When is a BJT switching current 1/10 versus 1/20?

Thread Starter

djsfantasi

Joined Apr 11, 2010
7,552
I learned the rule of thumb that when using a BJT as a switch, one should apply 1/10th of the desired collector current to the base. Recently, posts state that 1/20th current should be used.

Ok. But huh? When should either option be used? I guess by using 1/10th all the time, you can be assured of the transistor conducting. So, why use 1/20th ever? What’s the advantage and when should it be used?
 

dl324

Joined Mar 30, 2015
12,678
Some BC transistors specify a beta of 20 for saturation mode:
clipimage.jpg

You'd use whatever beta the datasheet specifies to guarantee saturation.
 

bertus

Joined Apr 5, 2008
21,240
Hello,

As @dl324 said, check the datasheets.
You will likely see that the low power transistors will use the 1/20 more often that high power transistors.
In high power transistors the 1/10 rule is almost always used.

Bertus
 

crutschow

Joined Mar 14, 2008
26,740
I guess by using 1/10th all the time, you can be assured of the transistor conducting. So, why use 1/20th ever? What’s the advantage and when should it be used?
If you want to save a little base current, you could likely use 1/20 with most low-power transistors and it would turn on fairly saturated, but to insure full saturation you should use the data sheet value.
 

ElectricSpidey

Joined Dec 2, 2017
1,532
For me its always been whether I needed hard or soft saturation.

Example: you have a 50 mA LED being driven from an NPN connected to a 40xx series IC, well there is really no need to go into full saturation when the transistor simply shares some of the voltage drop with the LED series resistor.

Now if you are driving a 500mA LED...that's a different story.
 

wayneh

Joined Sep 9, 2010
17,031
I prototype assuming 1:10 or 1:20 for small signals. IF I CARE about "wasted" base current (not an issue with emitter-follower where base current goes through the load), I then back it off until I observe a drop in function, or heat in the transistor or whatever. Then increase the base current back to "just enough".
 
Top