Transistors switch Problem

152

01:38:06

Hello Guys,
So I have designed this circuit as a transistor switch, where if you increase the resistor 1k, the red led would dim dramatically.
The problem is I just can't find the answer to: The current through the 1k ohm = 8.19mA, and current through the Q1= 8.03, and Q2 is roughly= 160uA. How is the current distributed between the transistors? I can't apply the current divider formula into this. How can I calculate the current through Q1, and Q2? 

published 10 years ago

hurz

10 years ago

Its because the second bjt in parallel is more than in saturation and overtakes all the base current. Spend a emitter resistor of e.g. 1k to this green-bjt and the other red-bjt behaves as expected. Hope this helps.

hurz

10 years ago

Ok, here we go http://everycircuit.com/circuit/6213958046842880

Lawliett

10 years ago

First of all, thanks for the reply. I do understand why it takes all the current Q1. However, what I am trying to understand here: is there a way to calculate that tiny amount of current passing through Q2 when most of the current is in Q1? And I saw your circuit, I do want to switch off the Red Led faster without increasing the Resistor to thousand K ohms. That's why I want it to be this way. because by knowing the tiny amount of current, I can calculate the Collector current and voltage drop.

abobaker

10 years ago

The only way i can think of is Ebers Moll Model

abobaker

10 years ago

I think your circuit can be made with an even more simple design though .. What's the intended job of the circuit ?

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