How to saturate at maximim BD135?

Using Arduino 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

What power supply will you be using to power the LED?
Lets say it is 5 volts.
When the transistor is saturated lets say VCEsat is .5 volts.
Your LED has a forward drop of 3.3 volts.
You want to let 400ma flow through the LED.
the LED series resistor has to be (5supply - .5sat - 3.3LED) / .4 = 3 ohms.
Let us assume the gain of the transistor is 25 form data sheet.
At a collector current of 400ma the base current will be .4I led / 25gain = 16ma
Lets us drive the base with 0 to 5 volts.
Assume Vbe of the transistor is .6V.
The base resistor will then be (5Drive voltage -.6Vbe) / .016 = 275 ohms
Now connect the circuit up and measure the voltage from the collector to emitter, is it at the saturation level of .5 volts?
Power for the transistor is .5V X .4A = 200mw

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3

Hi, LarryD. First of all I'AM SORRY for re-post it. I thought the post was on the wrong place and then I have posted in another category., because once you have created a post you can't delete it.

Well.

I thank you very much for spending your time with a great answer and image schema with someone you don't know and even so trying to help!

I Apreciate it!

I think Now I understand almost everything and I think Now I can do my own projects and I'll keep reading a lot.

Just last question.. (if you allow me to ask)..

Where did you take the gain of 25 from?

On Datasheet it says in hfe:
IC= 5 mA = Min:40, Typ:--, Max:--
IC= 150mA = Min:63, Typ:--, Max: 250
IC= 500mA = Min:25, Typ:--, Max:--

I will test it and let you know about if worked ok?

I'm sure it will work:)

4

Look in the datasheet for your transistor for "DC Current Gain" or "HFE" ("hfe" is AC gain, don't use this one for your type of circuit).

5

hi 123Splat!

Thank you for your answer!

So, the lowest value is the one that I can safelly use as the gain?

and Tks.. HFE then for DC