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Topic: Can an optoisolator do this or do I need a relay? (Read 2 times) previous topic - next topic


Well 175R gives you a current of 28mA so that should be OK, however you do need a diode across it or you will damage the output pin with revers voltage.

Are you thinking that the inrush current may be damaging to the Arduino

Inrush current concerns only really applies to capacitors and motors. A relay is an inductor and when you first apply apply a voltage to it there is a delay as the current increases. That is the inrush current starts off at zero and ramps up to the full value given by the resistance of the coil.

You might think a motor is an inductor but in that case the inrush current ramps up to the stall current, before the motor starts turning. Once going a DC motors take less current due to the constant change in direction of current in the rotor.


Jan 06, 2010, 05:36 pm Last Edit: Jan 06, 2010, 05:39 pm by cmagagna Reason: 1
Hi Mike and Ben,

I found a 5v relay (Hanlin Breed HE3621A0510), it has a built in diode and if I did the math right it draws 10 ma (5v and 500 ohms).

I'm not sure if it's appropriate to include vendor links on this forum or not, but I bought it from Jameco:


http://en.wiktionary.org/wiki/magagna <-- My last name.  Pretty apt.


Yes that relay looks OK for direct drive from an arduino. Things to note:-
1) You do order the one whose order code ends in 10 or 50 as that has the diode option.
2) This means the relay coil must be wired the right way round. If it is the wrong way round you will effectively short the output through the protection diode.
3) The contacts can only handle a small power 10W.

It is OK to post any relevant link on this forum.


http://en.wiktionary.org/wiki/magagna <-- My last name.  Pretty apt.

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