Transistor-amplification

Hey,

I´m considering bying some transistors (ST Microelectronics BD911, NPN) and there´re still some issues. I don´t quite get how the current-amplification works. If a want a collector-current of 6 ampere and my amplification factor is 250 I´d need a base-current of 6/250 => 24 mA, so in order to get this base current with a voltage of 5V, I´d need 200 ohm resistor.
So, does this work? What, happens when I increase the emitter-collector voltage and this whole thing seems a bit weird since the amplification-factor of the transistor is not even in the datasheet!!!! (SGS Thomson Microelectronics mXssxrw datasheet pdf) I found the amplification-factor on another site, but I´m not sure if it´s right, since it´s not even in the datasheet.

So how does the current-amplification work, how can I calculate it correctly? (which resistors I need....) p.s. I´ve heard of the 0.6 volts junction voltage I have to surpass in order to actually make the transistor conductive, where does 0.6 volts come from, is it just a silicium dependent value, or is it not always 0.6 volts?

Thanks in advance!

What is it that you are trying to accomplish?
Why not consider a MOSFET for such high current?

I want to control a DC-motor which requires 6 amperes with the Arduino.
I´m open to it, why not, but why is it better than a normal transistor and if I have a 11.1 volts power source => 11.1 volts emitter-collector voltage, how can I calculate the appropriate resistor for the base of the transistor or MOSFET to control it with an Arduino pin?

The “transistor amplification factor” as you are calling it is the DC CURRENT GAIN (hfe and it most certainly is on the datasheet (page 2). Moreover , the number you are citing (250) does not apply
because for one thing that is the MAX value and for another that applies to a collector current of 0.5 A.
The hfe for a collector current of 5 A is 150, (not 250) and the hfe (MIN) for a collector current of 10A is 5 (as opposed to 15 for a collector current of 5 A) . Also the hfe MAX for a collector current of 10 A is not listed. What this tells you is that there is a difference of 2 for every amp of collector current between 5 A and 10 A in the MIN rating so if you PARTICULAR transistor did IN FACT deliver THE MAX of 150 gain at 5 A, it would probably be (150-2) (or LESS) for 6 A. So your calculation for the base current should more likely be 6/140=42.8 mA and for an input voltage of 5V from an arduino output pin -0.7V Base-Emitter voltage drop = 4.3V/0.0428 A= 100 ohms. Since no two transistors are exactly identical, the only way to actually know what your dc current gain is , is to do a bench test using a load resistor and measure the collector current obtained with a 100 ohm base resistor and adjust the base resistor value until you obtain the desired collector current. I don’t know why you said the " amplification factor " was not on the datasheet. I can only assume you have never read a datasheet before and have no idea what hfe is or that it is called DC CURRENT GAIN and clearly on page 2 of the datasheet.

thank you very much for the precise explanation, you helped me a lot! : )

Your welcome. There is a special HELL reserved for people who don't read datasheets. It is a place where
nothing works and everything smokes when you turn on the power.... :smiley:

yeah, that´s why I at least try to read them, haha ; )

raschemmel:
Your welcome. There is a special HELL reserved for people who don't read datasheets. It is a place where
nothing works and everything smokes when you turn on the power.... :smiley:

Oh, my ... We need a 21st century version of Inferno :smiling_imp:

Seriously Mr. B,
Can you count on both hands the number of posts you've read that were caused by not reading datasheets ?

To be frank (sorry) building your own is a bad option take a look at this kind of thing. https://www.coolcomponents.co.uk/simple-high-power-motor-controller-18v15-fully-assembled.html?gclid=CK-U18iB-cICFdQZtAodg3oAHA

It will work out cheaper than trying to build your own.

Mark

If you need to drive over 40mA, you can't do that direct from an Arduino, you will need to amplify the current with a small signal transistor first that can deliver over 40mA.

The absolute maximum from an Arduino is 40mA, 20mA typical.

To be frank (sorry) building your own is a bad option take a look at this kind of thing.

Are you sure you have the right post ? There is nothing in this post about motor controllers .

Are you sure you have the right post ? There is nothing in this post about motor controllers .

Post #2 first line

I want to control a DC-motor which requires 6 amperes with the Arduino.

Mark

Edit - PS But why not control such a "high" load with a motor control, that card may have back EMF protection but would still work (off the peg) for say a big string of high power LED's or a heater!

Mark

raschemmel:
Seriously Mr. B,
Can you count on both hands the number of posts you've read that were caused by not reading datasheets ?

You must be tired, my response was a joke.

Ray

Oh please, like I don't know a joke when I see it. It was a good joke. I'll take that as a "no" to my question.