Dr. Slack wrote ...

light photons need a certain minimum energy, and in a LED, that needs to be provided by a single electron. The excitation voltage must be above that energy threshold, or we're back in perpetual motion territory again. However, below that voltage there is still current flowing by other leakage unwanted mechanisms, just no light production.

Dr. Slack wrote ...


Yes, check my long post and that's what I said too, by mechanisms other than the 'wanted' behaviour of splitting water molecules.

By the same token, dragging the subject back to the topic of LEDs, light photons need a certain minimum energy, and in a LED, that needs to be provided by a single electron. The excitation voltage must be above that energy threshold, or we're back in perpetual motion territory again. However, below that voltage there is still current flowing by other leakage unwanted mechanisms, just no light production.

Loading the LEDs with a 10M meter as I measure them tends to reduce the measured voltage at low currents, hence suggest that the string current is flowing at a lower device voltage. Measurement with a high impedance meter would sharpen the knee in the voltage/current curve. Thus my mid-impedance measurement conditions favour your 'no threshold' position.

I think if we haul the argument all the way back to the OP, he was talking about what conditions would give him visible light, not whether his LED had sub-threshold leakage currents. I don't think he'd be looking for remarks on his costume like 'Dude, I really like the leakage currents in your non light emitting things!'

Is it really the case that the only difference between our arguments is that you say there's no threshold for current flow, which I agree with, there isn't, due to leakage currents, where I say there *is* a threshold for useful function, light output or molecule splitting, which there is as it depends on the finite energy needed to make one quantum of stuff do its useful stuff?

Uspring wrote ...

Ash Small wrote:

The electrons around the junction have an 'average energy'. Some will have more, some will have less. They are all 'jiggling about', gaining and losing energy as they collide, etc.

Some will achieve a high enough energy level to 'jump' even when the 'average energy' is below the threshold.

If you talk about thermal jiggling, the energy is much too low for the LED to emit a meaningful amount of light at room temperatures. At sufficiently high temperatures the LED will glow red even without a voltage applied

Bored Chemist wrote:

The point is that it's a plot of log I vs V and it does not show a threshold.

By distorting the X axis you can make almost any function look linear in a plot. I think a threshold doesn't have an exact definition. I'd consider a threshold to be a sufficiently steep part in a function. Whatever that is.

Dr. Slack wrote ...



Any more discussion of electrolytic cells is way off topic, and should be taken to another thread, the equivalent of 'see you outside in the car park'. Whether the general science car park or the chatting car park depends on the level of scientific exchange. If we are going to keep shifting the ground between whether it's hydrogen and oxygen gas at STP in stoichiometric ratio, or just hydrogen, produced by a cell (I'd expect different thresholds for those), then I think chatting would be the one to go for.