I still struggle on reading datasheets.

I found this site:
http://www.petervis.com/GCSE_Design...ator/transistor_base_resistor_calculator.html

That helps you determine the base resistor to put the transistor into saturation. I am struggling to figure out where they get the values from the sheet.

1. I see where they are getting hFE but this is the min value. Why did they chose 110 for the example?

2. How did they determine a 770mv VBE from the datasheet?


3. How did they determine a 250mv VCE from the datasheet?

 

I'm sorry but that whole article is a bunch of misinformation (technically it's hooey).
You do not use hFE to calculate the required base current for saturation.
As you can see, the value of hFE is given for a Vce of 5V, not for the saturation voltage.
That value of hFE is used for linear transistor operation calculations, where the transistor is not saturated.

The required base current for good saturation is typically 1/10th or 1/20th of the collector current (forced Beta of 10 or 20), as shown in the data sheet where it gives the Vce(sat) voltage.
That is the base current value you should use.

How did they determine a 770mv VBE from the datasheet?

They apparently took if from the Vbe(on) value.
It should be the Vbe(sat) value.

 

I'm sorry but that whole article is a bunch of misinformation (technically it's hooey).
You do not use hFE to calculate the required base current for saturation.
As you can see, the value of hFE is given for a Vce of 5V, not for the saturation voltage.
That value of hFE is used for linear transistor operation calculations, where the transistor is not saturated.

The required base current for good saturation is typically 1/10th or 1/20th of the collector current (forced Beta of 10 or 20), as shown in the data sheet where it gives the Vce(sat) voltage.
That is the base current value you should use.
They apparently took if from the Vbe(on) value.
It should be the Vbe(sat) value.

Do you (or anyone) know of a good article that how to calculate the proper values for a the load and base resistors? Keep in mind I am analog handicapped.

 

Also from your post, I will need vce and vbe saturation voltages? How do I determine which values to use?

 

What is the application?
Have you considered Mosfets if just switching?
Max.

 

Also from your post, I will need vce and vbe saturation voltages? How do I determine which values to use?

For Ib you just divide the maximum Ic in your application by 20.
The maximum IC is given by the collector supply voltage divided by the collector resistor value.

For Vbe, you just interpolate (estimate) the value as being between 0.7V and 0.9V, again as determined by your maximum Ic.
For worst-case you can just use 0.9V.

Then you calculate the base resistor as (Vbs-Vbe) / Ib where Vbs is the base drive voltage and Ib is the base current calculated from above.

 

What is the application?
Have you considered Mosfets if just switching?
Max.

I want to switch 5V on/off with 2.4V to 0V. I only need 20ma or less.

I have a boat load of general purpose transistors (looking at some , 2n3904s right now) so I would like to use them. I only have a few logic level mosfets and they are way over kill load wise for my application right now.


But this is more of a learning inquiry than a project. . I would finally like to learn how to properly determine load and base resistors based off of what I read from the datasheet. So it is two fold. How to calculate and how to properly pull those values from the datasheet.

 

For Ib you just divide the maximum Ic in your application by 20.
The maximum IC is given by the collector supply voltage divided by the collector resistor value.

For Vbe, you just interpolate (estimate) the value as being between 0.7V and 0.9V, again as determined by your maximum Ic.
For worst-case you can just use 0.9V.

Then you calculate the base resistor as (Vbs-Vbe) / Ib where Vbs is the base drive voltage and Ib is the base current calculated from above.

Excellent!

My supply is 5V.
Base drive is 2.4v

Max IC = 5 / 10,000
Max IC = .0005

Ib = .0005/20
Ib= .000025

(2.4-.9)/ .000025

Rb=60,000


Does this sound right?

If I get an odd value for Rb, I assume it is best to round up?

 

I have a boat load of general purpose transistors (looking at some , 2n3904s right now) , I only have a few logic level mosfets and they are way over kill load wise for my application right now.
.

I do as well, but pretty much abandoned them where at all possible for mosfet, 2n7000 etc for Pic and logic applications etc.
Max.

 

I do as well, but pretty much abandoned them where at all possible for mosfet, 2n7000 etc for Pic and logic applications etc.
Max.

2n7000 is a logic level? What about the p channel variety? Perhaps I will pick some up.

 

Excellent!

My supply is 5V.
Base drive is 2.4v

Max IC = 5 / 10,000
Max IC = .0005

Ib = .0005/20
Ib= .000025

(2.4-.9)/ .000025

Rb=60,000


Does this sound right?

If I get an odd value for Rb, I assume it is best to round up?

I usually round down, but it probably doesn't matter.

 

There are P- channel Mosfet's but I rarely have use for them, if on the rare occasion I need a P channel I will either use two 2n7000's or a PNP bi-polar.
Max.

 

I usually round down, but it probably doesn't matter.

But is my math right?

 

But is my math right?

Perfect, I think.
My only question might be why did you need the transistor if your driving such a low current. But maybe just an example?

 

Does this sound right?

Yes, your math is correct.

If I get an odd value for Rb, I assume it is best to round up?

Since you already used the high value for Vbe (it's likely closer to 0.7V for that low value of Ice) you can safely round up.

 

And how does this change for MOSFETS?


BTW crutshow. Your explanation for calculation was very straight forward and understandable. Probably the best I have seen. Thanks!

 

And how does this change for MOSFETS?

For using MOSFETs as a switch you look at the Vgs voltage used to specify the Rds(on) in the data sheet.
That's typically a Vgs of 10V for standard MOSFETs and 5V or less for "logic-level" type MOSFETs.
[Many try to use Vgs(threshold) as the switch voltage but that's not correct. That's the voltage where the MOSFET just barely starts to turn on].

Of course MOSFETs require no DC current to stay on so a series gate resistor is not required (but often a few tens of ohm series resistor is used to minimize oscillations during the switching transition).