David, Indeed, everything has a limit, and I did not touch on that. An obvious (and significant) oversight. Indeed, many LEDs have a reverse voltage limit of around 5V, and as such my statements with respect to 5V logic circuits holds true. But that is not the end of the story either.
Some high-end LEDs actually have 2 back to back zener diodes (parallel with the LED die) to protect them against reverse power. For instance, the Everlight 334-15/T1C1-4WYA white LED also has a 5V reverse voltage limit, but allows a 100mA reverse zener current. That means that if the series resistor limits the forward current to its forward limit of 20 to 30mA, the reverse installation would be able to withstand whatever voltage is used in the "normal" forward position. Most LEDs do not have that kind of protection (although the designer could build in an external protection zener) and will not tolerate much more than 10uA or so in reverse. LEDs were not designed for reverse operation outside the 5V logic levels, and as such vary a lot as you already pointed out. For example, the Vishay TLUR6400 has a typical (guaranteed) reverse max of 6V, with a max value of 15V. Thanks for pointing that out. Bill -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of David Forbes Sent: Thursday, March 24, 2016 1:54 PM To: [email protected] Subject: Re: [neonixie-l] Re: Off topic, but it has an orange display.. Heathkit ID-1590 anemometer Bill, Thanks for the long rant, but typical LEDs have an absolute maximum reverse voltage rating of 5V. Exceeding this is not guaranteed to destroy the LED, but The LED is not guaranteed to work properly after such use. It is irresponsible to recommend that people ignore the datasheet ratings of components. On 3/24/16 10:35 AM, Bill van Dijk wrote: >>From time to time I read dire warnings about the death of a LED by reverse >>polarity. It does not happen. A LED is a diode, and is used normally in >>forward bias, with a resistor (or some other circuit) to limit the current. >>If the LED is reversed, it simply blocks the current, and precisely nothing >>happens. You can make a cool bridge rectifier with 4 LEDs, but although >>technically OK, that is not very practical in most applications due to their >>high forward drop and low current tolerance. Two wire, bi-color LEDS use this >>principle where two different color dies are connected back to back, >>depending on the polarity, one color or the other lights. A red and green LED >>back to back make therefore a great polarity indicator. Interestingly, if fed >>an AC current, a third color can be produced, i.e. a green / red combination >>will shine yellow on AC. RGB LEDs have 3 dies, but can display many colors by >>lighting one or more dies at varying intensity. This is done through PWM, >>somethin g a LED i s very much suited for. Dimming of LEDs, as well as multiplexing of LEDs is predicated on PWM. Many of the newer Ultra-bright LEDs will still light pleasantly on 5V with series resistors as high as 1.5 kOhm. > > The series diode does nothing to protect the LED, but does indeed drop the > voltage slightly due to its forward drop. From an efficiency perspective it > makes no difference, the diode also dissipates the drop Voltage times the > current in heat, just as the resistor does. Since the circuit has a diode it > is probably fine to leave it there, but adding one in a new design has no use > at all. > > -- David Forbes, Tucson AZ -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/56F429CC.2040003%40dakotacom.net. For more options, visit https://groups.google.com/d/optout. --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/007701d1860b%24f4d81860%24de884920%24%40com. For more options, visit https://groups.google.com/d/optout.
