Cant get transistor into saturation using an rc timer element

Thread Starter

Mrdouble

Joined Aug 13, 2012
105
image.jpeg
The circuit above energizes the relay after a sec or so. It works great except that I can drive the base any harder because of the resistor. I know I could reduce the resistor but then I won't have to timing element. The Vce is amost 2 volts and thus cooking my little 3904. I know saturation depends almost entirely on collector resistance which is the coil at 120 ohms. I'm sure there's a simple solution but boy am I stumped.
Thanks in advance
Micheal
 
Last edited:

AnalogKid

Joined Aug 1, 2013
8,251
Round numbers, the desired collector current is 100 mA so the base current should be 10 mA. But if the transistor is on the edge of satiration then it is running at a gain of around 100 so you need only 1 mA. 12 V / 1 mA = 12 K. You have 31 K so you don't have nearly enough base current. 10K resistor, 1500 uF cap.

ak
 

Thread Starter

Mrdouble

Joined Aug 13, 2012
105
Thank you all, I just decided to throw another 470uf cap in parallel and reduce the resistance. Not EXACLY saturated but good enough.

Spike
 
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