Hi Everyone, I've been a long time reader and now a first time poster.
I recently bought an Arduino Nano 3.1 and want to connect a digital pin (13) to control a relay that's rated with a coil voltage of 5V and coil resistance of 125 ohms (+/- 10%). The datasheet tells me that the nominal coil current is 40 mA which is the nano's max output pin current.
My relay is the OUAZ-SH-105L which falls under the OUAZ-L Sensitive category @ coil voltage = 5 V.
My question is, can I directly wire digital pin 13 to the relay coil? Or would a using a transistor switching circuit be better?
The part that confuses me the most is that I feel the transistor circuit would be safer for the nano, but wouldn't the current gain from the transistor damage the relay?
Better safe than sorry: use a transistor. Your relay will not feel the difference.
The standard schematic is : Arduino Playground - HomePage
Make sure you have the protection diode.
The part that confuses me the most is that I feel the transistor circuit would be safer for the nano, but wouldn't the current gain from the transistor damage the relay?
For sure use a switching transistor with that relay, 40ma continuous current would be working the output pin to hard. The 'current gain' (beta spec) of a transistor does not force that current into it's load circuit, the relay's resistance will determine the amount of current to flow, the current gain of the transistor just must allow that much current to be able to flow, any additional 'current gain' just insures that the transistor will be fully saturated on, allowing only the relay's coil voltage source and the relay's coil resistance to determine actual current flow amount.
Thank you both for your response. I did some more reading on transistors (google led me here: Transistor Circuits) and now better understand the need for the transistor switching circuit.