How to saturate at maximim BD135?

Other Hardware General Electronics
1

Hi. I'm sorry if its a stupid question, the problem is I'm searching on internet for 2 weeks or so and got at least 4 different formulas so far.

Well..

I'm using a bd135 as a switch. its working But I don't know if I'm working under the limits of device and things like that.

I want to know the "why of things".. I'm sorry if I'm in the wrong place for that.

Well. here's my case:

Devices:
5V VCC
400mA LED 3,3V;
BD135 NPN;

I want to drive the BD135 to its maximum saturation so I would like to find out (using formulas) to find out:
1 - Which Resistor to put on Base?
2 - Wich Resistor to put in series with the Led?

If it is not a stupid question, I would apreciate any help.

Tks a lot

2

Formulae aren't the answer - you look at the graphs on the datasheet and the
spec tables there.

The graph of Vce(sat) v. collector current suggest a Vce of 0.12V for Ib=40mA,
0.15V for Ib=20mA (both for Ic=0.4A). They also show Vbe=0.8V or so

However the worst case Vce(sat) for 0.5A is 0.5V, although that is across
the whole temperature range of course. At low temperatures lets assume the Vsat is in
the range 0.1 to 0.25V

Lets assume You are happy with Ib=20mA (the Arduino cannot source
much more safely), then the voltage across the base resistor = 5 - 0.8 = 4.2V,
so R = 4.2 / 0.02 = 210ohms. Allowing for the output resistance of the Arduino
pin of a few dozen ohms, that suggests the base resistor should be 180 ohms.

Now you have about 0.15V or a bit more across the transistor, 3.3V across the LED
leaving 5.0 - 3.3 - 0.15 = 1.55V for the current-limiting resistor. The current is 0.4A,
R = 1.55 / 0.4 = 3.9 ohms.

The power in this resistor = I^2 R = 0.25W

Thus you need at least a 1/3W resistor (allowing for variations in LED forward
voltage etc), preferably a 1W resistor so its not super-hot. A small heat sink on
the transistor may help keep it cooler too.

3

Hi!! First of all THANK you for sharing your knowledge!!!! :slight_smile:

And thank you for spending your time writing this much of info...

Well..

May I ask you some questions(some may be obvious. But I have some questions anyways)

The graph of Vce(sat) v. collector current suggest a Vce of 0.12V for Ib=40mA,
0.15V for Ib=20mA (both for Ic=0.4A). They also show Vbe=0.8V or so

I'm sorry but I didn't find it on Graphs. }

I'am with the datasheet of BD135 but couldn't understand how to get the info you provided(.....graph of Vce(sat) v. collector current suggest a Vce of 0.12V for Ib=40mA,
0.15V for Ib=20mA (both for Ic=0.4A). They also show Vbe=0.8V or so...)

However the worst case Vce(sat) for 0.5A is 0.5V, although that is across
the whole temperature range of course.

When you mean the worst case, you mean that all infos cames as Worst case in datasheet?
"VCEsat collector-emitter saturation voltage IC= 500 mA; IB=50mA ? ? 0.5 V"

At low temperatures lets assume the Vsat is in
the range 0.1 to 0.25V

Where did you take it from?

Lets assume You are happy with Ib=20mA (the Arduino cannot source
much more safely), then the voltage across the base resistor = 5 - 0.8 = 4.2V,
so R = 4.2 / 0.02 = 210ohms.  Allowing for the output resistance of the Arduino
pin of a few dozen ohms, that suggests the base resistor should be 180 ohms.

I understand this, and it has to to with the GAIN of Transistor, right?

Because you said to apply 0,20mA on the Base of Transistor, but it is relationed with the Gain, isnt?

Since I would like to have 400mA on LED, how to calculate to optain the 400mA that I need?

IN ADVANCE, THANK YOU SO MUCH!!!

Rodrigo

4

Posting twice is'nt very respectful.
http://forum.arduino.cc/index.php?topic=214129.msg1568307#msg1568307

5

Sorry, LarryD. I Posted on the first time by mistake and the site didn't let me delete it. I am SORRY. I will post a reply there referring to this one instead... Ok? Sorry again..