Hi all,

I am really sorry to bother you all with my calculations. Please bear with me one last time.

I have attached a very simple LED circuit. I just need to find the current it consumes.
I am not going to trouble you with my calculations. Please just check my approach.

1. Find the maximum current through the circuit without that LED and the Transistor.
2. Having found the maximum current through the circuit, I will take the forward voltage of the LED from the LED datasheet.
3. I will also find the collector-emitter voltage of the NPN transistor, as I know the collector current.
4. Now, using the voltage drop across the LED and the transistor, I will calculate the current through the circuit(Ohms law)

Is my calculation right?

Please let me know if there are any changes? And one more question, suppose, I dont know the resistance in this design but I have to chose one current limiting resistor as the one in the image, how do I calculate the resistance? Can someone help me with an example.

Thanks.

 

You might take a minute & check EDUCATION at top of page. VOL111, Diodes. LEDs.
No supply V listed. No LED or transistor = no current.
Supply = 5V, LED Vf = 3V @ 10 mA, transistor Vf = .1V, V to drop = 1.9V. R=V/I, 1.9 V / 10 mA = 190 ohms
or use 180 or 200 1/4 W, 5 % OK.

 

You don't need to bother with the current through the circuit without the LED. Just take your source voltage, subtract the LED forward voltage and the voltage across the transistor, then use that to calculate current across the resistor. This will be your current through the LED.

 

Just as an overview, in a series circuit the transistor collector current and the LED current as well as any series resistor current will be the same. Without a LED data sheet and knowing Vsupply voltage nothing can be solved for. Kirchhoff's Current Law would apply. The sum of the voltage drops will be Vsupply or Kirchoff's voltage law.

The forum exist for enthusiast like you to ask questions and learn. You ask as many questions as you like.

Ron

 

You can skip steps 1-3. Current through the LED will be:

\( \small I=\frac{V}{R}=\frac{V_{BAT3}-Vf_{LED}-V_{CE}}{750 \Omega} \)

If your assumptions for any of the quantities are way off, you need to do the calculations again.

A more typical approach would be to shoot for a desired LED current, use the typical Vf for that current, assume the typical saturation voltage for that current, and calculate the resistance.

\( \small R=\frac{V}{I}=\frac{V_{BAT3}-Vf_{LED}-V_{CE}}{10mA} \)

If you were using a 2N3904 at Ic=10mA, Vce would typically be around 0.1V with Ib=1mA.

 

You don't need to bother with the current through the circuit without the LED. Just take your source voltage, subtract the LED forward voltage and the voltage across the transistor, then use that to calculate current across the resistor. This will be your current through the LED.

Yes. But Initially, I do not know the forward voltage of the LED (and the Vce of the transistor, since both the forward voltage of LED and the Vce of the transistor, will vary depending on the current through them as per their datasheet, right?). So, I have assumed the maximum value of current by considering only the resistor at first. Later, with that known maximum current, I will look into the LED and the transistor datasheet for the current vs voltage graphs in them and select their forward voltage. That's how I have approached.
Two questions I have :

1. Is my approach correct?
2. How do we pick the LED and the transistor parts after I have know the max current (In this case, it has already been done for me)?

Thanks

 

You can skip steps 1-3. Current through the LED will be:

\( \small I=\frac{V}{R}=\frac{V_{BAT3}-Vf_{LED}-V_{CE}}{750 \Omega} \)

If your assumptions for any of the quantities are way off, you need to do the calculations again.

A more typical approach would be to shoot for a desired LED current, use the typical Vf for that current, assume the typical saturation voltage for that current, and calculate the resistance.

\( \small R=\frac{V}{I}=\frac{V_{BAT3}-Vf_{LED}-V_{CE}}{10mA} \)

If you were using a 2N3904 at Ic=10mA, Vce would typically be around 0.1V with Ib=1mA.
View attachment 166242
View attachment 166243

I understood. But I have done the first 3 steps because, Initially, I donot know the Vf of the LED and the Vce sat of the transistor (Assumed to be unknown, eventhough I have all the data, assuming I am starting from scratch). OK. One help.
Can you help me how to approach or design this type of circuit, if I have told you the required max load current to be 30mA through a collector of the transistor.
If I had given only this info to you, what would be your approach? How would you pick the LEDs and Transistor (Part numbers) and the base resistor values?

P.S. All I am doing is performing worst-case analysis. But I do not know how these are designed. Can you help me with the design steps? Please.

 

Is my approach correct?
2. How do we pick the LED and the transistor parts after I have know the max current (In this case, it has already been done for me)?

You're making it too complicated.

1. Pick an LED current. Consult the datasheet for the typical Vf.
2. Pick a transistor with a suitable maximum Ic. You may also need to consider breakdown voltages. Consult datasheet for required Ib to saturate the transistor at desired Ic.
3. Calculate base resistor.
4. If any parameter is too far from typical, do calculations with actual values.

 

You're making it too complicated.

1. Pick an LED current. Consult the datasheet for the typical Vf.
2. Pick a transistor with a suitable maximum Ic. You may also need to consider breakdown voltages. Consult datasheet for required Ib to saturate the transistor at desired Ic.
3. Calculate base resistor.
4. If any parameter is too far from typical, do calculations with actual values.

Thank you

 

Thank you

Forgot to mention that saturation voltage will also be in the datasheet.

 

In doing the calculation, the variation in the saturation voltage at typical LED currents will be tiny compared to the range of Vf for the LED. Do the calculation at both ends of the Vf range and you will have a good estimate of the range of current to expect.

Bob