement comes down to checking the transitions of the device. To do this, the tester switches to resistance testing mode at the smallest range. It is best to perform the check in the following sequence:
This behavior of the triac when tested by a tester indicates a high probability of its serviceability. It is also worth noting that during such a test it is not necessary to completely unsolder the radio element from the circuit, but rather just disconnect its control contact.
Using a schema
There are many different schemes used by radio amateurs to test the performance of a triac. But it is better to use a universal circuit that can test any element of the thyristor family, for example, BTB16-700BW. It does not require configuration and works immediately after assembly. In order to assemble it, you will need the following elements:
A KRONA type battery can be used as a power source.
The essence of the measurements comes down to the following actions: switch S3 is moved to the upper position, as a result, power is supplied to the device. After this, by briefly pressing the S2 button, current is supplied to the control output of the element.
If the BTB16-700BW is working, then its junction should open, which will be signaled by the VD3 LED. Then the switch is set to the middle position, the LED should go out. In the next step, S3 is switched to the down position and button S2 is pressed. The result of these actions will be the lighting of the VD4 LED. This behavior of the triac will allow us to declare with 100% confidence that it is working.
Checking a triac is not so difficult, especially if you use a tester, although it is better to assemble a special circuit. But it is worth noting that due to the high sensitivity of triacs to switching current, it is better to use pointer instruments as multimeters.
From the article you will learn about what a triac is, the operating principle of this device, as well as the features of its application. But first, it’s worth mentioning that a triac is the same as a thyristor (only symmetrical). Therefore, the article cannot do without a description of the operating principle of thyristors and their features. Without knowing the basics, it will not be possible to design and build even the simplest control circuit.
Thyristors
A thyristor is a switching device that is capable of passing current in only one direction. It is often called a valve and analogies are drawn between it and a controlled diode. Thyristors have three terminals, one of which is the control electrode. This, to put it roughly, is a button that switches the element into conductive mode. The article will consider a special case of a thyristor - a triac - a device and its operation in various circuits.
A thyristor is also a rectifier, a switch, and even a signal amplifier. It is often used as a regulator (but only when the entire electrical circuit is powered from an alternating voltage source). All thyristors have some features that need to be discussed in more detail.
Thyristor properties
Among the huge variety of characteristics of this semiconductor element, the most significant ones can be identified:
- Thyristors, like diodes, can only conduct in one direction. In this case they work in a circuit like
- The thyristor can be switched from off to on by applying a signal with a certain shape to the control electrode. Hence the conclusion - a thyristor, like a switch, has two states (both stable). A triac can function in the same way. The operating principle of an electronic key based on it is quite simple. But in order to return to the original open state, certain conditions must be met.
- The control signal current, which is necessary for the transition of the thyristor crystal from a locked mode to an open mode, is much less than the operating one (literally measured in milliamps). This means that the thyristor has the properties of a current amplifier.
- It is possible to finely regulate the average current flowing through a connected load, provided that the load is connected in series with the thyristor. The accuracy of the adjustment directly depends on the duration of the signal at the control electrode. In this case, the thyristor acts as a power regulator.
Thyristor and its structure
A thyristor is a semiconductor element that has control functions. The crystal consists of four layers of p and n type, which alternate. The triac is built in the same way. The operating principle, application, structure of this element and limitations in use are discussed in detail in the article.
The described structure is also called four-layer. The extreme region of the p-structure with the positive polarity terminal of the power source connected to it is called the anode. Consequently, the second region n (also extreme) is the cathode. A negative voltage from the power supply is applied to it.
What properties does a thyristor have?
If you conduct a complete analysis of the structure of the thyristor, you can find three transitions (electron-hole) in it. Therefore, it is possible to create an equivalent circuit using semiconductor transistors (polar, bipolar, field-effect) and diodes, which will allow us to understand how the thyristor behaves when the power to the control electrode is turned off.
In the case when the anode is positive relative to the cathode, the diode closes, and, therefore, the thyristor also behaves similarly. In the event of a polarity change, both diodes are biased, and the thyristor is also turned off. A triac functions in a similar way.
The principle of working on the fingers, of course, is not very easy to explain, but we will try to do this further.
How does thyristor unlocking work?
To understand, you need to pay attention to the equivalent circuit. It can be composed of two semiconductor triodes (transistors). Here it is convenient to consider the process of unlocking thyristors. A certain current is set that flows through the thyristor control electrode. In this case, the current has a forward bias. This current is considered the base current for a transistor with a p-p-n structure.
Therefore, the current in the collector will be several times greater (it is necessary to multiply the value of the control current by the gain of the transistor). Further, you can see that this current value is the base value for the second transistor with a pnp conductivity structure, and it is unlocked. In this case, the collector current of the second transistor will be equal to the product of the gains of both transistors and the initially specified control current. Triacs (the principle of operation and their control are discussed in the article) have similar properties.
Next, this current must be summed with the previously specified control circuit current. And the result will be exactly the value that is necessary to maintain the first transistor in the unlocked state. In the case when the control current is very large, two transistors are simultaneously saturated. The internal feedback continues to maintain its conductivity even when the initial current at the control electrode disappears. At the same time, a fairly high current value is detected at the anode of the thyristor.
How to turn off a thyristor
The transition to the locked state of the thyristor is possible if no signal is applied to the control electrode of the open element. In this case, the current drops to a certain value, which is called the hypostatic current (or holding current).
The thyristor will also turn off if there is an open circuit in the load circuit. Or when the voltage that is applied to the circuit (external) changes its polarity. This occurs at the end of each half-cycle when the circuit is powered by an alternating current source.
When a thyristor operates in a circuit, locking can be done using a simple switch or mechanical button. It is connected to the load in series and is used to de-energize the circuit. The principle of operation is similar, however, there are some features in the circuit.
Therefore, it is advisable to position the switch so that it is located between the cathode and the control electrode. This will ensure that the thyristor turns off normally and the holding current is cut off. Sometimes, for convenience and to increase speed and reliability, an auxiliary thyristor is used instead of a mechanical key. It is worth noting that the operation of a triac is in many ways similar to the operation of thyristors.
Triacs
And now closer to the topic of the article - we need to consider a special case of a thyristor - a triac. Its operating principle is similar to what was discussed earlier. But there are some differences and characteristic features. Therefore, we need to talk about it in more detail. A triac is a device based on a semiconductor crystal. Very often used in systems that operate on alternating current.
The simplest definition of this device is a switch, but controlled. When locked, it operates exactly the same as a switch with its contacts open. When a signal is applied to the control electrode of the triac, the device transitions to the open state (conductivity mode). When operating in this mode, you can draw a parallel with a switch whose contacts are closed.
When there is no signal in the control circuit, in any of the half-cycles (when operating in alternating current circuits) the triac transitions from open to closed mode. Triacs are widely used in relay mode (for example, in the designs of photosensitive switches or thermostats). But they are also often used in control systems that operate on the principles of phase control of voltage at the load (they are smooth regulators).
Structure and principle of operation of a triac
A triac is nothing more than a symmetrical thyristor. Therefore, based on the name, we can conclude that it can be easily replaced by two thyristors that are switched on back-to-back. It is capable of passing current in any direction. The triac has three main terminals - the control terminal, for supplying signals, and the main terminals (anode, cathode) so that it can pass operating currents.
The triac (the operating principle of this semiconductor element for dummies is presented to your attention) opens when the minimum required current value is supplied to the control output. Or in the case when the potential difference between two other electrodes is higher than the maximum permissible value.
In most cases, excess voltage leads to the fact that the triac spontaneously trips at the maximum amplitude of the supply voltage. The transition to the locked state occurs in the event of a polarity change or when the operating current decreases to a level lower than the holding current.
How the triac is unlocked
When powered from the mains, a change in operating modes occurs due to a change in the polarity of the voltage at the working electrodes. For this reason, depending on the polarity of the control current, 4 types of this procedure can be distinguished.
Let's say a voltage is applied between the working electrodes. And on the control electrode the voltage is opposite in sign to that applied to the anode circuit. In this case, the triac will shift along the quadrant - the principle of operation, as you can see, is quite simple.
There are 4 quadrants, and for each of them, the unlocking, holding, and switching currents are defined. The unlocking current must be maintained until it exceeds several times (2-3) the value of the holding current. This is precisely the switching current of the triac - the minimum required unlocking current. If you get rid of the current in the control circuit, the triac will be in a conducting state. Moreover, it will operate in this mode until the current in the anode circuit is greater than the holding current.
What restrictions are imposed when using triacs?
It is difficult to use when the load is inductive type. The rate of change of voltage and current is limited. When the triac goes from a locked mode to an open mode, a significant current appears in the external circuit. The voltage does not drop instantly at the power terminals of the triac. And the power will instantly develop and reach quite large values. The energy that is dissipated, due to the small space, sharply increases the temperature of the semiconductor.
Using a home tester (multimeter), you can check a variety of radio elements. For a home craftsman who is interested in electronics, this is a real find.
For example, testing a thyristor with a multimeter can save you from having to find a new part when repairing electrical equipment.
To understand the process, let’s look at what a thyristor is:
This is a semiconductor device made using classical single-crystal technology. There are three or more p-n junctions on the crystal, with diametrically opposed stable states.
The main application of thyristors is an electronic key. These radio elements can be effectively used instead of mechanical relays.
Switching on is adjustable, relatively smooth and without contact bounce. The load in the main direction of opening of the p-n junctions is supplied in a controlled manner; the rate of increase of the operating current can be controlled.
In addition, thyristors, unlike relays, are perfectly integrated into electrical circuits of any complexity. The absence of sparking contacts allows their use in systems where interference during switching is unacceptable.
The part is compact and is available in various form factors, including for mounting on cooling radiators.
Thyristors are controlled by external influence:
- An electric current that is supplied to the control electrode;
- A beam of light if a photothyristor is used.
In this case, unlike the same relay, there is no need to constantly supply a control signal. The working pn junction will be open even after the supply of control current has ended. The thyristor will close when the operating current flowing through it drops below the holding threshold.
Thyristors are available in various modifications, depending on the control method and additional capabilities.
- Direct conduction diodes;
- Reverse conduction diodes;
- Diode symmetrical;
- Direct conduction triodes;
- Reverse conduction triodes;
- Triode asymmetrical.
If you analyze the development path of semiconductor electronics, it almost immediately becomes clear that all semiconductor devices are created on junctions or layers (n-p, p-n).
The simplest semiconductor diode has one junction (p-n) and two layers.
A bipolar transistor has two junctions and three layers (n-p-n, p-n-p). What happens if you add another layer?
Then we will get a four-layer semiconductor device called a thyristor. Two thyristors connected back-to-back are a triac, that is, a symmetrical thyristor.
In English-language technical literature you can find the name TRIAC ( TRIAC– triode for alternating current).
This is how a triac is depicted on circuit diagrams.
The triac has three electrodes (terminals). One of them is the manager. It is designated by the letter G(from the English word gate - “shutter”). The other two are power electrodes (T1 and T2). On diagrams they can also be designated by the letter A (A1 and A2).
And this is the equivalent circuit of a triac made on two thyristors.
It should be noted that the triac is controlled somewhat differently than the equivalent thyristor circuit.
A triac is a rather rare phenomenon in the family of semiconductor devices. For the simple reason that it was invented and patented in the USSR, and not in the USA or Europe. Unfortunately, the opposite is more often the case.
How does a triac work?
If a thyristor has a specific anode and cathode, then the electrodes of the triac cannot be characterized in this way, since each electrode is both an anode and a cathode at the same time. Therefore, unlike a thyristor, which conducts current in only one direction, triac is capable conduct current in two directions. This is why the triac works great in AC networks.
An electronic power regulator can serve as a very simple circuit that characterizes the operating principle and scope of application of a triac. You can use anything as a load: an incandescent lamp, a soldering iron or an electric fan.
After connecting the device to the network, alternating voltage is supplied to one of the electrodes of the triac. A negative control voltage is supplied to the electrode, which is the control electrode, from the diode bridge. When the switching threshold is exceeded, the triac will open and current will flow to the load. At the moment when the voltage at the triac input changes polarity, it will close. Then the process is repeated.
The higher the control voltage level, the faster the triac will turn on and the duration of the pulse on the load will be longer. As the control voltage decreases, the duration of the pulses on the load will be shorter. After the triac, the voltage has a sawtooth shape with an adjustable pulse duration. In this case, by changing the control voltage, we can adjust the brightness of a light bulb or the temperature of the soldering iron tip.
The triac is controlled by both negative and positive current. Depending on the polarity of the control voltage, four so-called sectors or operating modes are considered. But this material is quite complex for one article.
If we consider a triac as an electronic switch or relay, then its advantages are undeniable:
Low cost.
Compared to electromechanical devices (electromagnetic and reed relays) long service life.
There are no contacts and, as a result, no sparking or rattling.
The disadvantages include:
The triac is very sensitive to overheating and is mounted on a radiator.
It does not work at high frequencies, because it simply does not have time to transition from open to closed state.
Reacts to external electromagnetic interference, which causes false alarms.
To protect against false alarms, an RC circuit is connected between the power terminals of the triac. Resistor value R1 from 50 to 470 ohms, capacitor size C1 from 0.01 to 0.1 µF. In some cases, these values are selected experimentally.
Basic parameters of a triac.
It is convenient to consider the main parameters using the example of a popular domestic triac KU208G. Having been developed and released quite a long time ago, it continues to be in demand among those who like to do something with their own hands. Here are its main parameters.
Maximum reverse voltage – 400V. This means that it can perfectly control the load on a 220V network and with some reserve.
In pulse mode the voltage is exactly the same.
The maximum current in the open state is 5A.
The maximum current in pulse mode is 10A.
The smallest direct current required to open a triac is 300 mA.
The smallest pulse current is 160 mA.
Opening voltage at a current of 300 mA is 2.5 V.
Opening voltage at a current of 160 mA – 5 V.
Turn-on time – 10 µs.
Turn-off time – 150 µs.
As you can see, to open a triac, a necessary condition is the combination of current and voltage. More current, less voltage and vice versa. Note the large difference between turn-on and turn-off times (10 µs vs. 150 µs).
A modern and promising type of triac is an optosimistor. The name speaks for itself. Instead of a control electrode, there is an LED in the triac housing, and control is carried out by changing the voltage on the LED. The image shows the appearance of the MOC3023 optosimistor and its internal structure.
Optosimistor MOC3023
As you can see, an LED and a triac are mounted inside the case, which is controlled by the radiation of the LED. Pins marked N/C and NC are not used and are not connected to circuit elements. NC is an abbreviation for N ot C onnect, which is translated from English as “does not connect”.
The most valuable thing about an optosimistor is that there is complete galvanic isolation between the control circuit and the power circuit. This increases the level of electrical safety and reliability of the entire circuit.
There have already been reviews on the site dedicated to the creation of spot welding machines. The item is very expensive when purchased ready-made, but is often very useful in the household for those who like to do something with their hands. Let me remind you that this device allows you to easily weld contact plates to batteries, weld thin sheets of metal, weld steel wire, etc. Below the cut is my version of the implementation of this unit. Readers can expect reflections, diagrams, circuit boards, programming, construction (all elements of collective farming) with many photos and videos...
Since many parts will be used in the review, I will provide links to them throughout the review; perhaps now the same parts are available cheaper from other sellers.
The subject of this review arrived in a hard plastic package containing 10 copies of the BTA41-800B triac.
We need this element to turn the welding machine on and off at the right moments.
Maximum reverse voltage 800 V
Maximum open current 40 A
Operating temperature -40 to 125 °C
TOP-3 case
A triac (symmetrical triode thyristor) or triac (from the English TRIAC - triode for alternating current) is a semiconductor device, which is a type of thyristor and is used for switching in alternating current circuits. It should be noted that the triac was invented and patented in the USSR (in Saransk at the Elektrovypryamitel plant in 1962-1963).
Block diagram of this element: 
A1 and A2 - power electrodes
G - control electrode
In the closed state, there is no conductivity of the triac, the load is turned off. When an unlocking signal is applied to the control electrode, conduction occurs between the main electrodes of the triac, and the load turns on. It is characteristic that a triac in the open state conducts current in both directions.
Detailed characteristics of the BTA41-800B can be found in.
To control a triac, special triac optocouplers (triac drivers) are usually used. Optosimistors belong to the class of optocouplers and provide very good galvanic isolation (about 7500 V) between the control circuit and the load. These radioelements consist of an infrared LED connected via an optical channel to a bidirectional silicon triac. The latter can be supplemented with an unlocking circuit that is triggered when the supply voltage passes through zero. 
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In most cases, it is preferable to use optosimistors with zero detection, for a variety of reasons. Sometimes (with a resistive load, zero detection is not important. And sometimes you need to turn on the load, for example, at the maximum sinusoid of the mains voltage, then you have to build your own detection circuit and, of course, use an optosimistor without zero detection.
Let's move on to our device. The stars just so happened that I needed to replace the cans in a couple of screwdriver batteries and came across a faulty microwave... And at the same time, the thought of the need to build myself a spot welder had been in my head for a long time. And I decided to take this step.
Next, you need to wind a thick wire instead of the removed secondary winding. I used this stranded wire with a cross section of 70 mm2: 
Its old name is PV3-70. Winding the wire did not require much effort, it turned out like this: 
I bought 2 meters of wire, I think I could have gotten by with one meter.
Cleaning the ends: 
We prepare soldering equipment (LTI-120 flux, a 2mm solder coil and a gas burner mounted on a gas cylinder): 
It is better to use a tip made of tinned copper for a 70 mm wire (TML 70-12-13): 
We generously moisten the inner surfaces of the tips and wires with flux. We insert the wire into the tip, bending the unruly wires (not a quick procedure), and heat it with a burner, applying solder from the side. The result is something like this: 
We’ll cover all the horrors with heat shrink: 
This one fits perfectly on my wire: 
At this stage, you can already connect the transformer to the outlet using a wire from the microwave (it already has terminals for connection) and even try to make the first welding, switching by pressing the ends of a thick wire, the only thing I recommend is screwing on some copper parts, since the tips will not be damaged desirable. You will only be able to cook some thick parts - since the switching capabilities are very limited.
Let's move on to the electrical part. I have already said that I decided to use a triac to switch the primary winding; it remains to decide which optosimistor to control it with. I decided to make a zero detection circuit, so I chose the option without zero detection, taking . for this chip. Typical inclusion is as follows: 
I decided to use the microwave fan to cool the transformer and board. Since it is also 220 V, I decided to use a relay to turn it on; it is compact and copes well with low-power loads.
To control the logic, I decided to use a controller in a QFP32 package.
The power supply is needed for 5 Volts, I used it. It is rated at 600 mA, which is quite enough.
The main focus in this matter is synchronization with a 220 V network. You need to learn how to turn on the load at the moment when the network voltage has a certain value. As a result, I came to this scheme: 
Features: VD1 - you need to choose a fast diode (I took MUR) - it is needed to bypass the optocoupler and avoid the appearance of a reverse voltage of more than 5 V on it, VD2 - any rectifier will do (1N4007 will do - it will significantly reduce the thermal load on R2, removing the extra half-wave ), R2- should be taken with a power of 1-2 W (I didn’t have it on hand, so I put 2 resistors in parallel, 90 KOhms per 1/4 W, the temperature turned out to be acceptable). A6 is the analog input of the controller that I used for these purposes. R1 pulls the controller input to ground. The rest of the scheme is quite simple.
I drew the board in Sprint Layout: 
We make a LUT board. After etching in ferric chloride: 
After washing off the toner: 
After tinning: 
Contrary to the usual tactics, I first soldered the power part in order to debug it independently of the controller; I decided to glue a radiator cut from an aluminum profile onto the triac: 
It turned out like this: 
I made sure everything was fine: 
The zero tracking circuit produces this: 
Soldered the remaining elements: 
We flash the bootloader (fortunately, I specially removed the SPI pins), and start writing, testing, fixing, resoldering... 
An oscilloscope was intensively used for debugging; I use it at the dacha; at home, of course, a stationary one is more convenient: 
Now you can solder the wires to connect the load (transformer and fan), I used wires with terminals from the same microwave, at that moment the thought flashed through not to confuse them during assembly... 
To check, I connected an incandescent lamp instead of a transformer, at this stage the welding looks like this:
A shift of 3 ms gives the following control pulses: 
And this is what goes into the load looks like (the scale of the mains voltage is deliberately taken to be different): 
And like this for a different duration: 
For visualization I used (I only used 2: blue and green), with a common cathode. When the welder is plugged in, the light is green; when welding is in progress, the light is blue. A sound alarm is also used using here, when you press the welding button, one melody is played, then another.
To visualize the setup process, I used a 1.3" diagonal. It is compact and clearly visible due to its brightness - in my opinion, the optimal solution.
The start screen looks like this: 
The working mode is like this: 
As you can see, three parameters can be set: the duration of the welding pulse, the number of pulses and the shift relative to the recognized beginning of the positive half-wave.
All parameters are customizable. I decided to make the following logic: switching setting modes is carried out by briefly pressing the encoder, changing the current parameter in a given range by rotating the encoder, and to save the current parameters you need to use a long press of the encoder, then they will be used when loading (default values).
Video of test welding with a screen and the use of an encoder, using the same 75 W light bulb as a load instead of a transformer:
First experience of welding on sheet metal from a tin can, still without a body: 
I was pleased with the result.
But you need a body. I decided to make the body out of wood. I had one furniture panel from Leroy and bought a second one. I figured out the location and sawed and cut it out (it didn’t turn out very neatly, but it suits me quite well as a housing for a spot welding machine: 
I decided to make all the controls in the front part of the case for ease of adjustment during operation: 
There are holes at the back for air intake: 
I installed a 10A circuit breaker as the power button and fuse.
The body was painted black: 
For protection, I installed grilles on the rear panel: 
A little about the power button. I decided to make it separately, and I wanted to have two options for the button: stationary - for long-term operation and mobile - for quick welding. Accordingly, a connector was required, which was a standard power connector (soldered wires to it and insulated with heat shrink): 
I decided to build a stationary version of the button in the form: 
There was a short wire leading to it, apparently it was supposed to be connected to the long one. Let's look at: 
Solder PVA 2x0.5: 
The original cable had three wires: 
We don't need black.
Let's put everything back together. And solder the plug to the other end of the wire: 
The mobile version was made quite simply: 
We attach the screen and connector for the button to the housing: 
We attach our board there: 
It's quite tight inside: 
Remember, I wrote about the idea of not mixing up loads... so I got it mixed up. OMRON G3MB-202P - went to its forefathers, starting to be turned on regardless of the control signal... In it: 
I had to remove the wall, then the board and resolder the relay. The process was accompanied by a small amount of obscene language. Moreover, before this I had already coated the board with 2 layers of protective varnish... But let’s not talk about sad things. Everything worked out, the device started working.
As you know, the rotation of a fan, especially one as small as in our case, is accompanied by vibration and load on the mount; the threaded connection gradually weakens and the process worsens. To prevent this from happening, in my crafts I try to use the domestic thread locker Automastergel from Region Spetstekhno. I even reviewed this wonderful gel: 
This fixative is anaerobic, that is, it polymerizes exactly where it is needed - in the tight twist of the thread.
I screwed glamorous legs to the bottom of the case: 
Test welding brought a lot of positive emotions: 
You need to use copper plates as electrodes, I didn’t have any, I flattened the tube from the air conditioner - quite normal.
This is what was cooked: 
Final view of the unit: 
Back view: 
Welds nails quite normally: 
A few measurements. Parameters of the country electrical network: 
Idle consumption: 
With the fan on: 
Due to the inertia of the device and welding with short pulses, most likely the device cannot determine the maximum power, this is how much it showed: 
My current clamps are not able to show a peak, but what I managed to fix with the button: 
In reality, I saw a figure of 400 A.
Contact voltage: 
Now for a useful application. One person (hello to him :)) had a screwdriver overwintered at his dacha and in the spring or even fall it was flooded. There were complaints about the very short operating time of the Akum, 1-2 screws and that’s it... Here’s a picture of the autopsy: 
Akuma felt clearly not okay, this was later confirmed by tests: 
New cans were ordered to replace them. And after finishing work with the welder, it was time to replace them: 
I couldn't tear off the strips with my hands. The scarf was washed and the wires were also replaced:: 
The battery started a new life: 
Battery welding video:
The result is always stable, the optimal time is 34 ms, the number of pulses is 1, the shift is 3 ms.
Thanks to everyone who read this huge review to the end, I hope someone finds this information useful. strong connections and good luck to everyone!
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