I have to switch on a relay based on a signal, and for this purpose, I am using an NPN transistor (BC-639G) in open-collector configuration, where the switching signal is applied to the base using a suitable current limiting resistor, and the relay coil sits between the collector and the power supply.
Switching signal frequency is random and is very very low.
simulate this circuit – Schematic created using CircuitLab
We are observing that this transistor is failing very often. The full load current of the relay coil is only 30mA, and the transistor is rated for 1A. We have not been able to pin-point the cause for this apparent failure. An earlier design used MJE802G (NPN Darlington 4A) device, and this kind of failure was not observed then.
Need expert comments on this to get to the bottom of this. All pointers are welcome.
Update from comment to an answer on Sept 8, 2017:
I forgot to put the diode in the schematic, but there is a 1N4007 diode in that place to allow the relay coil to freewheel when the transistor switches off. The failure (C-E short, or C-E open) is inspite of the above diode.
Best Answer
Add a flyback diode. When the relay gets turned off the coil acts as an inductor which creates a voltage spike.
Adding a diode will let the coil dissipate the stored energy in something other than the transistor.
simulate this circuit – Schematic created using CircuitLab