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problems about 12v relay on-off

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tej1200

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I use 12v relay in this circuit which is activated by controller 8051.controller o/p is given to base of transistor BY BC547 .i use base resister of 2.7k.but relay was not on.
only transistor was on.but when i gave 12v supply from directly lm317 to base of transistor then relay was on.
why?
 

if you have the transistor in common collector mode then there is no voltage gain - The controller is supplying 5V? You need to have both voltage and current gain.
 

Do you understand 8051 port operation? They are open drain with or without a weak pull-up (depends on the port). None of them is sourcing sufficient current to drive a NPN switch transistor without additional pull-up resistor.
 

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