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Ultimamente estoy recibiendo muchos pedidos sobre como construir una fuente con importantes valores de corriente de salida. El circuito que voy a presentar nos permitirá construir una robusta fuente de alimentación de las tensiones mas comúnmente usadas y un alto valor de corriente. A pesar de ser muy simple, hay que prestar atencion en algunos detalles, ya que estamos construyendo un circuito que maneja una potencia importante (para el caso de 24v - casi 150Watt de potencia!)

El diseño se basa en el bien conocido circuito integrado LM338 - regulador lineal de National Texas Instrument (National fue adquirida por TI). Este integrado nos va proveer una tensión fija a la salida, dada por la relación de las resistencias R1 y R2. En la entrada tenemos el transformador T1, puente rectificador B1 y un capacitor electrolítico C1 de 10.000 micro faradios. El capacitor C1 se encarga de mantener la tensión lo suficientemente continua en la entrada del regulador. En la salida del regulador vemos en serie las resistencias R1 y R2 que definen el valor de la tensión de salida, y el preset de ajuste R3. Los capacitores C2, C3 y C5 son recomendados por el fabricante para eliminar el posible ruido y mantener el circuito estable (bypass). El capacitor C4 ayuda a mantener la tensión de salida constante ante bruscas variaciones de carga.

Tensión de salida

Como ya mencionamos antes, la tensión de salida depende depende del valor de las resistencias R1, R2 y R3. En este diseño, el valor de R1 es fijo, 120 ohm. R2 y R3 nos van a determinar de que voltaje sera la fuente. En la siguiente tabla se presentan los valores ambos resistores, como también el valor del transformador de entrada.

tabla-tensiones

R3 es un preset o potenciometro variable que debemos ajustar una vez ensamblada la fuente para que a la salida tenga el valor de tensión deseado. Para poder tener el rango de ajuste adecuado, se debe implementar un preset del valor que se indica en la tabla para cada valor de tensión de salida.

Corriente de salida

Veamos, primero, el porque del alto valor en el capacitor de entrada C1. La tensión de entrada proviene de la linea de corriente alterna, que varía en el tiempo. A la salida del puente rectificador la tensión varía entre el valor pico máximo y 0v. Esta variación es periódica: la tensión crece desde los cero voltios hasta alcanzar el valor máximo, luego baja hasta cero, luego vuelve a crecer y asi sucesivamente. Este ciclo se repite 100 veces por segundo (considerando que la frecuencia de la tensión de linea es de 50Hz). La tensión de entrada estará entregando corriente cuando esta próxima a su valor pico. Dicha corriente fluye hasta la salida y además carga el capacitor C1. A medida que la tensión disminuye, cada vez se entrega menos corriente. A partir de cierto valor de tensión, la corriente entregada por el tensión de entrega es nula y toda la corriente de salida se mantiene gracias a la carga almacenada en el capacitor C1. El mismo deberá ser capaz de almacenar la suficiente cantidad de electrones como para poder proveer la corriente necesaria a la salida, de ahí su alto valor.

Debido a este efecto, en los instantes cuando la tensión de entrada pasa por los valores máximos de tensión, el circuito estará tomando un importante valor de corriente desde la linea de CA: deberá reponer toda la carga que perdió el capacitor, como también continuar aportando corriente de salida. En dichos instantes, se producen picos de corriente muy altos en la entrada del circuito, en la tabla siguiente presentamos los picos de corriente en la entrada, para diferentes valores de la corriente de salida:

tabla-corrientes

puente-rectificador-de-diodosPodemos observar que estos picos de corriente en la entrada superan en mas de tres veces la corriente de salida. Debemos tener mucho cuidado al respecto. En primer lugar, el puente de diodos. Seria muy recomendable utilizar puentes de diodos integrados, es decir, un componente de 4 terminales que ya tiene los 4 diodos en su interior. Estos dispositivos normalmente se piden por la máxima corriente que soporta en forma continua. Además, el fabricante especifica cual es el pico de corriente máxima que soporta el dispositivo y por cuanto tiempo (por ej., 20A durante 8,3ms). Lamentablemente, estos datos varían de fabricante en fabricante, por lo que, para una fuente de 5A a la salida, los puentes de 5A de diferentes fabricantes pueden tener distinta especificación de corriente máxima. Si estaríamos diseñando la fuente para una producción de miles de equipos, deberíamos estudiar en detalles estos datos y encontrar un componente mas económico que satisface los requisitos del diseño. Pero, muy probablemente, nosotros fabricaremos una sola fuente para uso personal, donde el ahorro de unos pocos centavos no tiene sentido. A cambio, tendremos un equipo altamente seguro, con muy bajas probabilidades de falla. Así que ahí va mi consejo: utilicen el puente de diodos de un valor igual o mayor a la corriente máxima indicada. Por ejemplo, para una corriente de salida de 5A no seria descabellado utilizar un puente de 20A.

Algo similar sucede con el transformador, solo que ahora no vamos a ser tan exigentes. Normalmente, dejando 1-2A de seguridad es suficiente. Algunas fuentes traen transformadores de igual valor a la corriente de salida. Sin embargo, no es muy recomendable, y es poco probable que puedan funcionar en forma segura durante largo rato.

El disipador

Otra cosa muy importante, poder disipar el calor del regulador LM338. La regla de siempre también vale aquí: cuanto mas grande - mejor. Para el caso de 5A es recomendable un disipador de 10 x 15 cm, con 3 o 4 aletas de 3 cm a lo largo seria suficiente. También se puede usar un disipador para microcontrolador de alguna PC antigua que esta en desuso, y de paso, aprovechamos el ventilador. Es recomendable, una vez ensamblada la fuente, colocarle una carga importante, en lo posible, que consuma una corriente próxima al valor máximo de salida. Deberíamos dejarla funcionar por un tiempo y verificar que el regulador no calienta demasiado. Si puedes apoyar el dedo sin quemarte, el disipador esta bien dimensionado.

Vale la pena aclarar cuales son los parámetros que hacen que el regular se caliente. Obviamente, uno de ellos es la corriente, a mayor corriente de trabajo, mayor es la temperatura. Pero, el otro parámetro no menos importante es la diferencia de potencial. Presten atención al siguiente detalle: en la entrada del regulador se aplica una cierta tensión, y a la salida tenemos otro. Cuanto mayor es la diferencia de tensión entre la entrada y salida del regulador, mas potencia debe disipar y por lo tanto, genera mas calor! Normalmente, la tensión en la entrada del regulador debe ser 3v mayor que la de salida. Además, el fabricante asegura el correcto funcionamiento del regulador cuando la diferencia de potencial no supera los 35v entre la en entrada y la salida. Esto nos da un importante margen de maniobra. Por ejemplo, podríamos usar un transformador de 15v para construir una fuente 5v. Pero, que pasa con la diferencia de tensión restante? Pues se aplica entre la entrada y la salida del regulador, aumentando la cantidad de calor generada y el tamaño del disipador. Por lo tanto, es muy recomendable utilizar los transformadores según se indica en la tabla anterior, justamente para prevenir el sobrecalentamiento.

El regulador

lm338-pinoutNo hay mucho que decir sobre el regulador. Solo debemos tener cuidado con el siguiente aspecto: el regulador se provee en dos tipos de encapsulados: TO-3 y TO-220. El primero tiene una gran ventaja sobre el segundo: disipa mucho mejor el calor. Para que tengan una idea, si un TO-3 se calienta 10ºC, el TO-220 se calienta 40º para la misma potencia. La diferencia es mas que notable, por eso es muy recomendable siempre que se pueda utilizar el encapsulado TO-3. A la derecha pueden ver el pinout del regulador (visto desde abajo). Tenemos:

Patilla Vin - es la tensión de entrada. Se conecta a la salida del puente. En el circuito se indica como (1)
Patilla Adjustment - es la pata de ajuste o tierra (GND), se conecta al resistor R1 y R2. En el circuito se indica como (2)
Carcaza del encapsulado - es la tensión de salida. Se conecta al terminal (+). En el circuito se indica como (3).
Aquí les dejo la lista de todos los componentes:

Lista de componentes

1 T1 ....... Transformador (Tensión / corriente segun tabla)
1 BR1 ...... Puete de diodos (corriente según tabla)
1 C1 ....... 10.000uF (tensión ver tabla)
2 C2,C3 .... 100nF
1 C4 ....... 2200uF (50v)
1 U1 ....... LM338 encapsulado TO-3
1 R1 ....... 120ohm 1/8W
1 R2 ....... (Ver tabla) 1/4W
1 R3 ....... Preset o Pensiometro (Ver tabla)
2 D1,D2 .... 1N4004

Para que están los diodos?

Veamos, por ultimo, para que están los diodos D1 y D2. Para el funcionamiento normal del equipo son completamente innecesarios, no alteran para nada el funcionamiento del circuito. Pero, ahora, pensemos por un momento, que sucede cuando se produce un cortocircuito en la entrada del regulador? Es como si aplicáramos 0v en la entrada. Pero, a la salida, tenemos el capacitor C4, que esta cargado y aplica la máxima tensión en la salida del regulador, por ej., si la fuente es de 24v, quedan 24v aplicados a la salida del regulador, mientras que a la entrada tiene 0v. Esta situación inyectaría corriente en sentido inverso y probablemente dañaría el regulador. Para prevenir esto, colocamos el diodo D2. Al producirse un cortocircuito en la entrada, el capacitor C4 se descarga rápidamente a tierra a través del diodo D2. Algo similar se puede decir respecto al capacitor C5, su carga atraviesa primero D1 y luego D2.

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