# need help with Transistor LED circuit

Thread Starter

#### zoodles

Joined May 26, 2019
4
Hi Guise

I have this circuit, and my main goal is to decide what voltage i should have on the voltage source to reach 15 +-10 % mA through the LED.
I realized some things that i think is correct but other than that I have no idea how to approach the problem.

1. The current through the resistor R1 is similar or equal to the LED current
2. Each diode has 0.7 V
3. I lose 0.7 V when going through the emitter on the transistor.

I would really appreciate if someone could give me any advice or formulas to work with, thanks in advance!

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#### WBahn

Joined Mar 31, 2012
26,398
You seem headed down a reasonable path. Assuming, for now, that your assumptions regarding the diode and base-emitter voltage drops are correct, what is the voltage drop across the emitter resistor? What does that resistance then need to be to get 15 mA flowing through it?

Once you've gotten that far, we can start looking at that 10% spec.

#### Zeeus

Joined Apr 17, 2019
615
Welcome to AAC

Not an expert
Your first point : Led is d4? Why similar current to R1?

Read your textbook for maybe 1hr then come back

#### WBahn

Joined Mar 31, 2012
26,398
To be sure that I understand the problem -- did YOU come up with that circuit, or was the circuit given to you and the only parameter you have control over is the voltage of the voltage source?

Thread Starter

#### zoodles

Joined May 26, 2019
4
Welcome to AAC

Not an expert
Your first point : Led is d4? Why similar current to R1?

Read your textbook for maybe 1hr then come back
well i meant R2, my bad.

Thread Starter

#### zoodles

Joined May 26, 2019
4
To be sure that I understand the problem -- did YOU come up with that circuit, or was the circuit given to you and the only parameter you have control over is the voltage of the voltage source?
Yes the circuit was given to me, exactly I can only change the voltage source.

#### WBahn

Joined Mar 31, 2012
26,398
Yes the circuit was given to me, exactly I can only change the voltage source.
Then you might have a problem, but let's work it and see.

If you were told what the voltage on the base of the transistor was, could you determine both the LED current and the current in the 1N4148 diodes?

If so, then we can break the problem into two smaller problems: First determine what the voltage on the base of the transistor needs to be to get the desired LED current and second to determine what the source voltage needs to be in order to set up the corresponding current in the diodes.

So try tackling that first question. This has moved away from being a problem in which you can make generic assumptions about voltage drops -- you need to delve into the data sheets for the transistor and the diodes to get the information you need for these components. Also keep in mind that, for common components sourced by multiple vendors, that not all of the specs are exactly the same and one vendor's data sheet might lack the information you need while another vendor's data sheet might have it. Be grateful for the days of Google and long-gone being the days of trying to drum up data books (though I admit that there are often times that I miss those days -- just not enough to want to go back to them).

#### WBahn

Joined Mar 31, 2012
26,398
well i meant R2, my bad.
Mistakes happen -- I didn't catch it and assumed that you were talking about the emitter resistor (which is why I prefer using more descriptive terms than just reference designators when practical). But the clarification was worth it -- it was conceivable that you were working with a circuit and were either told, or had come to believe, that the current in R1 and the LED should be the same).

Thread Starter

#### zoodles

Joined May 26, 2019
4
Then you might have a problem, but let's work it and see.

If you were told what the voltage on the base of the transistor was, could you determine both the LED current and the current in the 1N4148 diodes?

If so, then we can break the problem into two smaller problems: First determine what the voltage on the base of the transistor needs to be to get the desired LED current and second to determine what the source voltage needs to be in order to set up the corresponding current in the diodes.

So try tackling that first question. This has moved away from being a problem in which you can make generic assumptions about voltage drops -- you need to delve into the data sheets for the transistor and the diodes to get the information you need for these components. Also keep in mind that, for common components sourced by multiple vendors, that not all of the specs are exactly the same and one vendor's data sheet might lack the information you need while another vendor's data sheet might have it. Be grateful for the days of Google and long-gone being the days of trying to drum up data books (though I admit that there are often times that I miss those days -- just not enough to want to go back to them).
Thanks a lot for the tips, yes I'll work on figuring it out.
Those 0.7 voltage numbers were basically given to me, so they might not be entirely accurate but in this case i can assume they are correct just to solve the problem. Yes i'm grateful for google haha, anyway thanks!

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