how to enable mains to 5v to power 3 1.5v IR LEDS

[sterion70]

Hi all, my first post in my new hobby. How to wire up and safeguard a Led circuit containing 3 IR LEDS Using 5v from a plugin 230volt wall plug transformer? Many thanks for reading my post.

 

[Jeff7]

What's the power source specifically? (Brand and model#.)
Some supplies are well-regulated.
Others are not regulated, and might even put out 5V AC. LEDs run on DC only. They can tolerate a little bit of reverse-polarity voltage, but they're meant to be run on steady DC. Some diode manufacturers will say that up to a certain voltage is fine in reverse; others will just say "Don't reverse the voltage on these."


If it's regulated, one option is to just use a resistor in series. IR LEDs usually have a lower forward voltage than other visible-light LEDs. You might be looking at around 1.5V per LED, depending on the model. (Which IR LED are you using?)

If that's the case:
5volts - (3*1.5volts) = 0.5V.
If you want to drive them at...let's say 10mA.
V=IR
R=V/I
R = 0.5/0.010
R = 50 ohms.

However, with a simple resistor, the amount of current going through the LED string can change slightly. If it's sized properly though, the change shouldn't be much concern, especially with a string of only 3 LEDs.

A CL2 is another option.
It will automatically adjust to provide a constant 20mA through the LEDs, acting like a dynamic "smart" resistor.

When an LED turns on and the emitter starts to heat up, the forward voltage goes down: It doesn't take as much voltage to push current through it.
So at room temperature, it might take 1.5V to push 20mA through the LED.
But when the die gets hot, it might take only 1.3V to push 20mA through the LED. If you're still providing it with 1.5V, you will get more current going through it. That in turn produces more heat. That's why it's not a good idea to run LEDs without a resistor: Thermal runaway, which can destroy the LED.

A resistor is usually fine though. Something like the CL2 is used if you don't know the process for sizing resistors, or if you've got a poorly-regulated power supply.

If you have a power source that's only a step-down transformer and it's putting out 5V AC, then you're going to need more parts: Diodes, maybe a bridge rectifier, and maybe some capacitors if you don't want it to flicker.

 

[MrTeal]

Actually the reverse voltage on pretty much every LED is quite a bit higher than the forward voltage; it's pretty tough to torch an LED by reverse biasing it. That's why cheap LED strings are so inexpensive and have annoying flicker; they are essentially a whole bunch of low voltage LEDs connected in series with a dropping resistor connected directly to the AC mains. During the positive cycle they conduct, and during the negative cycle they're reversed biased and off, giving the 60Hz flicker you can see.

Same with these, a 5Vac transformer and 50 ohm resistor wouldn't damage anything, they would just flicker and the average brightness would be quite low. Whether that's acceptable depends what the application is.

@OP, what transformer, LEDs, and can you describe the application a bit?

 

[Jeff7]

Actually the reverse voltage on pretty much every LED is quite a bit higher than the forward voltage; it's pretty tough to torch an LED by reverse biasing it. That's why cheap LED strings are so inexpensive and have annoying flicker; they are essentially a whole bunch of low voltage LEDs connected in series with a dropping resistor connected directly to the AC mains. During the positive cycle they conduct, and during the negative cycle they're reversed biased and off, giving the 60Hz flicker you can see.

Same with these, a 5Vac transformer and 50 ohm resistor wouldn't damage anything, they would just flicker and the average brightness would be quite low. Whether that's acceptable depends what the application is.

@OP, what transformer, LEDs, and can you describe the application a bit?

The datasheets I see usually specify 5V reverse-bias voltage in the absolute maximum ratings section.
The Luxeon Rebel was the one I thought of that doesn't give a reverse voltage rating at all. "...not designed to be driven in reverse bias" says the datasheet.
Will they still act like diodes? Yup. Will the manufacturer do anything for you if you do damage a Rebel by reverse-biasing it? Nope.


(I hate those flickering LED Christmas lights. The strobing effect is awful to look at.)

Yes, much depends on the application.
Do you want it to last a really long time?
Critical operation?
Serviceability/replaceability?
Temperature range?

I guess I tend to be conservative about LEDs out of habit. I'm acclimated to designing things for an industrial temperature range and an extremely low failure rate. Absolute maximum rating of 1 amp at room temperture? Ok, that means I'll run it at 0.25A. (Numbers pulled out of nowhere.) Things where I work get derated to favor reliability and long life, rather than milking out every last micro-cent like you'd see in high-volume consumer-grade products.

 

[MrTeal]

The datasheets I see usually specify 5V reverse-bias voltage in the absolute maximum ratings section.
The Luxeon Rebel was the one I thought of that doesn't give a reverse voltage rating at all. "...not designed to be driven in reverse bias" says the datasheet.
Will they still act like diodes? Yup. Will the manufacturer do anything for you if you do damage a Rebel by reverse-biasing it? Nope.

(I hate those flickering LED Christmas lights. The strobing effect is awful to look at.)

Yes, much depends on the application.
Do you want it to last a really long time?
Critical operation?
Serviceability/replaceability?
Temperature range?

I guess I tend to be conservative about LEDs out of habit. I'm acclimated to designing things for an industrial temperature range and an extremely low failure rate. Absolute maximum rating of 1 amp at room temperture? Ok, that means I'll run it at 0.25A. (Numbers pulled out of nowhere.) Things where I work get derated to favor reliability and long life, rather than milking out every last micro-cent like you'd see in high-volume consumer-grade products.

Yeah, 5V or so is pretty common for an IR LED, or most standard 1-1.5V LEDs for that matter. Reverse voltage ratings stack the same way forward voltage ratings do though, so a string of three has a reverse voltage rating of 15V. A standard LED will handle forward voltage rating in reverse essentially indefinitely.

Interesting on the Luxeon LEDs though, that seems like a pretty big omission to leave off a datasheet. There obviously is going to be a reverse rating, but I wonder if they might have more diffused on there than just an LED that wouldn't handle reverse bias? Some of the high power modules have interesting construction. I have a sleeve of 8000lm 40V Cree XLamp's in the lab and they will handle being reverse biased just fine, though they appear to just be a large series string of LEDs. I might have to buy some of the Luxeon ones to play with and see.

 

[Jeff7]

Yeah, 5V or so is pretty common for an IR LED, or most standard 1-1.5V LEDs for that matter. Reverse voltage ratings stack the same way forward voltage ratings do though, so a string of three has a reverse voltage rating of 15V. A standard LED will handle forward voltage rating in reverse essentially indefinitely.

Interesting on the Luxeon LEDs though, that seems like a pretty big omission to leave off a datasheet. There obviously is going to be a reverse rating, but I wonder if they might have more diffused on there than just an LED that wouldn't handle reverse bias?

Maybe not an omission, so much as "Please just don't do this."
That's the exact wording on the datasheet: "LUXEON Rebel Color Portfolio LEDs are not designed to be driven in reverse bias."

Maybe they had intermittent issues and it just wasn't worth their time for the support that arose? Maybe the reverse leakage was enough to cause other issues like glowing LEDs in a string?
Some of their devices also have built-in zeners for ESD protection. That might play into it too. I think all of them with an InGaN die (blue/green/white/PC-Amber and lime) have the ESD protection. The red and standard amber colors use AlInGaP chemistry dies, and do not.


A Cree document says that as LEDs age, especially when driven at high current, the reverse-leakage current can change.

Osram: "Driving a LED in reverse direction is mostly forbidden, first of all as the LED is not designed for latched operation and because of the inherent ESD protection device."


So...beats me. As far as I knew, yeah, they're diodes, but they're primarily built to emit light. But I've also got the luxury of not having to design for the consumer market.

Some of the high power modules have interesting construction. I have a sleeve of 8000lm 40V Cree XLamp's in the lab and they will handle being reverse biased just fine, though they appear to just be a large series string of LEDs. I might have to buy some of the Luxeon ones to play with and see.

Those absurd-brightness modules are fun.
I've got some Philips CoB modules. Looks like they're on Gen3 now. I probably have Gen1, maybe 2, in the 1208 or 1211 package, I don't remember which.
They're also clusters of LED dies in series/parallel arrangements beneath a layer of phosphor: The dies are in series strings of 10 or 11, and then a few strings are paralleled, probably factory-matched for close forward voltages per string to keep them balanced.


Edit: Interesting, the XLAMP CXA3050 datasheet only says "Reverse current: 0.1mA."
There's no reverse voltage rating.
*shrug*