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relay driving problem ...plz help !!!!

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umairmian2002

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Dear ALL,
i want to drive relays from uln2003 but the problem is that in normal conditions when their is no connection between 8051 and uln2003 8051 will show 0 and 5v as per my logic but when uln is attached it will drop the 8051 to o and 3v and relay is not on which i want to on.
i am giving uln 5v and 12v for testing a relay operating at 6v ,but all invain please help how can i put on the relay wit 8051 and uln2003. thanks in advance.
 

I don't know which version of the 8051 you are using but the original Intel version was rubbish at sourcing current: 80uA with 2.4V output. The ULN2003 would want nearly 400uA at 2.4V so my guess is that poor old micro cannot drive enough current out of the output pins. One solution would be to put a pullup resistor from the micro output pins to 5V. Around 3.3k is about as much as the micro can sink. Try it and see if it helps.

Keith.
 

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