This power supply gives a voltage of 5 to 15V, stable installed in these limits, with a maximum current of 20A. At a current of more than 22A, protection is triggered.AC voltage 220V from the mains is supplied through a 4-amp fuse F1 to the primary winding of the T1 power transformer. This is a ready-made transformer with a primary winding at 230V and secondary at 20V at a current of up to 20A. If necessary, such a transformer can be manufactured independently on the basis of a power transformer from an old color lamp television, or on the basis of a low-frequency power transformer with a power of at least 500W to power halogen lamps (12V), or to obtain 36V for powering equipment by rewinding its secondary winding accordingly.
From the secondary winding, a voltage of 20V is supplied to the rectifier bridge VD1. This is a finished bridge assembly type MB356, designed for a maximum DC current of 35A. The ripple of the rectified voltage is smoothed by a capacitor C1 with a capacity of 22,000 microfarads. In the absence of a capacitor of such a large capacitance, it can be replaced by several capacitors of a smaller capacity, connected in parallel, so that in total they give at least 20,000 μF.
The constant voltage at capacitor C1 at idle is 28V.
The stabilizer consists of a stabilizer circuit on the IC A1 and an output voltage regulator on transistors VT1-VT5, the powerful transistors VT2-VT5 of which are connected in parallel. Resistors R5-R8 are used to equalize the current through the transistors, as as a result of differences in the transmission coefficients, they can, under equal conditions, open to different degrees. The resistors included in the emitter circuits help automatically set the base-emitter voltages under the action of the load current, at which the transistors open equally.
The LM723 is an integrated stabilizer with adjustable output voltage and an overload protection circuit. The output voltage is regulated by the resistor R3, which, together with the resistors R2, forms the output voltage divider. By adjusting, the dependence of the voltage at terminal 4 A1 on the output voltage is established. The comparator of the microcircuit works so that the voltage at the output (terminal 10) is regulated so that the voltage at its output 4 is unchanged. Accordingly, the voltage at terminal 10 is almost equal to the output. But the maximum permissible output current is small, therefore, to obtain a maximum load current of 20A, a current amplifier is needed, which is a VT1-VT5 transistor circuit.
The overcurrent protection circuit works by measuring the voltage across a resistance connected in series with the load. The inputs of the current sensor are terminals 2 and 3 A1. These pins are connected in parallel with the resistance formed by the resistors R9-R12, which is connected in series with the load. It is clear that, following Ohm's law, the voltage across the resistance will increase with increasing current. While the voltage between pins 2 and 3 is lower than 0.6V, the protection does not work, perceiving this as the fact that the load current does not exceed the maximum permissible value. At a current approaching the level of 22-23 A, the voltage between terminals 2 and 3 reaches a value of 0.6V or more. This leads to the operation of the protection, which reduces the voltage at terminal 10 A1 to zero, and thus disconnects the load.
The maximum output current can be set by another, accordingly changing the resulting resistance R9-R12, which in this case, provided that the upper threshold of the load current 23A is selected, is 0.025 Ohm. Or you can even organize the adjustment of the maximum output current if you turn on one low-resistance resistor R9-R12 with one variable resistor, somewhere around 10-100 Ohms, and remove the control voltage from its engine and one of the extreme terminals. The resistor will be a voltage divider on R9-R12. But in this case, the resistance R9-R12 must be calculated on the lower limit of the adjustment of the maximum load current.
The circuit provides a fairly good stability of the installed output voltage, for example, with an output voltage of 13V, under a load of 20A, the voltage decreases by only 40-60 mV.
LED HL1 is used to indicate the status connected to the network. The HL2 LED indicates the normal output mode of the power supply. That is, it burns when there is voltage at the output. If it is not lit, but HL1 is lit, this indicates that there is a short circuit on the load or overload and the stabilizer output is turned off by the current protection system, or that the fuse F2 is turned on at the output of the rectifier.
Transistors VT2-VT5 must be on volumetric radiators, ensuring their effective cooling. A good option is to use a plate aluminum radiator with a fan. In this case, the radiator and fan can be used from a faulty power supply of a personal computer of the AT or ATX type. The fan motor can be connected in parallel to the capacitor C1 through a resistor, which reduces the voltage on the fan to 12V.
Details The transformer is said at the beginning of the article. Capacitor C1 is an analogue of K50-35, imported, at 22,000 microfarads. Can be replaced by several capacitors of lower capacitance, connected in parallel, in the amount of not less than 20,000 μF.
The rectifier bridge can be replaced by another with a direct current of at least 30A, or assemble it on diodes designed for the same current, for example, 2D2997, KD2997, KD2998.
Transistors 1N3055 can be replaced by KT819. It is necessary to take transistors as close as possible in parameters. It is advisable, with one letter designation, from the same batch, and even better, before installation, select them according to the closest possible coefficients h21e.
LEDs - ordinary, indicator, almost any. You can use AL307. With insufficient brightness, the resistance of the resistors R1 and R13 can be reduced.
Resistors R5-R12 - five-watt, wire, resistance 0.1 Ohm.
If a pointer millivoltmeter is connected in parallel with resistors R9-R12, then on its scale it will be possible to determine the load current (accordingly, by redoing its scale in units of current strength).
Author: Gorchuk N.V.
