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Do-it-yourself impulse starting device for a car. Do-it-yourself start-up charger. Winding explanations

Last night I forgot to turn off the lights. In the morning the car did not start, and the car is urgently needed. While I was looking for someone to "light up" I remembered that there was a household welding MMA inverter in the trunk. That's what I thought

why not charge your car battery with a welding inverter?

You can charge the battery with an inverter if it is equipped with a start-charging function. For example, the device (pictured) is able to recharge the battery or start the engine. Set the output of your inverter to 12V, current 3A, if you need to charge the car battery. Amperage is calculated as 1/20*P, where P is the battery power. The exposure time is 30-40 minutes, this time will be enough to start the engine. To fully charge the battery, hold it at a current of 1.5 ... 2A for 3 hours.

If you have an ordinary household MMA welding inverter, it is not safe to try to start the car with it. You can damage the battery or the inverter itself. It is not capable of delivering a small current and voltage, usually 40 ... 60V and a current of 20 amperes are recorded at the output ... In the worst case, an acid battery can explode, and at best, a used battery will crumble and close, and in a new one the plates will deform. In order to get a current of 3A to an inverter or transformer power source, a ballast circuit is assembled that will limit the current (these can be resistors, diodes or incandescent bulbs of 60-100W).

Do-it-yourself microwave charger

You can build a simple and powerful battery charger from scratch. And it will cost practically nothing.

The diagram shows (from left to right)

  • A step-down transformer;
  • Diode bridge;
  • Ordinary computer fan;
  • Any voltmeter;
  • Electrolytic capacitor for 16V, more, for example, 25V. Capacitance from 3000uF to 10000uF. The higher the capacitance, the smoother the output current will be.

A 15A fuse is placed in the connection section of the primary winding of the transformer to protect against short circuits. in the primary winding, the voltage is high and dangerous. The diode bridge can be used from 10 to 50A, depending on which batteries you will charge with this device.

There is a lot of information on the Internet on creating a charger, as a rule, this is a remake of a computer power supply, which is rather unreliable and gives little power. They also offer to use ready-made step-down transformers, which are quite expensive in stores, and if you approach from this point of view, it is easier to buy a ready-made charger. They also offer to use transformers from old tube TVs, but today it is almost impossible to find such a rarity, except perhaps in a museum.

But the power source from the microwave oven can be easily found. There are a lot of old and broken microwaves. This is a high voltage source, but if you rewind it into a step-down transformer, you can use it in the proposed circuit.

The starter charger allows you to start the car engine in the winter. Since it takes a lot of time and effort to start an internal combustion engine with a dead battery. The density of the electrolyte decreases significantly in winter, and the sulfation process that occurs inside the battery increases its internal resistance and reduces the starting current of the battery. In addition, the viscosity of engine oil increases in winter, so the battery needs more starting power. To make it easier to start the engine in winter, you can warm up the oil in the car's crankcase, start the car from another battery, start it "from the pusher" or use a start-up charger for the car.

The starter charger for a car consists of a transformer and powerful rectifier diodes. For normal operation of the starting device, an output current of at least 90 amperes is required, and a voltage of 14 volts, so the transformer must be powerful enough at least 800 watts.


For the manufacture of a transformer, it is easiest to use a core from any LATR. The primary winding should be from 265 to 295 turns of wire with a diameter of at least 1.5 mm, preferably 2.0 mm. Winding must be carried out in three layers. Good insulation between layers.

After winding the primary winding, we test it by connecting it to the network and measure the no-load current. It should be in the range of 210 - 390 mA. If it is less, then unwind a few turns, and if more, then vice versa.

The secondary winding of the transformer consists of two windings and contains 15:18 turns of stranded wire with a cross section of 6 mm. The winding of the windings takes place simultaneously. The voltage at the output of the windings should be about 13 volts.

The wires connecting the device to the battery must be stranded, with a cross section of at least 10 mm. The switch must withstand a current of at least 6 amperes.

The starter charger circuit for a car contains a triac voltage regulator, a power transformer, a powerful diode rectifier and a starter battery. The charging current is set by the current regulator on the triac and is regulated by the variable resistance R2 and depends on the capacity of the battery. The input and output charging circuits contain filter capacitors that reduce the degree of radio interference during the operation of the triac regulator. The triac works correctly with a mains voltage of 180 to 230 V.

The rectifier bridge synchronizes the switching on of the triac in both half-cycles of the mains voltage. In the "Regeneration" mode, only the positive half-cycle of the mains voltage is used, which cleans the battery plates from the existing crystallization.

The power transformer is borrowed from the Rubin TV. You can also take the TCA-270 transformer. We leave the primary windings unchanged, but we will redo the secondary ones. To do this, we separate the frames from the core, unwind the secondary windings to the screen foil, and in their place are wound with a copper wire with a cross section of 2.0 mm in one layer until the secondary windings are filled. As a result of rewinding, approximately 15 ... 17 V should come out

When adjusting, an internal battery is connected to the starter charger, and the adjustment of the charging current by the resistance R2 is tested. Then we check the charging current in the charge, start and regeneration mode. If it is not more than 10 ... 12 amperes, then the device is in working order. When the device is connected to the car battery, the charge current at the initial moment increases by about 2-3 times, and after 10-30 minutes it decreases. After that, the SA3 switch is switched to the "Start" mode, and the car engine is started. In case of an unsuccessful attempt, we additionally recharge for 10 - 30 minutes, and try again.

The scheme contains: stabilized power supply(diodes VD1-VD4, VD9, VD10, capacitors C1, C3, resistor R7 and transistor VT2)

synchronization node(transistor VT1, resistors R1 / R3 / R6, capacitor C4 and elements D1.3 and D1.4, made on the K561TL1 chip);

pulse generator(elements D1.1, D1.2, resistors R2, R4, R5 and capacitor C2);

impulse counter(chip D2K561IE16);

amplifier(transistor VT3, resistors R8 and R9);

power unit(optocoupler thyristor modules VS1 MTO-80, VS2, power diodes V-50 VD5-VD8, shunt R10, devices - ammeter and voltmeter);

short circuit detection unit(transistor VT4, resistors R11-R14).

The scheme works as follows. When voltage is applied at the output of the bridge (diodes VD1-VD4), a half-wave voltage appears (graph 1 in Fig. 2), which, after passing through the circuit VT1-D1.3.-D1.4, is converted into pulses of positive polarity (graph 2 in Fig. 2). These pulses for the counter D2 are a reset signal to the zero state. After the reset pulse disappears, the generator pulses (D1.1, D1.2) are summed up in counter D2 and when the number 64 is reached, a pulse appears at the counter output (pin 6) with a duration of at least 10 generator pulse periods (graph 3 Fig. 2). This pulse opens the thyristor VS1 and voltage appears at the output of the ROM (graph 4 in Fig. 2). To illustrate the limits of voltage regulation, graph 5 of Fig. 2 shows the case of setting almost full output voltage.

With the parameters of the frequency-setting circuit (resistors R2, R4, R5 and capacitor C2 in Fig. 1), the opening angle of the thyristor VS1 lies within 17 (f = 70 kHz) - 160 (f = 7 kHz) electrical degrees, which gives the lower limit of the output voltage about 0.1 of the input value. The frequency of the output signals of the generator is determined by the expression

f \u003d 450 / (R 4 + R 5) С 2

,

where the dimension f is kHz; R - kOhm; C - nF. If necessary, the ROM can be used to regulate only the AC voltage. To do this, the bridge on diodes VD5-VD8 should be excluded from the circuit (Fig. 1), and the thyristors should be connected in anti-parallel (in Fig. 1 this is shown by a dashed line).

In this case, using the circuit (Fig. 1), you can adjust the output voltage from 20 to 200 V, but it should be remembered that the output voltage is far from sinusoidal, i.e. only electric heaters or incandescent lamps can serve as consumers. In the latter case, you can dramatically increase the life of the lamps, since they can be turned on smoothly by changing the voltage from 20 to 200 V with resistor R5. Adjustment of the ROM is reduced to detuning the level of operation of protection against short-circuit currents. To do this, we remove the jumpers between points A and B (Fig. 1) and at point B we temporarily apply voltage + Up. By changing the position of the resistor R14 engine, we determine the voltage level (point C in Fig. 1), at which the VT4 transistor opens. The level of protection operation in amperes can be determined by the formula I>k /R10, where k=Up/Ut.c., Up - supply voltage; Ut.s. - voltage at point C, at which VT4 is triggered; R10 - shunt resistance.


In conclusion, we can recommend the procedure for enabling the ROM to work and report possible replacement of components, tolerances and manufacturing features: the D1 chip can be replaced by the K561LA7 chip; chip D2 - chip K561IE10, connecting in series both counters; all resistors in the MLT type circuit are 0.125 W, with the exception of the R8 resistor, which must be at least 1 W; tolerances for all resistors, with the exception of resistor R8, and for all capacitors + 30%; the shunt (R10) can be made of nichrome with a total cross section of at least 6 mm (total diameter about 3 mm, length 1.3-1.5 mm). Turn on the ROM in operation only in the following sequence: turn off the load, set the required voltage with resistor R5, turn off the ROM, connect the load and, if necessary, increase the voltage with resistor R5 to the required value.

To solve the problem of starting the engine in winter, we use an electric starter that will allow motorists to start a cold engine even with an incompletely charged battery and thereby extend its life.

Payment. Carrying out an accurate calculation of the magnetic circuit of the transformer is impractical, since it is under load for a short time, especially since neither the brand nor the technology of rolling the electrical steel of the magnetic circuit is known. We find the required power of the transformer. The main criterion is the operating current of the electric starter Start, which is in the range of 70 - 100 A. Electric starter power (W) Rap = 15 Istart. Determine the cross section of the magnetic circuit (cm 2) S = 0.017 x Rep = 18...25.5 cm2. The electric starter circuit is very simple, you just need to correctly install the transformer windings. To do this, you can use toroidal iron from any LATRA or from an electric motor. For the electric starter, I used the transformer iron of an asynchronous electric motor, which I chose taking into account the cross section. Parameters S = av should not be less than the calculated ones.


The stator of the electric motor has protruding grooves that were used to lay the windings. When calculating the cross section, they are not taken into account. You need to remove them with a simple or special chisel, but you can not remove them (I did not delete them). This only affects the flow rate of the primary and secondary windings and the mass of the electric starter. The outer diameter of the magnetic circuit is within 18 - 28 cm. If the cross section of the stator of the electric motor is larger than the calculated one, it will have to be divided into several parts. With a hacksaw for metal, we cut the outer ties in the grooves and separate the torus of the required cross section. With a file, we remove sharp corners and protrusions. On the finished magnetic core, we carry out insulating work with varnished cloth or fabric-based insulating tape.

Now we proceed to the primary winding, the number of turns of which is determined by the formula: n1 = 45 U1/S, where U1 is the voltage of the primary winding, usually U1 = 220 V; S is the cross-sectional area of ​​the magnetic circuit.

For it, we take a copper wire PEV-2 with a diameter of 1.2 mm. Pre-calculate the total length of the primary winding L1. L1 \u003d (2a + 2c) Ku, where Ku - stacking factor, which is equal to 1.15 - 1.25; a and b - the geometric dimensions of the magnetic circuit (Fig. 2).

Then we wind the wire on the shuttle and install the winding in bulk. Having connected the leads to the primary winding, we process it with electrical varnish, dry it and carry out insulation work. Number of turns of the secondary winding n2 = n1U2/U1, where n2 and n1 are the number of turns, respectively, of the primary and secondary windings; U1 and U2 - voltage of the primary and secondary windings (U2 = 15 V).

The winding is carried out with an insulated stranded wire with a cross section of at least 5.5 mm2. The use of a busbar is preferable. Inside the wire we place a turn to turn, and on the outside with a small gap - for uniform arrangement. Its length is determined taking into account the dimensions of the primary winding. We place the finished transformer between two square getinax plates 1 cm thick and 2 cm wide more than the diameter of the wound transformer, having previously drilled holes in the corners for fastening with tie bolts. On the top plate we place the conclusions of the primary (isolate) and secondary windings, a diode bridge and a handle for transportation. We connect the outputs of the secondary winding to the diode bridge, and equip the outputs of the latter with M8 wing nuts and mark "+", "-". The starting current of a car is 120 - 140 A. But since the battery and the electric starter operate in parallel, we take into account the maximum current of the electric starter 100 A. Diodes VD1 - VD4 type B50 for a permissible current of 50 A. Although the engine start time is short, it is advisable to place the diodes on radiators. We install any switch S1 for a permissible current of 10 A. The connecting wires between the electric starter and the motor are stranded, with a diameter of at least 5.5 mm in different colors, and we equip the ends of the terminal lugs with crocodile clips.

Starter charger PZU-14-100

According to the start-up charger circuit, it is clearly seen that the thyristors are controlled by current pulses of the capacitance C4 circuit - transistors VT5, VT6, VT7 - diodes VD4, VD5. The thyristor unlocking phase and the current flow in the power circuit depend on the rate of increase in the voltage across the capacitance of the capacitor C4, that is, on the current through the resistance of the current regulator R23-R25 and through the start bipolar transistor VT3. VT3 turns on in the "start" mode if the voltage on the battery drops below 11 V. The key transistor VT4 turns on the control circuit when properly connected to the battery and protects it when the current is exceeded and the windings overheat. For reliable operation of this circuit, the most identical halves of the secondary winding are required, usually they are made by winding into two wires or by dividing the ends of the "pigtail" in two. The current flowing in the winding is measured by the voltage difference on the loaded and free halves, because - they are loaded in turn.

For some reason, in my car for the third winter, the battery stops turning the starter in great frosts. I decided to make life easier for the battery and make a starting device for the car. The cost of a factory-made launcher is quite high, and the output parameters leave much to be desired. Only a few parts are needed to make a launcher. All of them are expensive, but quite common. I managed to get them for almost nothing, I bought only a network and power wire.

Let's start with the transformer. I managed to find a transformer with a ready-made primary winding for 220V and sufficient power. We remove the secondary windings. On this transformer, the primary winding is divided into two parts, which are connected in passing. After removing the windings was the following picture:

Next, we wind 10 turns of any insulated wire, I took it from the old car wiring. We turn on the transformer in the network. We measure the voltage on the newly wound secondary winding. Calculate the voltage of one turn. At a voltage of 240V, this is considered the maximum voltage, the voltage of the secondary winding should be 14.5V. With a lower mains voltage, the output voltage must be correspondingly lower, the value is calculated as a proportion of the above values. We calculate the number of turns of the secondary winding, for this it is necessary to divide the resulting voltage, according to the reset, by the voltage of one turn.

The next step is to calculate the maximum wire diameter by the size of the window between the coils and the number of turns. It should be borne in mind that there will be two coils. My diameter is 5mm. The wire was taken from the AVVG 5x10 cable, with insulation, its diameter was 5 mm. The length of the wire can be calculated from the length of one turn. I didn’t have such a length, I had to twist it. We wind two secondary windings. One coil is wound on one half of the transformer, the other on the other. After winding, the end of the coil is bitten off with the calculation of winding a few more turns. The wound starter transformer is shown in the image below:

We install two powerful diodes together with radiators on a dielectric surface. Good fit diodes from the welding machine. Textolite 4-5 mm thick serves as a dielectric surface.

We connect the coils and diodes according to the diagram. The switch is optional, I did not.

Next, we make control measurements. The voltage on each secondary winding should be no more than 14.5V, respectively, between the extreme terminals of the two windings 29V. At the output of the starting device, due to the voltage drop across the diodes, the voltage will be slightly lower, about 14V. Let me remind you that these parameters should be at 240V in the network. If the voltage is greater, it is necessary to unwind the required number of turns according to the voltage of one turn. At a lower voltage, we wind, for this we left a supply of wire when winding.

The wires from the starter to the battery were taken from the so-called cigarette lighter. I do not advise anyone to do this, after two starts they melted, replaced with welding ones. After that, the losses in the wires decreased and the useful power increased.

This starting device starts a diesel passenger car, I haven’t tried trucks, but I would say in terms of rotation speed that they are trucks, with a completely zero battery.

All questions on calculations and assembly of the starting device can be set to .

As soon as the cold comes, the owner of the car is faced with some problems associated with starting the car. So, the most important load is placed on the battery with a starter. And for such unpleasant situations, a start-charger was invented.

You can buy it in an online store or where they sell auto parts. But usually such devices cost a lot of money and can cause a lot of damage to your wallet.

But these devices have a very limited output parameter in start mode. Because of this, the battery takes on the entire load, and the help from such a device receives little.

But this miracle device can be made with your own hands. This does not require special knowledge in electronics, but some experience is still needed.

Interesting! You will also need a diode bridge and a core from a transformer or the transformer itself. The power of the finished device will have at least 1.4 kilowatts. This is quite enough to start the weakest power source.

For the convenience and ease of assembling a car device with your own hands, we recommend using a conditional drawing. The circuit of the starting-charging device will clearly demonstrate what and how it works. It will greatly simplify assembly. Owners of knowledge in electronics will be able to create the necessary drawing with their own hands.

  • transformer;
  • diode bridge;
  • cooling device;
  • voltmeter;
  • electrolytic capacitor.

The break in the connection of the primary winding of a 220 volt transformer should be 15 amperes. Since there is a very high voltage, the fuse will be able to protect against a short circuit.

The diode bridge must be chosen between 10 and 50 amperes. It all depends on which batteries will be started using the device.

Any cooler (fan) from a personal computer is suitable for cooling. You also need to find a voltmeter, no matter what.

The electrolytic capacitor should be 16 volts, but more is possible. Its capacitance can vary from 3,000 to 10,000 microfarads. Important: the output current will be smoother if the capacitance is larger.

There are many instructions on the Internet for creating a start-charger for a car using a computer power supply. But its power is too small, and the use will be extremely unreliable.

For our device, a transformer from microwave ovens is best suited. Probably every third person has an old unnecessary microwave oven. But before assembling the ROM, the transformer must be redone with your own hands. But before altering, be sure to check it for operability. You can do this by connecting the terminals to the network with your own hands. If it starts to emit a slight hum, then the device is working properly.

You should start assembling the charger with your own hands from the high-voltage winding. It needs to be cut down. For these purposes, a simple hacksaw for metal is perfect. During sawing, the main thing is not to damage the primary winding.

After the high-voltage winding has been cut in its place, it is necessary to drill holes. They must be made with a thick drill. Through the holes formed, you need to pull out the remnants of the winding. Any blunt object can knock them out.

After the internal cavities have been freed from debris, it is necessary to create a secondary winding. Somewhere you need to make 16 turns and wind a turn to turn. The voltage will directly depend on the cross section of the wire. After that, you need to measure the voltage at the output. Should be 16 volts after the diode bridge.

I would like to clarify that it is easier to wind with a flexible wire and ideally use a single-core one. Also use copper wires, because they conduct current better and do not heat up, unlike aluminum ones.

As a case for a starter-charger, a former case from a personal computer power supply is suitable. In it, it will be necessary to unscrew the fan and install it the other way around so that it does not blow out the air, but blows it inward.

Across one of the wires, you need to insert a 15 amp fuse, you can use any from the car.

The transformer in the housing must be installed on a thick cardboard gasket. This is necessary so that during the occurrence of magnetic induction, the case does not vibrate and does not create an additional hum. Also put a thick pad on top. It will not be necessary to screw the transformer, because it is massive and when closed with a lid, it will press tightly.

Now you need to install the diode bridge. If the choice fell on low-power, then it can then be installed inside. Cooling from a fan will be enough.

Important! If you use a power of more than 10 amperes, then it must be installed on a radiator. Otherwise, it may simply burn out.

The radiator for the diode bridge is suitable from a computer, which serve to cool the microprocessor. The cooler is not needed, it can be removed. No other cooling is required for it. True, I would like to say that it will not work to install it in the case, and it will be necessary that the bridge be outside the case.

Now it remains to install only the cover. It can be put on the glue moment, but better on silicone or sealant. All device for the car is ready.

So, a diagram and minimal knowledge will help you assemble a budget device for charging or starting a car with your own hands. Without connecting our start-charger to the network, it can be used as a tester.

Reliable engine starting a passenger car in winter can sometimes turn into a problem. This issue is especially relevant for powerful auto-tractor equipment of agricultural enterprises, road and communal services that operate it in conditions of garageless storage. This will not happen if there is an electronic assistant at hand, which a medium-skilled radio amateur can make.

Fig.1 Scheme of a single-phase starting device.

Sct = 27 cm2, Sct = a? c (Sct - cross-sectional area of ​​the magnetic circuit, cm2)

Fig.3 General view of a single-phase starting device.

The described method for calculating the starting device is universal and applicable to engines of any power. We will demonstrate this using the ST-222 A starter used on tractors T-16, T-25, T-30 of the Vladimir Tractor Plant as an example.

Basic information about the ST-222 A starter:

  • rated voltage - 12 V;
  • rated power - 2.2 kW;
  • battery type - 2? 3ST-150.

Means:
Ir \u003d 3 C20 \u003d 3 150 A \u003d 450 A,
The power supplied to the starter will be:
Rst \u003d 10.5 V 450 A \u003d 4725 W.
Given the power loss:
Rp = 1–1.3 kW.
Starting device transformer power:
Rtr \u003d Rst + Rp \u003d 6 kW.
The cross section of the magnetic circuit Sct = 46–50 cm2. The current density in the windings is taken equal to:
j = 3 – 5 A/mm2.

The short-term operation mode of the starting device (5–10 seconds) allows its use in single-phase networks. For more powerful starters, the starting device transformer must be three-phase. Let's talk about the features of its design on the example of a starting device for a powerful diesel tractor "Kirovets" (K-700, K-701). Its ST-103A-01 starter has a rated power of 8.2 kW at a rated voltage of 24 V. The starting device transformer power (including losses) will be:

Рtr \u003d 16 - 20 kW.

A simplified calculation of a three-phase transformer is carried out taking into account the recommendations set out in. If possible, you can use industrial step-down transformers such as TSPC-20A, TMOB-63, etc., connected to a three-phase network with a voltage of 380/220 V and a secondary voltage of 36 V. Such transformers are used for electric heating of floors, premises in animal husbandry, pig breeding, etc. .d. The circuit of the starting device on a three-phase transformer is as follows (see Fig. 4).



Fig.4 Starting device on a three-phase transformer.

MP - magnetic starter type PML-4000, PMA-4000 or similar for switching devices with a power of 20 kW. Start button SB1 type KU-121-1, KU-122-1M, etc.

A three-phase half-wave rectifier is used here, which makes it possible to obtain an open-circuit voltage of 36 V. Its increased value is explained by the use of longer cables connecting the starting device to the starter (for large-sized equipment, the cable length reaches 4 m). The use of a three-phase transformer provides more opportunities to obtain the required voltage of the starting device. Its value can be changed, including windings with a “star”, “triangle”, apply one-half-wave or two-half-wave (Larionov's circuit) rectification.

In conclusion, some general tips and recommendations:

- The use of toroidal transformers for single-phase starting devices is not necessary and is dictated by their best mass-dimensional indicators. At the same time, the technology of their manufacture is the most labor-intensive.

– Transformer calculation The launcher has some features. For example, the calculation of the number of turns per 1 V of the operating voltage according to the formula: T \u003d 30 / Sst, is explained by the desire to “squeeze out” the maximum possible from the magnetic circuit to the detriment of efficiency. This is justified by its short-term (5–10 seconds) mode of operation. If the dimensions do not play a decisive role, you can use a more gentle mode by calculating according to the formula: T \u003d 35 / Sst. The cross section of the magnetic circuit is taken 25–30% more.

– The power that can be “removed” from the existing toroidal core is approximately equal to the power of the three-phase asynchronous electric motor from which this core is made. If the engine power is not known, then it can be approximately calculated using the formula:

Rdv \u003d St? Juice,

where Rdv is the engine power, W; St is the area of ​​the cross section of the magnetic circuit, cm2 Sst = a?v Sok is the area of ​​the window of the magnetic circuit, cm2 (see Fig. 2)

Sok = 0.785 D2

– The transformer core is attached to the base frame with two U-shaped brackets. With the help of insulating washers, it is necessary to avoid the appearance of a short-circuited coil formed by a bracket with a frame.

– Given that the open-circuit voltage in the three-phase starting device is higher than 28 V, the engine is started in the following sequence:

  • 1. Connect the starter clamps to the starter leads.
  • 2. The driver turns on the starter.
  • 3. The assistant presses the start button SB1 and, after stable operation of the engine, immediately releases it.

- When using a powerful starting device in a stationary version, according to the requirements of safety regulations, it must be grounded. The handles of the connecting pliers must be rubber insulated. To avoid confusion, it is advisable to mark the “plus” tick-nude, for example, with red electrical tape.

– When starting, the battery may not be disconnected from the starter. In this case, the clamps are connected to the corresponding terminals of the battery. To avoid overcharging the battery, the starting device is turned off after starting the engine.

– To reduce magnetic leakage, it is better to wind the secondary windings of the transformer first on the core, and then wind the primary winding.