Zener and Transistor Power Supply
Considered below stabilized power supply is one of the first devices that are collected by novice radio amateurs. This is a very simple, but very useful device. Its assembly does not require expensive components that are easy enough for a beginner to pick up depending on the required characteristics of the power supply unit. The material will also be useful to those who want to understand in more detail the purpose and calculation of the simplest radio components. In particular, you will learn in detail about the components of the power supply, such as:
- power transformer;
- diode bridge;
- smoothing capacitor;
- zener diode;
- resistor for Zener diode;
- load resistor;
- LED and a resistor for it.
The article also describes in detail how to pick up radio components for your power supply and what to do if not desired value. The development of the PCB will be clearly shown and the nuances of this operation will be disclosed.A few words are said specifically about testing the radio components before soldering, and also about assembling the device and testing it.
A typical scheme of a stabilized power supply
There are a lot of various schemes of power supplies with voltage stabilization today. But one of the simplest configurations, with which it is worth starting a newcomer, is built on only two key components - a stabilitron and a powerful transistor. Naturally, there are other details in the scheme, but they are auxiliary.
Diagrams in radio electronics are generally disassembled in the direction in which current flows through them. In a power supply with voltage stabilization, everything starts with a transformer (TR1). It performs several functions at once. First, the transformer lowers the mains voltage. Secondly, it ensures the operation of the circuit. Third, it feeds the device that is connected to the unit. The diode bridge (BR1) - is designed to straighten the lowered mains voltage. Speaking in other words, an alternating voltage enters it, and the output is already constant.Without a diode bridge, neither the power supply unit itself nor the devices that will be connected to it will work. A smoothing electrolytic capacitor (C1) is needed in order to remove the pulsations present in the home network. In practice, they create interference that adversely affects the operation of electrical appliances. If, for example, take a sound amplifier powered from a power supply unit without a smoothing capacitor, then these same pulsations will be clearly heard in the columns in the form of extraneous noise. In other devices, interference can lead to malfunction, malfunction and other problems. Zener diode (D1) is a component of the power supply that stabilizes the voltage level. The fact is that the transformer will produce the desired 12 V (for example) only when there is exactly 230 V in the outlet. However, in practice there are no such conditions. Voltage can both subside and increase. The same transformer will give and output. Due to its properties, the zener diode balances the undervoltage regardless of the jumps in the network. For this component to work correctly, a current limiting resistor (R1) is needed.It is described in more detail below. Transistor (Q1) - needed to amplify current. The fact is that the Zener diode is not able to pass through itself all the current consumed by the device. Moreover, it will work correctly only in a certain range, for example, from 5 to 20 mA. To power any devices of this frankly little. This problem is handled by a powerful transistor, the opening and closing of which is controlled by a zener diode. Smoothing capacitor (C2) - designed for the same as the above-described C1. In typical circuits of stabilized power supplies, there is also a load resistor (R2). It is necessary for the circuit to remain operable when nothing is connected to the output terminals. Other components may be present in such circuits. This fuse, which is placed in front of the transformer, and the LED signaling the switching on of the unit, and additional smoothing capacitors, and another amplifying transistor, and a switch. All of them complicate the circuit, however, increase the functionality of the device.
Calculation and selection of radio components for the simplest power supply
The transformer is selected according to two main criteria - the secondary voltage and power.There are other parameters, but within the framework of the material they are not very important. If you need a power supply, say, 12 V, then you need to select a transformer so that you can remove a little more from its secondary winding. With the power all the same - we take with a small margin. The main parameter of the diode bridge is the maximum current that it can pass. On this feature and should be guided in the first place. Consider the examples. The unit will be used to power the device consuming a current of 1 A. This means that the diode bridge should be taken approximately by 1.5 A. Assume you plan to power any 12-volt device with a power of 30 watts. This means that the current consumption will be about 2.5 A. Accordingly, the diode bridge should be at least 3 A. Its other characteristics (maximum voltage, etc.) in such a simple circuit can be neglected.
In addition, it should be said that the diode bridge can not be taken already ready, but to assemble it from four diodes. In this case, each of them must be designed for the current flowing according to the scheme. To calculate the capacity of the smoothing capacitor, rather complex formulas are used, which in this case do not apply to anything.Usually, a capacity of 1000-2200 microfarads is taken, and this will be quite enough for a simple power supply. You can take a larger capacitor, but this will significantly increase the cost of the product. Another important parameter is maximum voltage. According to it, the capacitor is selected depending on what voltage will be present in the circuit. Here you should take into account that the voltage between the diode bridge and the zener diode after switching on the smoothing capacitor will be about 30% higher than at the terminals of the transformer. That is, if you make a power supply unit at 12 V, and the transformer issues with a margin of 15 V, then at this section, due to the operation of the smoothing capacitor, it will be approximately 19.5 V. Accordingly, it should be designed for this voltage (the nearest standard value 25 V). The second smoothing capacitor in the circuit (C2) is usually taken as a small capacitor - from 100 to 470 microfarad. The voltage on this section of the circuit will be already stabilized, for example, to a level of 12 V. Accordingly, the capacitor must be designed for this (the nearest standard nominal is 16 V.). And what if the capacitors do not have the required nominal and store reluctance to go (or banally there is no desirebuy them)? In this case, it is quite possible to use the parallel connection of several smaller capacitors. It should be noted that the maximum operating voltage with such a connection will not be summed up! The stabilitron is selected depending on what voltage we need to get at the output of the power supply. If there is no suitable value, you can connect several pieces in series. The voltage to be stabilized will be added together. For example, take the situation when we need to get 12 V, and there are only two Zener diodes at 6 V. By combining them, we will consistently get the desired voltage. It is worth noting that to obtain an average nominal parallel connection of two Zener diodes will not work. Selecting the current-limiting resistor for the Zener diode as accurately as possible can only be done experimentally. For this, a resistor of approximately 1 kΩ is included in the already working circuit (for example, on a breadboard), and between it and the Zener diode an ammeter and a variable resistor are placed in the open circuit. After turning on the circuit, you need to rotate the variable resistor knob untiluntil the required rated stabilization current flows through the circuit section (indicated in the characteristics of the Zener diode). The amplifying transistor is selected according to two basic criteria. First, for the scheme under consideration, it must necessarily be an n-p-n structure. Secondly, in the characteristics of the existing transistor, you need to look at the maximum collector current. It should be slightly larger than the maximum current for which the assembled power supply will be designed. In typical circuits, the load resistor is taken from 1 kΩ to 10 kΩ. You should not take less resistance, because in the case when the power supply is not loaded, too much current will flow through this resistor and it will burn.
Design and manufacture of a printed circuit board
Now briefly Consider a good example of the development and assembly of a stabilized power supply with your own hands. First of all, it is necessary to find all the components present in the circuit. If there are no capacitors, resistors or zener diodes of the required nominal values, we exit the situation in the ways described above.
Next, you will need to design and manufacture a printed circuit board for our device.For beginners it is best to use for this simple and, most importantly, free software, for example, Sprint Layout. We place all components on the virtual board according to the chosen scheme. We optimize their location, we adjust depending on what specific details are available. At this stage, it is recommended to double-check the actual dimensions of the components and compare them with those added to the scheme being developed. Pay special attention to the polarity of electrolytic capacitors, the location of the leads of the transistor, the Zener diode and the diode bridge. If you go to add a signal LED to the power supply, it can be included in the circuit both before and after the Zener diode (preferably). To select a current-limiting resistor for it, you need to perform the following calculation. From the voltage section of the circuit subtract the voltage drop across the LED and divide the result by the rated current of its power. Example. In the area to which we plan to connect the signal LED, there is a stabilized 12 V. The voltage drop in standard LEDs is about 3 V, and the nominal supply current is 20 mA (0.02 A).We get that the resistance of the current-limiting resistor is R = 450 Ohm.
Component Testing and Power Supply Assembly
After the board has been developed in the program, we transfer it to fiberglass laminate, etch, pave the paths and remove excess flux. br>
After this we perform the installation of the radio components. Here it is worth saying that it is not superfluous to immediately check their performance, especially if they are not new. How and what to check? The windings of the transformer are checked by an ohmmeter. Where resistance is greater, there is primary winding. Next, it must be included in the network and be sure that it delivers the required undervoltage. When measuring it, use extreme caution.Also note that the voltage at the output is variable, so the appropriate mode is turned on at the voltmeter. Resistors are checked with an ohmmeter. The Zener must "ring" only in one direction. The diode bridge is checked according to the scheme. The diodes built into it should conduct current only in one direction. To test the capacitors, you will need a special instrument to measure the electrical capacitance. In the n-p-n transistor, the current must flow from the base to the emitter and to the collector. In other directions, it should not leak. It is best to start assembly with small parts - resistors, Zener diode, LED. Then the capacitors, the diode bridge are soldered. Pay special attention to the installation process of a high-power transistor. If you confuse his conclusions - the scheme does not work. In addition, this component will be quite hot under load, so it must be installed on the radiator. The last part is the biggest component - the transformer. Next to the conclusions of its primary winding soldered power plug with a wire. The output of the power supply unit also provides wires.
It remains only to thoroughly check the installation of all components, wash off the flux residues and turn on the power supply in the network. If done correctly, the LED will glow, and the output of the multimeter will show the desired voltage.
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