Table of contents:
- Power supply settings
- How to choose a transformer
- Production of a rectifier
- Filter block
- Output voltage stabilization
- Voltage adjustment 0-12 Volt
The 12 Volt power supply will allow you to power almost any household appliance, including even a laptop. Please note that up to 19 volts is applied to the laptop input. But it will work fine if you power it from 12. True, the maximum current is 10 amperes. Only the consumption reaches such a value very rarely, the average is kept at the level of 2-4 Amperes. The only thing to consider is that when replacing a standard power source with a home-made one, you will not be able to use the built-in battery. But still, a 12-volt power supply is ideal even for such a device.
Power supply settings
The most important parameters of any power supply are the output voltage and current. Their values depend on one thing - on the wire used in the secondary winding of the transformer. How to select it will be discussed below. For yourself, you must decide in advance for what purposes you plan to use a 12-volt power supply. If it is necessary to power low-power equipment - navigators, LEDs, and so on, then2-3 amps is enough at the output. And that will be a lot.
But if you plan to use it to carry out more serious actions - for example, to charge a car battery, then you will need 6-8 Amperes at the output. The charging current must be ten times less than the battery capacity - this requirement must be taken into account. If it becomes necessary to connect devices whose supply voltage differs significantly from 12 Volts, then it is wiser to set the adjustment.
How to choose a transformer
The first element is the voltage converter. The transformer contributes to the conversion of an alternating voltage of 220 volts into the same in amplitude, only with a value much less. At least you need a smaller value. For powerful power supplies, you can take a transformer of the TS-270 type as a basis. It has high power, there are even 4 windings that give out 6.3 volts each. They were used to power the glow of radio tubes. Without much difficulty, you can make a 12 Volt 12 Amp power supply out of it, which can even charge a car battery.
But if you are not completely satisfied with its windings, then you can remove all the secondary ones, leaving only the network one. And wind the wire. The problem is how to calculate the required number of turns. To do this, you can use a simple calculation scheme - count how many turns the secondary winding contains, which produces 6.3 Volts. Now just divide 6, 3 by the number of turns. And you will get the amount of voltage that can be removed from onecoil of wire. It remains only to calculate how many turns you need to wind in order to get 12.5-13 Volts at the output. It will be even better if the output is 1-2 Volts higher than required.
Production of a rectifier
What is a rectifier and what is it for? This is a semiconductor diode device that is a converter. It converts alternating current into direct current. To analyze the operation of the rectifier stage, it is more visual to use an oscilloscope. If you see a sinusoid in front of the diodes, then after them there will be an almost flat line. But small pieces from the sinusoid will still remain. Get rid of them after.
The choice of diodes should be taken with the utmost seriousness. If a 12 volt power supply is used as a battery charger, then you will need to use cells with a reverse current of up to 10 amperes. If you intend to power low-current consumers, then a bridge assembly will be quite enough. This is where you should stop. Preference should be given to a rectifier circuit assembled as a bridge - from four diodes. If applied on a single semiconductor (single-wave circuit), then the efficiency of the power supply is almost halved.
Now that there is a constant voltage at the output, it is necessary that the 12 volt power supply circuit be improved a little. For this purpose it is necessary to use filters. For nutritionhousehold appliances, it is enough to apply an LC chain. It is worth talking about it in more detail. An inductance - a choke is connected to the positive output of the rectifier stage. The current must pass through it, this is the first stage of filtering. Next comes the second - an electrolytic capacitor with a large capacity (several thousand microfarads).
After the inductor, an electrolytic capacitor is connected to the plus. Its second output is connected to a common wire (minus). The essence of the operation of an electrolytic capacitor is that it allows you to get rid of the entire variable component of the current. Remember, there were small pieces of a sinusoid at the output of the rectifier? Here, it is precisely from it that you need to get rid of, otherwise the 12 Volt 12 Amp power supply will interfere with the device connected to it. For example, a radio or radio will produce a strong hum.
Output voltage stabilization
To stabilize the output voltage, you can use just one semiconductor element. This can be either a zener diode with a working voltage of 12 volts, or more modern and advanced assemblies such as LM317, LM7812. The latter are designed to stabilize the voltage at 12 volts. Therefore, even under the condition that there are 15 volts at the output of the rectifier stage, only 12 will remain after stabilization. Everything else goes into heat. This means that it is extremely important to install the stabilizer on the radiator.
Voltage adjustment 0-12 Volt
For greater versatility of the device, it is worthuse a simple scheme that can be built in a few minutes. This can be done using the previously mentioned LM317 assembly. Only the difference from the switching circuit in the stabilization mode will be small. A 5 kΩ variable resistor is included in the wire break that goes to minus. A resistance of about 220 ohms is included between the output of the assembly and the variable resistor. And between the input and output of the stabilizer, reverse voltage protection is a semiconductor diode. Thus, a 12-volt power supply, assembled with your own hands, turns into a multifunctional device. Now it remains only to assemble it and calibrate the scale. Or you can even put an electronic voltmeter at the output, by which you can see the current voltage value.