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1.INTRODUCTION atx power supply

you treats if of modifying a feeding power supply computer ATX to be used in the supported to test sound automotive and any equipment that needs 12 volts for 10 Amperes. In general equipments that are fed by car batteries. Mainly transceptor of radio. In the market exists this type of power supply of several marks and qualities, that idea is for the technician that it likes to set up your own equipments, usually sets up your own feeding power supply. Normally schematic of power supply's of discharge same current the that introduces an enormous transformer some potency transistors and other components usually found in power supply's. That type of power supply's, thoroughly used, it has some unfavorable characteristics, as, great weight and volume, and on the risk de to place the transistors regulators in tan circuit, and you read " they toast ".

Some of these power supply's incorporate protection circuits that, in case of it unites on tension, theoretically they disconnect the power supply to avoid damages. In any case, so that the protection circuit acts it is necessary that this is produced on tension, with consequent risk to the equipment.

Unite solution it is to use a power supply switch. In that power supply type the revenue it is larger. The problem is difficulty of construction of that power supply type, since we needed transformers with nucleus of ferrite component other e that are not found at the electronics stores. But a type of power supply switch that we can find with great easiness exists. it you treats if of the computer power supply. That is capable to supply considerable current in a volume and reduced weight and also of low cost.

2. - power supply ATX.

You find them power supply ATX in the computer science stores, if we observe the characteristics of those power supply's, we see that she supplies an amount of considerable current. For instance we can have the exit of 12 volts with current capacity of up to 12 amperes, what gives to feed most of the equipments of 12 volts. As we won't use the other exits of the power supply she will work with rest.

To tie a power supply ATX we needed you connect her/it in the electric outlet and we tie a resistor of 2.2k between the green wire and any black (ground). It is immediately the fan it will begin to rotate, confirming that the power supply is working.

To test the power supply.

With the power supply in operation can measure the exit tensions, below we have the table with the values of the tension found in the wires.

WIRE COLOR	Nominal tension VCC	Measured tension VCC
RED	5	5,05
YELLOW	12	11,40
ORANGE	3.3	3,35
BLUE	-12	-11.05
WHITE	-5	-4,57
Gray		3,05
VIOLET	5VSB	5,08

With can be observed, the tension in the yellow wire, that nominal should be of 12 volts, it is below the appropriate value to feed an equipment that needs 12 volts as a transceptor. Many transceptor need a tension of 13.8 v more or less 15% of tolerance. Like this if we want to feed an equipment of those with a power supply ATX we should elevate your tension.

The regulation of the exit tension takes place taking a sample of that tension and applying in the pin one of CI TL494, whose internal diagram can see in the image. This integrated it incorporates all the functions necessary to construction of a circuit of Modulation for Pulse (it Pulse Width Modulation, PWM) and it was developed for the control of feeding power supply's. As one can see below in the image, count in your interior two mistake amplifiers, an adjustable oscillator, a comparator, a control flip-flop, exit circuits and an internal regulator.

tl494 information

Without entering into more details, we can it accomplishes if taking unites sample of the exit tension and applying to the entrance of the mistake amplifier, in that case, the pin one of integrated TL494. For so much, to adjust the exit tension, the first that have to do it is a connection to the pin one. we will locate the pin one of the integrated circuit TL494 and on him we will weld a wire, being the careful of not doing any short circuit with the other trails of the circuit printed. It will surely be convenient to use a welder of fine tip for this operation. In the illustration below have the connection schematic .

It is convenient that the color of that wire is different from the other, for it had improved the identification. I use brown that a color different from the other wires of the power supply.

Unite time welded this wire, we will place the circuit printed in your place. We will place the brown wire with the rest of the wires.

We will connect a resistor of 4.7Kohm in series with a potentiometer of 10K ohm and we will connect this group among the wire brown soldier in the pin one of the integrated circuit TL494 and any black wire, that is the general earth. The Schematic is below in the image.

atx power supply

Place the potentiometer of 10Kohm in a position would intermediate and tie the power supply. Measuring the tension between any brown wire and any black wire, preferably with a digital multimeter, adjust the potentiometer of 10Kohm to obtain a tension of exit of 13,8 volts. It is possible that, depending on the power supply model used, be necessary to vary the values of the resistor and do potentiometer, not to get the wanted tension.

the equipment (transceptor) and the power supply. It suits to use two the three yellow wires and two or three black wires in parallel so that the tension fall is the possible smallest. Leave the connected power supply in the apparel which she will be coupled, after some time without verifying any anomalies can close the power supply in an appropriate cabinet (I use the power supply in the original box), in whose front can place a voltmeter, ammeter, connection Jacques, net switch, etc. ,e also all those accessories habitual in the feeding power supply's.

Reference: Fuente of alimentacion of Luís Sánchez Pérez. EA4-NH. In the site in Spanish has Schematic of how to place three power supply ATX in parallel. For currents of up to 30 ámperes, it is just Worth to check.

That project is just goes ends of didactic test, that doesn't implicate in any responsibility of the projectist of the non operation of the circuit or damages caused by the same.