1
\$\begingroup\$

I have ten LEDs connected in series that all have the same characteristics. When I slowly raise the voltage, nine of them turn on but one of them in the middle of the circuit stays off. The LED which stays off passes the current, I presume. I say this because I don’t have any extra diodes in place to pass the current in case of a broken LED.

When I turn on the voltage a bit more, then the previously unlit LED turns on too.

One more thing: Turning off the voltage makes the poorly-performing LED turn off later than the others, for about 2 seconds.

\$\endgroup\$
6
  • 2
    \$\begingroup\$ Please post a schematic. \$\endgroup\$
    – winny
    Feb 12 at 12:28
  • \$\begingroup\$ 1. What colour LEDs? 2. Are they all the same type / batch? 3. What voltages do these effects occur at? \$\endgroup\$
    – Transistor
    Feb 12 at 12:40
  • 1
    \$\begingroup\$ Turning voltage off makes [one particular LED turn] off [seconds] later than [all?] others That would imply they are arranged in one line rather than connected in series - assuming turning off means instantly rather than over more than 10 seconds. \$\endgroup\$
    – greybeard
    Feb 12 at 12:46
  • \$\begingroup\$ Do you have a capacitor directly attached to the nonconforming LED? \$\endgroup\$ Feb 13 at 6:57
  • \$\begingroup\$ As already mentioned, it behaves as if there were a capacitor in parallel with this LED. Or this LED doesn't have the same characteristics. Or there is a defective connection. \$\endgroup\$
    – Fredled
    Feb 13 at 9:38

1 Answer 1

2
\$\begingroup\$

All LEDs really have different characteristics, even LEDs from the same reel, even from the same piece of crystal. They can be binned, which does better, but you will still have variations.

If you carefully measure the point at which each individual LED lights up to the same level (place it next to a "standard" LED that is lit by a power supply and a series resistor set to -- do not use a battery, as the voltage may change more than you want over the course of your testing.)

The LED which illuminates last will have a higher forward voltage when it reaches the illumination of your standard.

Think of the water analogy. The LEDs that light up are like sprinklers with their orifices at a lower height, so as you turn the water spigot slowly on, the water finds the easiest path, and comes out of the lower sprinklers. The one up on the hill gets nothing until you increase turn the spigot on even more, and the pressure "reaches" the lone sprinkler up on the hill, where it finally starts emitting water.


I expected this behavior with the LEDs in parallel, and was surprised at the behavior with the LEDs in series, but it happens in both cases. My explanation is that the energy being expended at the LED has to overcome the band-gap in order to emit light, and that energy gap slightly varies from LED to LED.

Full disclosure, I'm still not a full EE, so hopefully this explanation will be corroborated by other answers.


Finally, the Joule thief circuit handles this "problem" somewhat by providing pulses, and where the pressure (voltage) is high enough, the "dimmer" LED still lights up, but more dimly. I've even had LEDs where I could tell that the bonding wire was loose, and the Joule thief made it light up fairly evenly. This should work with PWM as well because PWM uses the maximum voltage and turns it either all-on or all-off, and the high pressure (voltage) of the all-on part should (usually) turn all of the LEDs on.


Commenter Fredled said:

Your explanation are exact but it would be surprising to see such a big difference in behavior from LEDs "exactly the same" like staying completely off while others are all completely on. You could expect, at worse, one led a bit dimmer than the others. – Fredled

At the boundary where it happens, the relatively small differences get greatly magnified. To me, this seems very similar to what happens with a constant current going into a long-tailed pair, making a very small difference in current greatly magnified, making a comparator. If using a two discrete transistors long-tailed pair, any poor matching, or temperature difference, should be very evident. Also accentuating the effect is our own eyesight, when our pupils dilate in the dim light.

Also, I have found this to be common -- the rule, rather than the exception to the rule. Take any dollar store LED light bulb, dim the lights, and light up the circuit board until they just start to illuminate, and you will see variations in brightness, and frequently significantly so.

enter image description here

enter image description here

enter image description here

\$\endgroup\$
10
  • 1
    \$\begingroup\$ Your explanation are exact but it would be surprising to see such a big difference in behaviour from LEDs "exactly the same" like staying completely off while others are all completely on. You could expect, at worse, one led a bit dimmer than the others. \$\endgroup\$
    – Fredled
    Feb 13 at 9:33
  • 1
    \$\begingroup\$ LEDs requiring a minimum amount of current to start emitting light was a thing maybe 50 years ago (due to the state of GaAs semiconductor technology the time), but now that is not a thing, at least in undamaged LEDs. I've seen some strange behaviours in damaged GaAs and especially heterojunction LEDs so maybe something is wrong with that particular unit. For example, there may be literally a short on the tiny die that acts as a resistor in parallel with the junction. \$\endgroup\$ Feb 13 at 19:14
  • 1
    \$\begingroup\$ @MicroservicesOnDDD Be sure to study (inadvertently) self-flashing el-cheapo LEDs (I have several, from failures of cheap 'Harbor Freight' style flashlights, I even rebulbed a couple, just for the heck of it). \$\endgroup\$ Feb 13 at 19:55
  • 1
    \$\begingroup\$ BTW, some LED suppliers have a 'B' grade of LEDs that slightly or completely don't meet specs, something like 2/3 or 3/4 of them are 'usable' (maybe a bit dim or have a higher than spec Vf). These are never offered directly to rich countries (maybe some get sent via dodgy dealers on eBay etc) but they can sell well to poor countries where the labor to sort them out is inexpensive. \$\endgroup\$ Feb 13 at 20:01
  • 1
    \$\begingroup\$ @SpehroPefhany -- I have seen those self-flashing LED's, with the bond wire actually vibrating visibly inside. And thanks about the 'B' grade LED information, which was very interesting. \$\endgroup\$ Feb 13 at 20:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.