If you put too little voltage across an LED, nothing happens, as I would expect. However, I've seen a demo where a red LED is powered with a 1.5V cell, and it lights. I assumee that red LEDs are 1.9v or so, and was wondering whether there is a range in which they can glow a little bit, and if so, what accounts for it.
3 Answers
I'm betting what you saw was a joule thief.
However, small amounts of current do flow below the forward voltage (see image) of an LED so perhaps it could very dimly light before hitting Vf.
-
1\$\begingroup\$ That Joule Thief link wasn't very useful, I found thejoulethief.com/joule.html (Schematic images here and here, Eagle files also on that page) much more helpful. There's a Wikipedia article, but that isn't particularly good. \$\endgroup\$ Jul 25, 2011 at 20:23
-
\$\begingroup\$ @Kevin It was the most applicable one for a 1.5V AA battery I thought... perhaps not the device as a whole \$\endgroup\$ Jul 25, 2011 at 20:44
-
\$\begingroup\$ @Kevin Funny how the kit has two \$2.2k \Omega\$ resistors and one \$1k \Omega\$, but the schematic has the opposite... \$\endgroup\$ Jul 26, 2011 at 17:00
-
1\$\begingroup\$ No, I meant you could see a AA battery wired to an LED, no other circuitry involved, but I like the Joule thief so much I'm giving this answer the credit \$\endgroup\$– DovApr 2, 2012 at 15:03
-
1\$\begingroup\$ Some LEDs do very definitely glow quite noticeably in dim environments at voltages well below Vf; the real "culprit" is the dynamic range of the human eye. The amount of light required for a glowing object to be visble in a dark room is often well below 1/10,000 of the amount required to be noticeable in bright sunlight, so it doesn't take much current at all to make things glow when they shouldn't. \$\endgroup\$– supercatFeb 11, 2016 at 17:36
Some LEDs have a forward voltage of 1.5V or less at reasonable currents. Look at the datasheet of some red LEDs for example. The forward voltage will also be lower at higher temperature.
-
\$\begingroup\$ This is almost certainly what was happening to me, but the Joule thief posting was highly useful! \$\endgroup\$– DovApr 2, 2012 at 15:02
The reason, that you can use diode below saturation voltage (knee voltage) is that the current value depends on voltage by Shockley diode equation. See wiki