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14Vf LED question

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Uspring

Fri Sept 25 2015, 11:05AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
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Posts: 711

@Bored Chemist:
I guess we're argueing about the definition of the term "threshold". I believe it to be vague enough to be wasting our time to come up with something we agree upon.

@Ash:

Udo, my point was that as the 'average energy' approaches the threshold voltage, more 'jiggling' electrons will gain enough energy to jump. I was suggesting this as an explanation (or partial explanation) of why PicoAmp readings can be obtained at voltages below the 'threshold voltage'.

Ok, I misunderstood your point. Thermal effects definitely play a role wrt to the current. They don't help very much wrt the voltage, that is needed to push electrons to an excited state that is necessary for light emission.

Ash Small

Fri Sept 25 2015, 11:56AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Uspring wrote ...

Ok, I misunderstood your point. Thermal effects definitely play a role wrt to the current. They don't help very much wrt the voltage, that is needed to push electrons to an excited state that is necessary for light emission.

Maybe I've not explained it very well, Udo.

I agree that thermal effects themselves won't excite many electrons sufficiently for them to jump, but any potential difference accross the junction is going to raise the average energy levels of the electrons waiting to 'jump'.

As the voltage approaches the 'threshold voltage', a greater percentage of the 'jiggling electrons' will attain sufficient energy.

One point I'd like to make is that, even when the 'average electron energy' reaches the threshold, some of the electrons will still have less energy than that required to 'jump'. As the voltage rises still further, more and more electrons will be above the 'threshold energy', resulting in more current flow at higher voltages.

Newtonian physics only looks at 'the average energy' of the electrons, so doesn't explain why some electrons can 'jump' before the average energy reaches the threshold (as observed in the experiments above). In order to explain what's actually happening, you need to start thinking more along the lines of QM, in my opinion.

The 'average' way of looking at it is still useful when using components to design circuits, as long as you take into account 'leakage currents', etc., which aren't really explained by using Newtonian Physics alone.

krenshala

Fri Sept 25 2015, 01:49PM

Registered Member #143 Joined: Sat Feb 11 2006, 04:25PM
Location: Austin TX, NorAm, Sol III
Posts: 28

Conundrum wrote ...

Its worth mentioning that blue LEDs can fail short, I now put resistors across mine in high reliability applications to mitigate against ESD.
Works for laser diodes too, a good value to start with is 4.7K or 4K7 which only uses <1mA at a typical Vf of the diode with a current limiting resistor or other current control mechanism feeding that.
Also just because it seems to work with two in parallel doesen't mean that it will not burn up with a fresh battery (learned that the hard way, use a resistor!!)

Most of the sellers use 47 or 100 ohms which with your setup and a plausible 1.7V drop from 9.8V in at 2.7V Vdiode would give an individual LED current of 54mA which is high but acceptable.
there will be some diode heating which could be significant as thermal runaway can damage them.

Since the LEDs are listed as 14Vf (not 1.4) on the packaging, would that mean it needs a larger or smaller resistor to accomplish the same task?

Conundrum wrote ...

I've seen some circuits that use a 555 timer and MOSFET (cough road lamp /cough) to pulse the LED and reduce current drain, in fact used a variant of this on my christmas tree lights to get a tiny fraction of the normal current used for only maybe 20% drop in visibility.
Others have used a flickering candle diode between D and G of an N channel MOSFET and a variable resistor from G to S to tune for maximum flickerness (tm)

While tempting just for the additional experience it would provide me, i think that is more than I want to put into this particular project.

Conundrum

Fri Sept 25 2015, 04:21PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

1.4V Vf is a typo. I have never seen a blue LED run much below 2.2V and in fact they typically need a lot more.

Unless they actually mean 4.1V which is possible, they might be older technology such as silicon carbide (SiC)

krenshala

Tue Sept 29 2015, 10:52PM

Registered Member #143 Joined: Sat Feb 11 2006, 04:25PM
Location: Austin TX, NorAm, Sol III
Posts: 28

They are fourteen (14) volt LEDs.

Dr. Slack

Wed Sept 30 2015, 06:25AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

krenshala wrote ...

They are fourteen (14) volt LEDs.

Would you post the data sheet pdf, or a link to it, please. Any more discussion on this device is superfluous without it.

Bored Chemist

Wed Sept 30 2015, 12:33PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

A LED that actually drops 14 v while working properly would be very strange indeed. The same is true of an LED that drops just 1.4 volts and produces much light.
But (as was pointed out earlier) you can get packages with an LED and a resistor in them- the case illustrated was a 12 Volt Led.
Now 14 volts is an odd choice to cater for but, if that's correct and it's a combination of an LED and a resistor that works nicely at 14 volts then I'd expect it to work (albeit less brightly) at 9 volts.

Another possibility is that this "14 volt" LED is a cluster of, for example, seven LEDs in series, each rated for about 2V

Has anyone got any reason to rule those ideas out?

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