Instructables

Step 2: Calculate how much current will flow

The formula you need is a simple one:

(maybe people in England or the US know the Voltage as "V" but I will refer to it as "U" as we call it in here)

U = R * I

OK, but we want the current "I" right ? So just divide through the Resistance "R".

U = R * I / :R

I = U/R

For my relay that would be:

I = 12V / 400Ohm
I = 0.03 A => 30 mA (That is Ic)

The Arduino can handle up to 20mA but its better to use a transistor even if your current is only 20mA. So for 30mA you definitely need one.

 
Remove these adsRemove these ads by Signing Up
jashan5102 years ago
my coil resistance is just 71ohm so Ic comes really high
if i put a resistor in series wid d coil would it work?