I need to controll multiple relays (10) using Arduino. These relays should control circuits supporting 350mA and 30-40V.
I'm pretty sure I should also use transistors for giving the needed coil power to the relays, but have problems choosing the right combinations and, most of all, have some doubts about one things: in case I choose 5V coil relays and and power them using the +5V Arduino PIN using the right transistors, will I be able to give enough current?
In the specs on the Arduino web page there's no indication of the maximum current supported by the +5V pin, because, I think, it depends by the VIN power. Am I right?
See the datasheet for the relays you are considering. They will list the coil voltage and current (or resistance) value. Add up the coil current times the number of relays for total current needed. Plan on around 400ma maximum of +5vdc current avalible from your arduino board. And yes the voltage of the Vin will limit the current because of too much heat dissipation on the on board +5vdc voltage regulator, The lower the Vin voltage the better, as long as it's at least +7.5vdc. The USB current is limited by the on-board 500ma thermofuse.
I have used small +5vdc reed relays that can be driven directly from arduino output pins and have contact ratings within your needs. You should also add reversed biased diodes wired across the relay coil terminals to deal with transient protection.