Re: will it work ??
FMradio,
The circuit will work. However there are some severe practical problems. By varying the input voltage source, you are controlling the base current. The collector (motor) current will depend on the hFE of the transistor. The hFE is a function of temperature; it increases with increasing temperature. The junction temperature will rise, resulting in an increased hFE and a consequent increase in collector current for a given base current. Lets assume you are not using a heatsink. The data sheet that I found for the D313 does not give the junction - to ambient thermal resistance. However, a typical value is around 60 degC/watt for a TO220 package. Worst case power dissipation will occur at 1/2 the supply voltage (6V). therefore the power transistor dissipation would be 3W. This results in a temperature rise of 60 X 3 = 180 Deg C, which is unnacceptable, so you must use a heatsink. You must calculate the temperature rise using the junction to case thermal resistance (typically around 2 degC/watt), and the case to ambinet thermal resistance of the heat sink. After you have done that, you can calculate the junction temperature rise, and determine if the variation of motor current vs temperature (time) is acceptable. What will happen is that you will have to constantly vary the input current to compensate for changes in junction temperature with time. As a result, this is not a practical circuit.
Regards,
Kral