El termostato de uso general mostrado en la figura 1 utiliza un termistor de alto valor resistivo como sensor. Los termistores en el rango de 47 k ohms a 220 k ohms se pueden utilizar y en su función el valor de R3 debe ser cambiado para obtener la sensibilidad deseada y el punto de disparo del relé.

 


 

 

   

Con el uso de un paso amplificador transistorizado, las pequeñas variaciones de resistencia del termistor se detectan llevando el circuito al punto de disparo. El ajuste de este punto de disparo es dado por el trimpot conectado a la entrada de referencia del comparador. Para mayor precisión se debe utilizar un trimpot del tipo multivoltas.

   El relé utilizado es del tipo sensible con una bobina de 50 mA en el máximo y tensión de acuerdo con la usada en la alimentación.

   Si el sensor se queda lejos del circuito (más de 2 metros), se debe utilizar un cable blindado con la malla debidamente puesta a tierra. El capacitor de 100 nF en la entrada del termistor evita que transitorios o pulsos de corta duración provoquen el disparo errático del circuito.

   La fuente de alimentación debe estabilizarse para no afectar el punto de disparo del circuito. Una posibilidad que puede ser analizada para algunas aplicaciones está en el uso de un diodo común polarizado inversamente como sensor de temperatura.

 

Semiconductores:

CI-1 - LM339 o equivalente - ¼ utilizado - comparador

Q1, Q2 - BC548 - transistores NPN de uso general

D1 - 1N4148 - diodo de uso general

 

Resistores: (1/8 W, 5%)

R1 - 22 k ohms

R2 - 2,2 k ohms

R3 - 3,3 k ohms

R4, R5 - 4,7 k ohms

R6 - 5,6 k ohms

R7 - 1,2 k ohms

P1 - 10 k ohms - trimpot multivoltas

 

Capacitores:

C1, C2, C3, C4 - 100 nF - cerámico o poliéster

 

Varios:

X1 - Termistor - ver texto

K1 - Relé de 12 V x 50 mA

Placa de circuito impreso, cables blindados, fuente de alimentación, soldadura, etc.

 

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