NPN Transistor current leak?

[newInRobotics]

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I did not use pull-down resistor - that was my problem
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Hi everyone

Background:
Yesterday I have connected a 12V PC fan through a NPN transistor (BC549C). Collector was connected to a negative wire of the a fan and Base was connected to regulated 5V from USB via Button and 15ohm Resistor.


My question is:
How come fan started to spin very slowly with Base of the transistor disconnected. After pressing the button fan got to full speed; after realeasing the button fan slowed down but did not stop.

[Soeren]

Hi,

Background:
Yesterday I have connected a 12V PC fan through a NPN transistor (BC549C). Collector was connected to unregulated 12V from battery and Base was connected to regulated 5V from USB via Button and 15ohm Resistor.

Always use a PNP in that position.
And always use a base tier if you don't drive the base continually. Making it float means it will act as an "antenna" picking up noise and such.

My question is:
How come fan started to spin very slowly with Base of the transistor disconnected. After pressing the button fan got to full speed; after realeasing the button fan slowed down but did not stop.

Because you didn't use a base pull down and used an NPN on the high side.
Change to a PNP and pull it's base high with 5k to 100k and it won't leak 

[newInRobotics]

Because you didn't use a base pull down and used an NPN on the high side.

I did use NPN on low side   however I did not use pull-down resistor - that was my problem   Thanks  

Edit: I have corrected my previous post as it said that NPN was used as high side switch which it wasn't.