# help understanding some transistor features

#### primal001

Joined May 13, 2017
1
Hello,

So I was playing around with a bc337-25 a npn bjt.
I am trying to power a 3.3V circuit and use the transistor as a switch. The thing is I don't get 3.3v to power the circuit.
The 3.3V comes from a regulated 5V. 3.3V is supplied to the circuit and base of the bjt.

So I have Ib=Ic/beta, IB should be 100mA (what the circuit needs)/160(worst beta of bc337-25) = 0,6mA should be supplied to achieve the 100mA.
Rb=(3.3V-Vbeo)/0,0006A = (3.3V-1.2V)/0,0006A = 3,5kO.

so now I try to hook up the circuit but I get around 1,2V and around 16mA.

Any help is appreciated.

datasheet: https://www.onsemi.com/pub/Collateral/BC337-D.PDF

#### Dodgydave

Joined Jun 22, 2012
11,234
Post your circuit so we can see more, might be better using a mosfet or darlington.

#### dl324

Joined Mar 30, 2015
16,669
I am trying to power a 3.3V circuit and use the transistor as a switch.
When using a transistor as a switch, the general rule of thumb is to assume a beta of 10. This excerpt from the datasheet confirms that:

Note that the 100mA curve shows base current of 10mA for Vce = 0.04V. Less base current will give a higher saturation voltage.