The use of computer power supplies. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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The use of computer power supplies. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Computers

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Paying attention to the cheapness of computer (PC / AT) power supplies (the result of huge "circulations"), they are very easy to adapt to other amateur radio needs. In a typical circuit (Fig. 1), the mains voltage is immediately rectified to a constant 320 V, half of which is applied to the primary winding of the transformer T1 through key Q1 / Q2 (they act alternately). The secondary windings of T1 have a common ground point, and the voltages are formed by full-wave two-diode (D1D2 for +12 V and D3D4 for +5 V) rectifiers with LC smoothing filters.

The four remaining unmarked diodes in the diagram are used to generate negative polarity voltages. The key transistors are controlled through the second transformer T2, the primary winding of which is connected to the output of a specialized IC TL494. This is an integrated PWM controller with a carrier of about 30 kHz (set by the C1R1 circuit at pins 5 and 6), the duration of the switching pulses at the output of which depends on the result of comparing the voltages at the +5 V and +12 V outputs with the internal reference voltage. If the output voltage of the power supply is less than necessary, then the duration of the pulses increases and vice versa; thus, effective stabilization of the output voltages is ensured.

Application of computer power supplies

Typical specification of a standard power supply: +5V/3...22A, +12V/0,5...8A, -12V/0,5A, -5V/0,5A. the following requirements for possible modifications: the total power of all loads should not exceed 200 W (these are limitations Q1, Q2, T1); the current through any of the 12-volt windings should not exceed 4 A, and 5-volt -10 A (limitations of coils L1a / b and diodes D1-D4); each of the halves of the secondary winding T1 must be loaded symmetrically, i.e. mandatory full-wave rectification; the ratio of the voltages of the five and 12-volt outputs cannot be changed due to the fixed ratio of the number of turns of the secondary windings T1 - the absolute values of the voltages can be changed by ± 30%, or 3,5 ... 6,5 V and 9 ... 15 V, but their the ratio will always be 5:12. Example 1: 6V/16A. A voltage of 6V fits into the 30% corridor for a 5V output, and 16A corresponds to (2x)8A from each leg of the rectifier, i.e. requirements are met.

Application of computer power supplies

The scheme for such an option is shown in Fig. 2; here, +12/-12/-5 V outputs with a frame are eliminated as unnecessary. The output voltage is determined by resistors R4 and R5. The TL494 regulates the output so that the voltages on its pins 1 and 2 (reference 5 V) are equal, i.e. the voltage across R5 is 5V. Obviously, the voltage across R4 is 6-5=1V and its resistance is R4=R5(6B-5B)/5V. If R5=4,7 kΩ, then suitable R4=1 kΩ. Example 2: 24 V / 4 A. 24 V does not fit into the 30% corridor of any of the standard voltages, therefore, we apply the modification of switching on the secondary windings - fig. 3. D5 and D6 are added here, forming a voltage of -12 V. Let's re-mark -12 V to "common", then +12 V will become +24 V (such a "focus" is possible due to the galvanic isolation of the circuit from the network and the load). Resistance R3=R5(12V - 5V)/5V, i.e. if R5 \u3,3d 3 kOhm, then R4,7 \uXNUMXd XNUMX kOhm.

Application of computer power supplies

Literature

Elektor Electronics №6, 2003, p.46-50

Author: D. Begebeger; Publication: cxem.net

See other articles Section Computers.

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