# Single Transistor Circuit Current Question

#### chopnhack

Apr 28, 2014
1,576
Self assigned work, not homework, but seemed more appropriate to post here. I built a small circuit from a video course I have been watching. The assignment is below, but I modified it to work with parts I had on hand:

Namely, I didn't have an LDR nor a small incandescent bulb, so I just built a simple circuit using a switch (not shown) to mimic the on/off input of the LDR. For the two source voltages, I used a 5vdc source and a 9v battery (8.7vdc). The circuit is below. According to the program, the current at R1/D1 is 1.6mA, in reality when I built the circuit, I measured 0.34mA. The circuit functions as far as I can tell, the three LED's light when power is applied to the base of the transistor, so I assume the their is flow from collector to emitter. I was expecting to see more current.

Is the current of the top rail derived by dividing the final voltage (after the voltage drop of each diode and resistor) by the 1k resistor?

As for the paper exercise of the above - I assumed a beta of ~100 for the transistor and then set the resistor attached to the base to allow 2.5mA to pass. I used 4.3v/1720Ω = 0.0025A and with a 100 gain, it should allow 0.25A to pass between collector and emitter.

#### duke37

Jan 9, 2011
5,364
Add up the voltage across D1, D2 and D3 and Q1 then subtract this from V1. This voltage is across the 1K resistor so you can calculate the current.

#### chopnhack

Apr 28, 2014
1,576
Add up the voltage across D1, D2 and D3 and Q1 then subtract this from V1. This voltage is across the 1K resistor so you can calculate the current.
Thanks Duke! I keep forgetting about that diode in the transistor!

Assuming a 0.7 volt drop for the transistor and the rated drop for each LED of 2-2.2v, max v drop is 7.3v. Leaving ~1.4v across resistor, 1.4mA (matches roughly what the simulation is stating of approximately ~1.5mA). I wonder why I read 0.34mA in the real, physical model I made?

#### KrisBlueNZ

##### Sadly passed away in 2015
Nov 28, 2011
8,393
Measure the actual voltages across the LEDs. Conventionally, red LEDs are around 1.8~2.0V but they can differ quite a lot between different types. Also measure the voltage across the resistor, and across the transistor collector-emitter. That will tell you what is limiting the current.

If you measure a current of only 0.34 mA then you should expect to see 0.34V across the 1 kΩ resistor. If you don't, then something is very wrong, as resistors tend to obey Ohm's Law pretty strictly!

If the transistor is saturated, you should see less than 0.3V across it. (Not 0.6~0.7V; that is the base-emitter voltage but the collector-emitter voltage goes a lot lower than that when the transistor is saturated.) If there's significant voltage across the transistor, you need more base current (or perhaps your transistor is damaged or connected wrong).

#### chopnhack

Apr 28, 2014
1,576
Measure the actual voltages across the LEDs. Conventionally, red LEDs are around 1.8~2.0V but they can differ quite a lot between different types. Also measure the voltage across the resistor, and across the transistor collector-emitter. That will tell you what is limiting the current.

If you measure a current of only 0.34 mA then you should expect to see 0.34V across the 1 kΩ resistor. If you don't, then something is very wrong, as resistors tend to obey Ohm's Law pretty strictly!

If the transistor is saturated, you should see less than 0.3V across it. (Not 0.6~0.7V; that is the base-emitter voltage but the collector-emitter voltage goes a lot lower than that when the transistor is saturated.) If there's significant voltage across the transistor, you need more base current (or perhaps your transistor is damaged or connected wrong).

Wow, you were right! The voltage across the LED's was 2.7-2.8 (they were all green, same lot)! And there is 0.35V across the resistor.

Vbe = 0.7V
Vce = 3.7mV

Thanks Kris, it makes more sense now, I didn't realize the LED's had a higher potential across them - I took the data sheet info for granted! Now with a potential drop of 8.4 for the leds we have 8.7-8.4v=0.3v 0.3/1k = 0.3mA

#### Foxkoun

Dec 17, 2014
15
We were just covering transistors in class, however in spite managing to get through it I'm still not grasping fully. What resources are there for aiding on this topic?

#### chopnhack

Apr 28, 2014
1,576
We were just covering transistors in class, however in spite managing to get through it I'm still not grasping fully. What resources are there for aiding on this topic?
I guess that depends on how deep down the rabbit hole you want to go:

If after reading the above and the links included in the posts, you are more confused, you might want to elaborate further on what part you are having difficulty with in a new thread so that other members can see it as well.

#### Foxkoun

Dec 17, 2014
15
I'd gone back over the material and I have a better grasp of it. If anything it's just frustrating to be on the ball in class and then come next morning it's all maddeningly indecipherable.

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