Why is this getting a current at all

132

01:26:32

The supplied voltage does not exceed the combined Vf of the diode plus LED but still there is some current going through the (simulated 1W 3.3Vf) LED...???
Also the voltage drop on the LED doesn't match the "Voltage" setting for the LED, so that value is not used for Vf or Vd.. WTF?

Part of my problem was my misconception about how LEDs/diodes work.
As long as they are "forward biased" with as little as 0.2 - 0.7 V (or any voltage at all?) they start do conduct a very small current, and that I vs V characteristic/"graph" spikes very sharply at a certain point so that it acts almost like a "short circuit" path above higher "forward bias" voltages.
The region were they act as an "open circuit" is much smaller (non-existant?) than my original view implied.

Here's an xcellent circuit which visualizes the I V relationship of LEDs: 
http://everycircuit.com/circuit/6430643947307008

I am still trying to figure out if the Vd of each component in my circuit is correct but I have also learned that Vd of a diode/LED is linked to the passing current so it is not quite as simple as "Vd = Vf" (which might be true in certain ideal circuits?).

published 9 years ago

MmmAaaXxx

9 years ago

Dude, there's a resistor, that means it has to have a current no matter what.

ohmster

9 years ago

What is puzzling here are the voltages listed. The LED shows of voltage across it at 1.05 millivolts. That is below the threshold of most light-emitting diodes which is typically 1.2 volts. The circuit does indicate a current flow in the LED, however the LED does not appear to be lit. So I think the Opie has a valid point, this is a very puzzling circuit if not totally inaccurate for the app.

ohmster

9 years ago

1.05 volts, my mistake. But the circuit doesn't appear to follow the laws of electricity.

hurz

9 years ago

For me it looks quite plausible. Think about a little longer. Simulation works fine!

rich11292000

9 years ago

http://everycircuit.com/circuit/6430643947307008

sagnik2305

9 years ago

U changed the LED Configuration in a random way.... Thats why itz working like this

Tommy_2Tall

9 years ago

Sagnik2305: trying to simulate a highpower LED so "random" is actually "best effort to approximate values of the real thing".

Tommy_2Tall

9 years ago

rich... solved most aspects of my "mystery" (/my lack of knowledge about semiconductors). Thanks!

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