I am building an Arduino circuit to drive a solenoid door lock. I have been following this http://www.arduino.cc/playground/uploads/Learning/solenoid_driver.pdf schematic on the tutorials page, and I have a few questions.
If the transistor was to fail for some reason, due to a short or over heating, would the collector-emitter continue to conduct, keeping the solenoid powered permanently ? This is an important design consideration as this scenario could potentially leave the door unlocked constantly - I would rather the door remained locked until the circuit could be fixed.
The solenoid lock I'm using is much lower power than the one in the above schematic - 250mA 12V - and will probably only be active for 60 seconds, once or twice a day, would a heat sink on the transistor still be advisable? I was of the belief that if the correct resistors were used, the voltage drop across C-E would be minimal, and so the transistor would not dissipate that much heat.
I'm not too hot on transistors. I have a BC489 http://uk.rs-online.com/web/p/general-purpose-transistor/5450040/ , would that be an acceptable substitution for the TIP102.
Thanks in advance,