Need Help with Transistor for switching a relay

[Kimly]

hi
i want to use this surface mount npn transistor to switch a relay for a arduino project im building, i want to use a so23 surface mount transistor.  Could someone please tell me the symbols for the max current and max voltage for the base pin to work out what value resistor to use?
thanks

http://www.rapidonline.com/pdf/47-3372.pdf

[Jerry]

There is a rule of thumb to use that says your Base current should be about 1/10 your Collector current.  The reason for this is that this would typically drive your transistor well into saturation causing enough current to activate your relay.  Your data sheet shows the continuous Collector current rated at 1A, so you don't want to exceed this.  Doing so would burn up your transistor.  I would suggest backing off to 0.8A or 0.9A to build in some safety net.  Let's go with the 0.8A, assuming that would be enough to drive your relay.  I don't know what your relay requires.  So, with 0.8A Collector current we would need approximately 80mA of Base current to fit our 1/10 rule of thumb.  So you would select your Base biasing resistor to provide you with the required 80mA of current.

The other thing to consider would be heat dissipation.  Your transistor is rated for 500mW.  To operate your relay a little surface mount 1/2 Watt transistor might not handle the job.  You may want to consider a transistor with more power dissipation capability so that it doesn't overheat and burn out.  Look into the specks for your relay.  Maybe you don't need 0.8A of current to drive it.