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Reversible LED

Diver300

Well-Known Member
I bought a 12 V relay, and found it has an LED indicator for the coil voltage. (This one if anyone is interested).

I noticed that the 3 mm LED is in fact two LEDs in one package, in inverse parallel. The centre of illumination is slightly off-centre, nearer one pin, and then nearer the other pin when the polarity is reversed. The tops of the leads are the same size, when in normal LEDs one is larger, and is usually the negative.

It makes perfect sense in this application. A bridge rectifier could be used, but that would be an extra component to fit.

I'd not come across LEDs like that before. I've seen the red/green ones with different colours in each direction, but this is red both ways. I've also seen opto isolators with two input LEDs ( TLP 182 is an example ) but wondered if the visible LEDs were commonly available. What is it that I should search for to find one?
 

schmitt trigger

Well-Known Member
That is interesting.

I have also seen and used inverse-parallel LEDs in a single package, but these tend to be different colors, red/green a common pair.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I bought a 12 V relay, and found it has an LED indicator for the coil voltage. (This one if anyone is interested).

I noticed that the 3 mm LED is in fact two LEDs in one package, in inverse parallel. The centre of illumination is slightly off-centre, nearer one pin, and then nearer the other pin when the polarity is reversed. The tops of the leads are the same size, when in normal LEDs one is larger, and is usually the negative.

It makes perfect sense in this application. A bridge rectifier could be used, but that would be an extra component to fit.

I'd not come across LEDs like that before. I've seen the red/green ones with different colours in each direction, but this is red both ways. I've also seen opto isolators with two input LEDs ( TLP 182 is an example ) but wondered if the visible LEDs were commonly available. What is it that I should search for to find one?
I would presume it's simply an AC LED?, saves you using a bridge to feed it.
 

ronsimpson

Well-Known Member
Most Helpful Member
I can't find any at the usual suspects
Digikey.com has over a 1000 part numbers for "AC/DC input isolators" with transistor or Darlington outputs. Not counting those with TRIAC or digital outputs.
Search for "AC input".
 

Diver300

Well-Known Member
An AC LED? A PN-junction suddenly becomes an NP-Junction?
AC input opto isolators have two input LEDs, in reverse parallel, so there are two separate PN junctions, only one of which is forward biased at any one time.

The red indicator that I saw in the relay is actually two LEDs in one package, also in reverse parallel.

The acronym "LED" for Light Emitting Diode" isn't strictly accurate here, or in a bi-colour LED, or in many other light emitters that contain multiple light emitting diodes, protection devices or control circuits. However "LED" has become the term used for many devices that contain a more that just one light emitting diode.
 

gophert

Well-Known Member
Most Helpful Member
However "LED" has become the term used for many devices that contain a more that just one light emitting diode.
No, it hasn't.
- An LED is an LED,
- a bicolor LED is a bicolor LED
- an LED bulb is an LED bulb
- an LED display is an LED display

People who call everything an LED are as ignorant or lazy as people who cannot differentiate silicon and silicone or refer to every carbonated beverage a "coke".

Don't be lazy and don't encourage other people to be lazy.
 

dr pepper

Well-Known Member
Most Helpful Member
I've never seen those, I probably have some relays with them in though, will pay attention next time I see or use one.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
Last edited:

gophert

Well-Known Member
Most Helpful Member
You find the truth funny?.
I do if your truth involves calling a pair of antiparallel diodes an "an AC" diode. There, it just made me laugh again.

Future readers of this thread, please note, "An LED" is not an AC device. A pair of LEDs can be configured but, as suggested in post above, a single LED cannot be an AC device.

simply an AC LED
 

gophert

Well-Known Member
Most Helpful Member
I've not seen LEDs like that, but "AC input" type optocouplers are a fairly common item.
They just have two LEDs in inverse parallel, so it does not seem an unreasonable description for a LED with inverse parallel elements.

eg. https://uk.rs-online.com/web/p/optocoupler-ics/9034217
"AC input" vs "AC LED" are completely different. And even an AC Input optocoupler uses "LEDs", not "an LED" as the laugh-inducing post above stated.
 

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