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resistor for the base of the npn transistor

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newbie111

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if the PIC and the relay share the same power supply

the equation of calculating the resistor for the npn transistor's base

is 0.2 x relay resistance x hfe


but if the pic uses 5v and the relay (separate) uses 12 v

how do i calculate the resistor for the base of the npn transistor

?

Thanks
 

I think different supply is not a problem. connect two ground each other .Use Resistor as per the circuit
 

hi
the 5v for pic and 12v for realy must share same ground.
to calculate the the resistor for the base of transistor
in first measure the coil resistance using ohm meter ....
to get realy current use I=12v/Rcoil I here mean IC
then find IB=IC/hfe....
the vcc for pic 5v then RB=5v-0.6v/IB
but in practice u must use double of that IB to saturate transistor....

u can use compelete driver for relay like ULN2003,ULN2804
 

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    newbie111

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instead of using 4 transistors + resistors u can use ULN2003 to drive relayz with more eaze
 

    N

    newbie111

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Good idea, but i have no idea how to use ULN2003.


Thanks
 

Your formulas for the base resistor do not make sense:
1) The resistance of the relay coil means nothing if you don't calculate how much current it needs.
2) The hFE of a transistor is not used when it is a switch.

The datasheet for every little transistor shows its max saturation voltage when its base current is 1/10th the collector current.

We don't know the collector current because we don't know how much current the relay coil uses.
The base current is about 5.1mA since the base resistor is 820 ohms from 4.9V.
 

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