transistor base voltage

[Alistair Ballantyne]

Hi,

I’m trying to calculate the voltage needed at the transistor base to “open” the transistor. I know it’s got to be ≥ 0.7v.

Saw the schematic above. I calculate the resistance in parallel as R = 470x1000 / 470+1000 = 320Ω. Therefore current is I = 7v (9v – current drop of LED) 7/320 Ω = 0.02ma. Therefore voltage going through transistor base is V = 0.02x320 = 6.4V.

Is that calculation correct and is 6.4V too high a voltage for the transistor to handle?

Thank you!

 

[KlausST]

Hi,

Your descriptiin is confusing, I can't follow it.
R1 and R2 is already given....so what "R" do yo try to calculate?

R = 470x1000 / 470+1000 =

= 2000
How do you come to 320 Ohm?

I = 7v (9v – current drop of LED) 7/320 Ω = 0.02ma.

What is this? It seems you are mixing something. Please go step by step so we can follow your ideas.

Klaus

 

[barry]

your calculations make no sense. First of all, there’s no “parallel restance”. And I don’t think you want to “open” the transistor, you want turn it on, i.e, drive it into saturation.. When using a transistor as a switch, the rule of thumb is that Your base current should be one-tenth the collector current.

For your case, assume 2volt drop across the LED. So, Ic=(9-2)/470= 14mA. Ib should then be 1.4 mA.
(9-0.6)/1.4mA=Rb=8.4K.

 

[00kam]

Hello
you have two closed loop current path when you close the switch one from B-->E and from C-->E so you have two closed circuits the first path is (+ 9 battery-->R1-->LED-->C-E transistor junction-->-9 battery which is the end loop) the second path is (+9 battery -->R2-->B-E transistor junction-->-9 battery which is the end loop) the two circuits's components are connected in series if you want to make current,volt, resistance calculations

kamal

 

[Audioguru]

Why is the base resistor value as low as 1k ohms? Then the base current is (9V - 0.7V)/1k= 8.3mA which is almost the same as the collector current of (9V - 2V)/470 ohms= 14.9mA.

EDIT: why do you want to calculate the base voltage? Look in the datasheet.
The base-emitter is a diode with a voltage of 0.6V when the base current is low and is 0.8V when the base current is very high.

 

[danadakk]

A general rule of thumb is to operate the transistor as a switch, to turn it on and put it
into saturation, is to set Ib = Ic / 10. Most datasheets specific the Vcesat of the
transistor at this level of base current.

So nominally you would compute R1 for a design Ic, then compute R2.

R 1 = (Vbatt - Vled - Vcesat ) / Ic and R2 = (Vbatt - Vbe ) / (Ic / 10 )

Vcesat is a transistor spec, saturation V collector to emitter, and graphs
of it versus Ic in datasheet. Also base current levels. Here is a curve for 2N3904
where Ib = Ic / 10 (term is "forced beta") -

Regards, Dana.

 

[Alistair Ballantyne]

Many thanks for your responses.

I see I have confused a number of issues and used a bad schematic example.

I have made a number of simple circuits where I have overheated the transistor which I thought was due to excess voltage going through the base. I see now I have been looking at this the wrong way.

Very useful insights – thank you!

 

[Audioguru]

The base of a transistor is usually the input that uses a low current. Then the base-emitter is delicate and is easily damaged by too much current.
A little 2N3904 transistor has a maximum allowed collector current of 200mA. Then as a switch its base current is never more than 20mA.

If the base current is too low then the transistor will not be a good switch and will have voltage across it and current through it which causes too much heat in it.

 

[barry]

At the risk of sounding pedantic, voltage doesn’t “go through” the base, or through anything, for that matter. CURRENT goes through, voltage is across.

Also, transistors are current, not voltage, devices. It’s not the voltage AROSS the base that controls the collector current; it’s the current into to the base that matters.

 

[crutschow]

Also, transistors are current, not voltage, devices

You do realize that may generate a response from other pedantic types who will say that the solid-state physics equations of a BJT shows that it is a voltage controlled device, not current controlled.

Of course, for large-signal and switching applications it's much easier to use a current-controlled black-box model,.
The voltage-controlled model is really only useful for the design of small-signal BJT circuits.

 

[danadakk]

V vesus I control, like wave particle duality in physics, the coin has two sides. V is
the one, but don't throw away your current controlled tools and understanding,
both very useful.

Is BJT a Voltage controlled device or Current controlled device?

I have searched already for this question, but everywhere I see debate instead of clear answer. Though it is not that important of a matter whether we call it a voltage controlled device or current

electronics.stackexchange.com

.

Regards, Dana.

 

[MichalP]

I think this kind of tasks will help you. This tasks is same as I had at university to know about bipolar calculation
https://911electronic.com/bipolar-junction-transistor-tasks/

 

[Ratch]

barry

" Also, transistors are current, not voltage, devices. It’s not the voltage AROSS the base that controls the collector current; it’s the current into to the base that matters.

Let me explain why you are wrong about that statement. It's all in the physics. A BJT NPN transistor operates by diffusion, which causes charges to flow by diffusion from the emitter to base and onward to the collector and circuit by the positive collector voltage. But the electrons leave behind charged ions which counter the flow of electrons. Applying a positive base voltage counters this effect and speeds the electrons on their way. However, some the the electrons that should go to through to the collector are attracted into the base circuit by the positive voltage. This is wasted current. By happy fortune, the collector current is a fairly constant ratio of the base waste current called "beta" . Now you can hook up a current generator to a transistor and plot some useful curves and think you have a current controlled device. But you are fooling yourself. You have a current controlled CIRCUIT. The virtual resistor of the current generator is part of the circuit. The transistor itself still dances to the voltage it receives from the current generator. A transistor is voltage controlled.

Another example. No one denies that an ordinary op-amp is a voltage amplifier. Yet, within the proper circuit circuit it can be part of just about anything else. A device that is truly current controlled is a magnetic amplifier. They are big, ugly and rugged compared to solid state.

Ratch