We are working on a circuit in which we are controlling two way solenoid valve (24V, 10W). We are using 3.3V based controller. The solenoid valve is connected to the controller via TIP 122 (NPN Darlington) transistor.
When the solenoid valve was connected in the collector end of the TIP 122, it was working fine. When the valve was connected to the emitter of the transistor, it was not switching ON.
Why there is discrepancy, when connecting the valve on the emitter end. In the end the transistor has to turn-ON and has to allow current to pass between collector and emitter end.
Answer: When connecting load across emitter and the ground: The VBE required to maintain the forward bias will go away as voltage will drop across the load. If VBE is set at 0.7 V, and then as soon as the voltage across load becomes 0.7V transistor will no longer act as a switch. As their will not be potential difference across VBE. Th