In my project I'm using GSM module for controlling the AC operated devices using cell phone. For this purpose I'm using following components:
- Controller : ATMEGA32A
- GSM module : SIM800C
- Relay : 5VDC coil, 230V/7A Goodsky make
- Transistor used for switching : 2N3904
- Freewheel diode : 1N5819
- Power IC: LM2596,adj
I have designed the circuit so that base current should be 8.4mA in the saturation mode. Overall circuit works on 4VDC(Pull in voltage for relay is 3.75V). The circuit worked fine. But after some time around 8-10 months some modules fail because of transistor failure. All the other components are perfectly fine. I have not included any resistor between base to emitter is this the issue for transistor failure.
Thanks in advance.