Electronics > Beginners
Need trouble shooting help (pics)
ArthurDent:
I would not replace the fixed 4.7K resistor with a 10K pot as suggested because when the pot is turned to one end there will be approximately zero ohms in series with the transistor base which will have a maximum drop to ground of about 0.7 volt and you could damage the pot.
A better way for speed control is to connect a 1K to 10K pot (it's not that critical) between +5 VDC and your common point and leave the 4.7K resistor in series with the pot wiper and the transistor base. You can now adjust the speed of the motor.
What you will now find is that instead of the transistor acting as a switch and dropping about .7 volts, it is acting as a resistor between the +12 VDC and the motor. When the motor is receiving a low voltage and turning slowly, the transistor will be dropping the majority of the voltage at whatever current the motor is drawing at that speed. This power must be dissipated as heat and the transistor may get quite warm. That is why audio amps and your bench supply have large heatsinks and often fans to remove the heat from the transistors they use.
billbyrd1945:
Thanks Arthur. Will do.
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