Muchos lectores que poseen transformadores con tensiones diferentes de 6 o 12V de secundario nos preguntan cómo "alterar" tales componentes para poder ser usados en fuentes de 6, 12 o incluso otras tensiones en voltios. Todo esto es simple, si hacemos uso de reguladores, como explicaremos a continuación.

   Para tener una fuente de 12 V o de 6V, no necesitamos necesariamente un transformador con secundario de esta tensión. En realidad es incluso mejor que la tensión de secundario sea mayor para que la etapa reguladora que sigue la rectificadora funcione mejor, dentro de ciertos límites, por supuesto.

   Si tenemos un transformador que da 6 V de tensión de secundario, este valor es RMS (valor eficaz), lo que significa que en realidad tenemos un valor de pico que llega a 6 x 1,41 = 8,46 V. (figura 1)

 

 

Figura 1 - La tensión de pico
Figura 1 - La tensión de pico

 

   

Así, si rectificamos esta tensión, el capacitor de filtro, cuando cargado, tendrá entre sus armaduras prácticamente el valor de pico.

   La conexión de una carga en un circuito de este tipo no será alimentada con 6 V.

   También no será de 8,46 V la tensión aplicada, sino un valor intermedio que dependerá de su corriente y del propio valor del capacitor usado.

   Cuanto mayor sea el capacitor, más cerca del valor de pico será la tensión en la carga, y cuanto mayor el Consumo, más cercano a los 6 V estará la tensión en la carga. (fig. 2)

 

Figura 2 - Variación de la tensión en la salida
Figura 2 - Variación de la tensión en la salida

 

   

Por supuesto, no todos los aparatos electrónicos admite una gama de variación de tensión de alimentación tan amplia como ésta.

   Es por este motivo que, normalmente, intercalamos entre la salida de tensión continua (después de la rectificación y filtrado) una etapa reguladora. Con el fin de garantizar una tensión fija a la carga independiente de la corriente requerida.

   Una de las series más populares de circuitos integrados reguladores es la 78XX que tiene en los "XX" el valor de la tensión de salida. Así, el 7805 proporciona 5 V de salida; el 7806 proporciona 6 V y el 7812 proporciona 12V. La corriente máxima para estos integrados es de 1A. (figura 3)

 

Figura 3 - Integrados de la serie 78xx
Figura 3 - Integrados de la serie 78xx

 

  Para utilizar estos integrados lo importante es que la tensión máxima que se aplique a su entrada no supere los límites establecidos por el fabricante.

   También es importante garantizar que la tensión de entrada del regulador sea mayor que el mínimo requerido por el integrado para un buen funcionamiento.

   Teniendo en cuenta los integrados de 5, 6 y 12 V tenemos las siguientes especificaciones dadas por Texas Instruments:

 


 

 

   

Entonces, para una fuente de 12 V es conveniente que el transformador usado no sea simplemente de 12 \ /.

   Si utilizamos uno de 15 V, estaremos con una tensión RMS por encima del mínimo exigido que es de 14,5 V, y con una tensión de pico de 16,92 V que es mucho menos que el máximo permitido.

   Podemos incluso utilizar un transformador de 18 V que estará con un valor RMS por encima del mínimo, y con un valor de pico de 25,38 que todavía está por debajo del máximo permitido.

   Este transformador puede perfectamente ser utilizado en, nuestra fuente de 12 V. (figura 4).

 

Figura 4 - Fuentes con los reguladores
Figura 4 - Fuentes con los reguladores

 

   

El mismo es válido para otras tensiones, con una aplicación simple y segura.

   El capacitor de filtro recomendado para los diversos casos garantizando un buen filtrado es:

a) Cantidad:

1 000 uF a 12 V

1 500 uF a 6 V

1 500 uF para 5 V

b) Tensión de trabajo: dos veces la tensión RMS del secundario del transformador.

 

 

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