Electronic – Why is there no current flowing through another parallel wire in this video

circuit analysiscircuit-design

In his video Making logic gates from transistors Ben Eater draws a following scheme

schematic

simulate this circuit – Schematic created using CircuitLab

When the switch is on, the LED stops emitting light (to demonstrate that there is no current on the wire the LED is connected to). I wonder why there is no current on the LED wire. In theory, there should be at least SOME current flowing through LED.

Is it assumed that the resistance of wire going through the transistor is negligible as compared to a huge resistance that is provided by LED and therefore the current going through LED is negligible too?

Best Answer

When the transistor is turned on in that manner it can be switched into saturation if the base current is sufficient. The collector-emitter emitter voltage will drop to about 0.2 V. This voltage will be applied to the LED.

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Figure 1. Current through various colours of LED as a function of forward voltage. Image source IV curves.

Figure 1 shows that none of the LEDs from infrared to ultra-violet will pass any significant current at 0.2 V. There just isn't enough voltage to get the charge carriers to jump the P-N junction.

enter image description here

Figure 2. A water check-valve analogy. Image source: What is an LED?.

If you look at the check-valve in the figure above, it should be clear that the spring normally keeps the ball in position and prevents back-flow. When “forward-biased” the ball shut-off can be moved against the spring but it will take some initial pressure to move the ball. This results in a pressure drop across the valve: the pressure downstream will be less than the inlet pressure.

In a similar manner the PN junction in an LED causes a voltage drop. For a red LED it is about 1.5 V to 2.0 V. You need to exceed the Vf to get enough current to flow and light the LED.

The links are to articles by me and may help you further.

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