Si queremos medir una corriente mayor que la de fondo de escala de un instrumento debemos conectar en paralelo una resistencia de derivación o "shunt", como muestra la figura 1.


 

 

 

 

   Esta resistencia desviará el exceso de corriente, multiplicando así la escala del instrumento. Como se muestra en la figura 2, podemos utilizar un instrumento que tenga un fondo de escala de 1 mA, es decir, que mide de 0 a 1 mA para medir corrientes hasta 100 mA, si conectamos un "shunt" que desvía 99 mA.


 

 

 

 

Para calcular el valor del shunt es necesario conocer la resistencia que la bobina del instrumento tiene. Así, teniendo en cuenta que, para hacer circular una corriente en el instrumento, se aplica una tensión entre sus extremos y que esa tensión es la misma en el shunt, podemos usar la siguiente fórmula, partiendo de la figura 3:


 

 

 

 

Rs = (Ri x Ii) / Ir

 

Donde:

Rs es la resistencia del shunt en ohms

Ri es la resistencia interna del instrumento usado en ohms

Ii es la corriente de fondo de escala del instrumento en milampères

IR es la corriente ue debe ser desviada hacia el shunt en miliampères

Recordamos que Ii + IR es la nueva corriente de fondo de escala del instrumento.

 

   Ejemplo de cálculo:

   Deseamos utilizar un instrumento de 1 mA de fondo de escala y resistencia interna 100 ohms para medir corrientes hasta 100 mA. ¿Cuál debe ser la resistencia del shunt para esta aplicación?

Tenemos: Ii = 1 mA

             IR = 99 mA

             Ri = 100 ohms

             Rs =?

 

Aplicando la fórmula:

       R = (100 x 1) / 99

       R = 100/99 = 1,010 ohms

   Evidentemente la precisión en la medida dependerá de la precisión del shunt. Como se trata de resistencia de valor muy bajo, normalmente se utilizan trozos de hilo que se cortan para presentar la resistencia deseada.

 

 

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