Power supply on the LM723 chip, 12 volts 25 amps. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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When operating automotive audio equipment or communication equipment in stationary conditions, an appropriate power source is required, which should provide a voltage of 12 to 14,5 V, stable within these limits, at a maximum current of 20 A. The power supply, the circuit of which is given here, has exactly these characteristics .

AC voltage 220 V from the mains is supplied through a 5-amp fuse F1 to the primary winding of the power transformer T1. This is an imported transformer with a primary winding of 230 V and a secondary winding of 20 V at a current of up to 25 A. If necessary, such a transformer can be made independently on the basis of a power transformer from an old color lamp TV, or on the basis of a low-frequency power transformer with a power of at least 500 watts for power supply halogen lamps (12 V), or to obtain 36 V to power the equipment by rewinding its secondary winding accordingly.

(click to enlarge)

From the secondary winding, a voltage of 20 V is supplied to the rectifier bridge VD1. This is a finished type bridge assembly. MV356, designed for a maximum direct current of 35 A. Ripple of the rectified voltage is smoothed out by a capacitor C1 with a capacity of 22000 microfarads. In the absence of a capacitor of such a large capacity, it can be replaced by several capacitors of a smaller capacity, connected in parallel, so that in total they give at least 20000 microfarads (more is possible, but not less).

The constant voltage across capacitor C1 at idle is 26 V.

The stabilizer consists of a stabilizer circuit on IC A1 and an output voltage regulator based on transistors VT1-VT5, the powerful transistors VT2-VT5 of which are connected in parallel.

Resistors R5-R8 serve to equalize the current through the transistors, since as a result of differences in the transmission coefficients, they can, under equal conditions, open to varying degrees. Resistors included in the emitter circuits help to automatically set the base-emitter voltages under the influence of the load current, at which the transistors open equally.

The LM723 is an integral regulator with adjustable output voltage and overload protection circuit. The output voltage is adjusted using the resistor R3, which, together with the resistors R2 and R4, forms an output voltage divider. The adjustment sets the dependence of the voltage at pin 4 A1 on the output voltage.

The comparator of the microcircuit works in such a way that the output voltage (pin 10) regulates in such a way that the voltage at its pin 4 is unchanged. Accordingly, the voltage at pin 10 is almost equal to the output. But the maximum allowable output current is small, therefore, to obtain a maximum load current of 20 A, a current amplifier is needed, which is the circuit on transistors VT1 ... VT5.

The overcurrent protection circuit works by measuring the voltage across a resistance in series with the load. The current sensor inputs are terminals 2 and 3 A1. These pins are connected in parallel with the resistance formed by 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 terminals 2 and 3 is below 0,6 V, the protection does not work, perceiving this as the fact that the load current does not exceed the maximum allowable value. With a current approaching the mark of 23-24 A, the voltage between terminals 2 and 3 reaches a value of 0,6 V 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 to others, respectively, by changing the resulting resistance R9-R12, which in this case, provided that the upper load current threshold of 23 A is selected, is 0,025 Ohm.

Or you can even organize the adjustment of the maximum output current, if you connect one variable resistor with a resistance of about 9-12 Ohms in parallel with the low-resistance resistors R10-R100, and remove the control voltage from its engine and one of the extreme conclusions. The resistor will be a voltage divider across R9-R12. But in this case, the resistances R9 ... R12 must be calculated on the lower limit of the maximum load current adjustment. Thus, with the help of this resistor, it will be possible to regulate the current of protection operation.

The circuit provides a fairly good stability of the set output voltage, for example, at an output voltage of 13 V, under a load of 22 A, the voltage decreases by only 40 ... 60 mV.

The HL1 LED is used to indicate the state connected to the network. The HL2 LED indicates the normal power supply output mode. That is, it lights up when there is voltage at the output. If it is not lit, but HL1 is on, this indicates that there is a short circuit or overload on the load and the stabilizer output has been turned off by the current protection system, or the fuse F2 has blown at the rectifier output.

Transistors VT2-VT5 must be on volumetric radiators to ensure their efficient cooling. A good option is to use an aluminum plate heatsink in conjunction with a fan. In this case, the radiator and fan can be used from a faulty AT or ATX type personal computer power supply. The fan motor can be connected in parallel with capacitor C4.

Details. The transformer is mentioned at the beginning of the article.

Capacitor C1 - analogue of K50-35, imported, 22 thousand microfarads. Can be replaced by several smaller capacitors connected in parallel.

The rectifier bridge can be replaced with another one for a direct current of at least 30 A, or it can be assembled on diodes designed for the same current, for example, 2D2997, KD2997, KD2998.

Transistors 1N3055 can be replaced with KT819. It is necessary to take transistors as close as possible in terms of parameters. It is desirable, with one letter designation, from one batch, and even better, before installation, select them according to the closest h21e coefficients.

LEDs - ordinary, indicator, almost any. You can use AL307. With insufficient brightness of the glow, you can lower the resistance of resistors R1 and R13.

Resistors R5-R12 - five-watt, wire, resistance 0,1 ohm.

If a pointer millivoltmeter is connected in parallel with the resistors R9-R12, then it will be possible to determine the load current on its scale (respectively, by redoing its scale in units of current strength).

Author: Gorchuk N.V.

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