Hello How would one figure out the resistor to be used with a bjt like the 2n3904 transistor. Also looking at the data sheet what would be the most important information needed. Thanks for all the help

 

Transistor circuits use many resistors.
You forgot to tell us which resistor and forgot to tell us the function, load and power supply voltage of the transistor.
Attaching a schematic of whatever your transistor is doing will help us help you.

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Uncle $crooge

 

Basiclly I am tring to us it as a simple switch to control a light. The supply voltage is 28 volts. Thankd for all the help

 

you'll not find that kind of information directly in a datasheet, any more than you'd find directions to an out-of-restaurant in your car's owners manual!

try the hFE curves as a starting point, from that see how much mininum base drive current you'll need for your collector lamp load. If the base series resistor can overdrive that by at least 5x you're ok. If not you'll be better with a mosfet instead.

Ohm's Law is very useful to calculate the value with

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"I think, therefore I toast" ( Mr. T. Toaster, artificial life form, kitchen appliance & Philosopher)

 

You must limit the base current with a resistor.
For a transistor used as an on-off switch, the amount of base current should be 1/10th of the collector current as shown in the datasheet of the transistor. It is shown as "Collector-Emitter Saturation Voltage".

The base voltage is about 0.7V so with a 28V supply the base resistor will have 27.3V across it. Ohm's Law is used to calculate its value.

A light bulb is almost a dead short when it is cool. Your tiny transistor might burn out trying to drive it.

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Uncle $crooge

 

Thanks for the reply. I guess what I was looking for was the formula. Or how would I calulate these values.

 

The formula for calculating the value of a resistor is Ohm's Law. Look for it in Google then learn it.

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Uncle $crooge

 

Thanks for the reply. I know understand ohms laws but appling them is the problem. Just need to know how to apply them to real word electronics. To many of you this is childs play.

 

Yes, just like Law Law, Ohm's Law requires an Ohm's Lawyer

Normally you need to go to Ohm's Law School to learn how to use it for manipulating things and people (just like Law Law!), but it's amazing how much you can pick up, by carefully asking the right questions.

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"I think, therefore I toast" ( Mr. T. Toaster, artificial life form, kitchen appliance & Philosopher)

 

Are Ohm's Lawyers bottom feeding scum also?

 

Voltage = Resistance * Current
V=IR
Your resistor will have a value in ohms
Your current will have a value in ampere
Your voltage will have a value in Volts
That's traditionally in SI system.
How to use it?
In an extremely simple circuit, locate the Voltage Source. Hopefully, it will be in DC otherwise it gets complicated.
Then locate the resistor, that is the value of the squiggly symbol that looks like a wave.
Finally V/R= Current
That's how to use it. In the case of a transistor, you want to look up KVL and KCL.
Hopes this narrows down your search ...
Good luck from california, JJVC