Reversible LED

Diver300 · May 14, 2020

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 · May 14, 2020

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 · May 14, 2020

Diver300 said:
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.

Diver300 · May 14, 2020

Nigel Goodwin said:
I would presume it's simply an AC LED?, saves you using a bridge to feed it.

Yes, I think it is. My question is whether they are available. I can't find any at the usual suspects (RS, Farnell, Mouser or Digikey). Maybe I'm not searching for the right term.

AC LED just brings up light bulbs.

ronsimpson · May 14, 2020

Diver300 said:
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 · May 14, 2020

ronsimpson said:
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".

Yes, I've seen those. I mentioned on of those in the first post of this thread.

It's AC input, visible LEDs that are new to me.

gophert · May 14, 2020

Nigel Goodwin said:
I would presume it's simply an AC LED?, saves you using a bridge to feed it.

An AC LED? A PN-junction suddenly becomes an NP-Junction?

Diver300 · May 14, 2020

gophert said:
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 · May 14, 2020

Diver300 said:
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.

Nigel Goodwin · May 15, 2020

gophert said:
An AC LED? A PN-junction suddenly becomes an NP-Junction?

Perhaps you didn't bother reading the original post? - simply reverse parallel connected LED's in one encapsulation.

dr pepper · May 15, 2020

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 · May 15, 2020

This http://lednique.com/leds-with-more-than-two-pins/ page states:

Be aware that bi-colour LEDs are also sold in this package. Some are dual-colour so that reversing the current through them changes the colour. Others may have both LEDs the same colour and this can be useful in AC applications as it can conduct on both cycles of the mains and eliminates the need for a rectifier.

Here are some: http://www.solitronics.com/products/multichip-led/CAT-BASED-LED-PACK01.pdf

gophert · May 15, 2020

Nigel Goodwin said:
Perhaps you didn't bother reading the original post? - simply reverse parallel connected LED's in one encapsulation.

I read it. Your AC comment still made me laugh

Nigel Goodwin · May 15, 2020

gophert said:
I read it. Your AC comment still made me laugh

You find the truth funny?.

gophert · May 15, 2020

Nigel Goodwin said:
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.

Nigel Goodwin said:
~~simply an AC LED~~

Visitor · May 15, 2020

And if it is "simply an AC LED", please post a datasheet.

rjenkinsgb · May 15, 2020

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

gophert · May 15, 2020

rjenkinsgb said:
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.

Reversible LED

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