doubt regarding resistance value in relay connection

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bobdxcool

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i am connecting p 1.2 of 8051 to a relay using a transistor setup as shown in figure below!

what value of resistance should i use between pin 1.2 and base of transistor..i am using bc 547!

 

and the relay specs are

7a/240 v ac
7a/24v dc
coil 12v dc
 

Generally, RB is calculated by:

RB = (Vcc x hFE) ÷ (5 x IL)

where:

Vcc is the IC's power supply (5V)
hFE is the transistor's gain (check your exact transistor(A,B,C) and datasheet, but let's say 120)
IL is the load current - see below.

You've missed out a critical parameter - the relay coil's resistance. With that, we can calculate the load current (I = V / R) then use the above to get the resistor value.

P.s. Don't cross post your questions to other forums.
 
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but i am connecting pin 1.2 of 8051 to transistor base...so should'nt we take 5 volts of 8051 instead of 12 volt??

and in load current calculation I = V / R....is V the rating (24 v dc or 240 ac) of relay or the 12 volts vcc applied??
 

lol -yes - the Vcc is the IC's supply. Just testing!

The load current involves the voltage you give to the coil - so it's 12 V. The relay's switch rating has nothing to do with it.
 

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