Saturation BJT

Thread Starter

SpNw

Joined May 27, 2020
11
Hi guys, I need some help.
I'm trying to make a led driver with a transistor(BC548C) but something is wrong in my opinion, and I dont understand why.

So, as I said i'm trying a transistor as a switch (BJT) where I want to control a 20mA led when voltage is applied Base,


My calculations to get the Saturation:

1º - In my Collector I have 12.53V so the first thing I did is to calculate the collector resistor:
(12.53V - 1.8V (led forward voltage drop) / 0.02A) = 536 Ohm


2º - Once calculated the collector resistor I need to know how much current base need to saturate my transistor so:

Assuming Beta = 420
1619564033925.png

Base Current = Ic/Beta (0.02A/420 = 47uA)
That means that I need 47uA in my transistor base to saturate right?

3º Base resistor Vin with 3.3V

(3.3v - 0.7) / 47uA = 55K


The Problem is even with this calculation I cant get the saturation and I dont kow why, still have voltage in Vce as I show bellow.

1619565162204.png


I hope I explained it well to make you understand my doubts. Thanks everyone! (And sorry for my bad english)
 

Papabravo

Joined Feb 24, 2006
16,835
You are operating the transistor in its linear range and assuming that it actually has a beta equal to the model parameter BF. This is in fact a fantasy. BF is not the same thing as beta. It is used to calculate beta at a particular set of conditions - that is all. In order to guarantee saturation you must overdrive the BJT. The LED and the Resistor will limit the collector current to the design value and the base current needs to be on the order of 1/20th to 1/10th of the collector current. In this manner, no matter what value of beta the transistor actually has, it WILL be in satuaration. Try for a base current of 1.5 mA to see what happens. Playing around with the base resistor will work for one transistor in one circuit. If the circuit is to be replicated you have to choose a value that is "close" the the current you want. this process is often described a "forcing" the beta to a particular value like 10, or 12, 15 or some other number way lower than the beta you would get in a linear amplifier.
 

dl324

Joined Mar 30, 2015
13,143
The Problem is even with this calculation I cant get the saturation and I dont kow why, still have voltage in Vce as I show bellow.
With most general purpose BC transistors, you use a beta of 20 for saturation mode.

What makes you think you use 420? Whoever told you that is wrong.

From Fairchild's datasheet BC546-50 datasheet:
1619573836569.png
 

Papabravo

Joined Feb 24, 2006
16,835
Reading the datasheet carefully is a highly recommended activity. The major benefit is that it prevents you from asking questions like your original one, which must, at this moment, be just a bit uncomfortable. You can ask much better questions like: "What does this mean"?
 

MrChips

Joined Oct 2, 2009
24,237
It even spells it out in the datasheet.
For Ic = 10mA you need IB= 0.5mA.
Hence you can extrapolate, for Ic = 20mA you need IB= 1mA.
 

Audioguru again

Joined Oct 21, 2019
3,550
The Philips, Mororola and the datasheet shown above say that the typical hFE for a BC548C at an Ic of 2mA is 420. But it shows that then its collector voltage is 5V then obviously it is not saturated.
 

Thread Starter

SpNw

Joined May 27, 2020
11
Hmm I get it now, thanks for your time guys, looks like someone need start to pay attention when is reading the datasheet, sorry for "dumb" question, but not so dumb, made you guys teach me something!
 

crutschow

Joined Mar 14, 2008
27,761
Hmm I get it now, thanks for your time guys, looks like someone need start to pay attention when is reading the datasheet, sorry for "dumb" question, but not so dumb, made you guys teach me something!
Don't feel too bad.
We have gotten a lot of posts on this site, even from those who should know better, thinking they can use the data-sheet current gain value (Hfe or Beta) to calculate the base current needed for transistor saturation.
 

dl324

Joined Mar 30, 2015
13,143
looks like someone need start to pay attention when is reading the datasheet
When I studied this in school, we only used 2N prefix transistors which are common in North America. I didn't learn about BC until several decades later. 2N transistors usually use a beta of 10 for saturation mode. BC are usually 20, so I started using BC547 for discrete logic gates.

Some people recommend "pushing" 2N transistor beta to 20, but I was taught the more conservative number and am sticking to it. I see little benefit to designing a circuit that someone else might find to be marginal because the design wasn't sufficiently conservative.
 

Papabravo

Joined Feb 24, 2006
16,835
Don't sweat the small stuff. You realized something was wrong that you did not understand. It seems natural to ask for an explanation. The best engineering happens in the "no judgement" zone where asking questions is essential to refining ideas, just like we turn crude oil into gasoline.
 
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