I have a pile of bluish white LEDs salvaged from Christmas lights. They look like ordinary LEDs, with one electrode having a little cup with the light emitter, and the other with a tiny wire going to the top of the emitter. When I apply a milliamp or so from a 9v supply, they light up just fine. If I apply 10 microamps or so, they barely light when power is applied, then get brighter over the next few seconds. It is clearly visible to the eye, and makes them useless for making optisolators with. Obviously the LEDs used in commercial optoisolators don't have this problem. Why do these LEDs do this, and what kinds of LEDs do it?

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    \$\begingroup\$ How do your control 10 uA , from a 9V supply? \$\endgroup\$ Sep 3, 2017 at 21:44
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    \$\begingroup\$ 1 megaohm will not excite/activate photo-emission to the LED sufficiently and no, optoisolators would have the same problem. two false assumptions \$\endgroup\$ Sep 3, 2017 at 21:46
  • \$\begingroup\$ Which commercial opto-isolators work at 10uA? \$\endgroup\$ Sep 3, 2017 at 21:47
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    \$\begingroup\$ the one KFW is trying to imagine \$\endgroup\$ Sep 3, 2017 at 21:51
  • \$\begingroup\$ the lag between blue emission and phosphor photo absorption converted emission depends on temperature for threshold which varies widely with process control and then reduced to microsecond durations at high energy levels at normal operating range \$\endgroup\$ Sep 3, 2017 at 21:55

3 Answers 3


Bare LEDs are generally quite fast. The ones with phosphor to re-emit a different color are slower. How much slower depends on the phosphor. White LEDs, for example, have phosphors. Those would be silly in a opto-isolator.

Commercial opto-isolators use infrared LEDs, usually in the 9xx nm wavelength.

Look at the current spec for commercial opto-isolators. Note that they require significantly more than 10 µA of input current to work. What a LED does with only 10 µA is irrelevant to opto-coupler use since they aren't being run at such low currents. Usually a few mA is expected.

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    \$\begingroup\$ The only physical process I know that operates on the order of seconds (or even milliseconds, to be honest) is phosphorescence. This depends upon illegal quantum spin state transitions, which being illegal really just means slow. Photon re-emission at the atomic level operates on the order of \$\tau\approx 10^{-10}\:\textrm{s}\$, plus or minus a few orders. Fluorescence and molecular can be a little slower but really is still quite fast and doesn't get into milliseconds (to my knowledge.) Only phosphorescence yields delays like that. Charge \$\tau\$ similar to discharge \$\tau\$. +1 \$\endgroup\$
    – jonk
    Sep 3, 2017 at 23:43
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    \$\begingroup\$ @jonk has a good point, and I have bluish-white LED bulbs that emit for a few seconds after switching off, but much less blue than when on, as the phosphor emits. With some phosphors you can even see different lifetimes for different components as a further change in colour. (I've seen it in CFLs as well) \$\endgroup\$
    – Chris H
    Sep 4, 2017 at 8:45
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    \$\begingroup\$ @jonk Just for the sake of clarity: the transitions are called "illegal" because in an ideal, simplified world, they are indeed illegal; there is no way the system can transition from one state to the other on its own. However, in the real world, interactions with all of the other matter in the universe can give the system a little nudge in the "illegal" direction. \$\endgroup\$
    – Arthur
    Sep 4, 2017 at 10:06
  • \$\begingroup\$ @jonk um, “only” seems a bit strong... what about radioactive decay – can take anything from picoseconds to terayears? (The reason for this slowness being actually pretty much the same as in case of phosphorencence: quantum transitions which are exponentially unlikely – tunnelling.) \$\endgroup\$ Sep 4, 2017 at 16:23
  • \$\begingroup\$ The optoisolators I was constructing substitutes were Vactrols. They control a light-dependet resistor over a 10000:1 range, with a maximum LED current of 10ma or so, and hence require the LED current to vary over approximately the same range. Vactrol makes a whole range of commerical parts that do this, so there's nothing particularly unusual about my specs. \$\endgroup\$
    – KFW
    Apr 27, 2020 at 8:15

The most plausible reason for this behavior is that you only think the current though the LED is constant where in fact it isn't. Plot the current with a scope for the few seconds while the brightness changes: I'm pretty sure you won't see the flat line you'd expect.

  • \$\begingroup\$ True, and I'd like to know what's going on inside the LED to modulate the current like that, and why the LED is doing it. That's why I asked the question in the first place. \$\endgroup\$
    – KFW
    Apr 27, 2020 at 8:13
  • \$\begingroup\$ @KFW Without a datasheet, who knows. Maybe they have a built-in timing circuit which changes their brightness periodically. Or perhaps they have some sort of current limitation circuit which misbehaves at very low currents. \$\endgroup\$ Apr 27, 2020 at 9:17

They are UV LEDs with multiple phosphors, designed for efficiency at medium current even more than high brightness (i.e. they're not illumination but decoration). The UV is detectable if I look for it. They do, in fact, take observably long times for the phosphors to both activate and deactivate.

re LED illumination, I found this in another forum, and a densely mathematical article that backs it up down to the quantum level, where individual photons are emitted.

"There is no absolute minimum current required to make an LED emit light; using a sensitive photodetector, I've detected light output from LEDs (green "ultrabright" indicator lamps) driven by currents as low as one nanoampere. They probably emit at even lower currents, but that's where my photodetector bottomed out."

A datasheet for something similar to what I build only shows the LED current/opto resistance graph out to 100k, also states that the off resistance is 50MEG, and suggests contacting the factory for specially selected units if you want more accuracy at low current levels.


And a commercial use of such an optoisolator shows a maximum current through the LED of less than 1ma, linear use rather than on-off switching, and an active resistance range for the optoresistor up to 680k or so, successfully running it at microamp levels.

So it appears that both the LEDs and photoresistors work at those levels, just with less precision.


  • \$\begingroup\$ White LEDs use a blue emitting diodes (~450nm) not UV while the phosphor lifetime is on the order of 100ns, so while the phosphor is charging up the diode would appear blueish white for several hundred nanoseconds but would essentially by at equilibrium after 1-2 microseconds. I think your second explanation with the contact resistance is much more likely. \$\endgroup\$ Feb 2, 2022 at 15:14
  • \$\begingroup\$ The current is constant, and set by a constant-current source - I tried that to be sure. The visible time constant is significant fractions of a second. Literally dozens of these LEDs, all from the same string, do the same thing in separate test setups, all of which don't have the same resistance. Other white LEDs that I have don't. They show measurable UV with the appropriate light meter and filter. The light output bears little relationship to the current drive at low currents, unlike other LEDs. The ones you're examining are clearly built differently than these. \$\endgroup\$
    – KFW
    Feb 2, 2022 at 23:32

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