Why

137

01:44:39

What is the difference .. Why this is happening

published 10 years ago

alexbecker

10 years ago

Reduce the resistance of the base resistor, the transistor isn't saturated

jchmiel

10 years ago

Because some voltage is going from the collector to the base on the npn transistor that is above the load. It is also using more of the available power so there is less available to the load below. On the left your pulling to ground and most of the power is being used by the load vs the transistor.

Chryseus

10 years ago

The lamp on the right can have no more than 2.2V across it due to the base-emitter junction whereas the one on the left gets almost the full 3V, in addition 5k is a little too high, assuming the lamp draws 30mA at full brightness you want at least 1mA base current to ensure saturation, 2.2k being a good value, the lamp on the right has no need of a base resistor since the base current flows out the emitter so it will limit itself the resistor in this case is just dropping more voltage V = IR

hunar

10 years ago

Thank you all .. I got it

selman

10 years ago

I think it's like this, when you put a lamp which has a resistance 75Ohms at the emitter, what happens at the base is that this resistance gets larger by a factor of beta+1 which is about 7.5K, now this resistance appears in series with the 5K base resistance lowering the base current to ((3-0.85)/(5K+7.5K)) = 175uA and a collector current of 17mA which is no where near the full brightness, and lowering the 5k wont help scince the 7.5K is still there, even if you set the 5K to 0ohm the collector current will be only at 27mA

add comment in editor

EveryCircuit is an easy to use, highly interactive circuit simulator and schematic capture tool. Real-time circuit simulation, interactivity, and dynamic visualization make it a must have application for professionals and academia. EveryCircuit user community has collaboratively created the largest searchable library of circuit designs. EveryCircuit app runs online in popular browsers and on mobile phones and tablets, enabling you to capture design ideas and learn electronics on the go.