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.

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?