Hey,
I recently took a 12V 0.2A 92mm fan out of my broken DVD player. i tested it and it was absoultly fine but it was a bit too powerful so i decided to build a 555 astable circuit for PWM. (R1=10K,R2=10K,C1=0.1UF). The output was connected to a BC547C Transistor which was sinking the neutral of the fan. the positive was connected to 12v. However when i turned it on i got a High Pitched tone coming from the fan so i imediatly turned it off. i then got my multimeter and measured the frequency to be approximatly 460hz and the duty cycle was approximatly 64%. why is this noise being produced and how can i stop it