Challenge: Simple power-line LED

[carbonzit]

I've just built a simple power-line operated LED, and it's been lit for a day now:

**broken link removed**

I built this as a challenge to all the nay-sayers here who claim this can't be done, that it violates all the laws of electronics, even physics, etc., etc. I'm going to see how long it'll remain illuminated, and will report back here periodically.

The circuit is safely enclosed in a clear plastic (thick polystyrene) case.

Disclaimer:

• This project is not UL, CSA, etc., approved.
• The author makes no claims as to the merchantability of this circuit.
• Wear approved safety glasses at all times.
• Observe all recommended safety procedures.
• Yada yada yada ...

 

[tcmtech]

And?

I was building stuff like that years ago and from what I recall most decent more robustly built red, green, or yellow LED's can easily take the same reverse currents as they can forward current the only difference is the reverse breakdown voltage is higher making them less efficient when conducting back wards. Not all LED's can handle it but suprizingly many can.

Odds are it will stay lit for a very long time in my opinion. Maybe a few years even!

 

[carbonzit]

I should note that in testing this circuit, I caused the death of several poor li'l LEDs, of various colors, that just couldn't take the stress. The only ones that seemed to work (with my 0.47µF cap, anyhow) were low-intensity red and green ones.

I was disappointed that the dead LEDs didn't go out in a blaze of glory: just a brief light-up, then nothing. No sparks, fireworks, bright flashes, nothing. No magic smoke released.

They could at least have had the decency that my blown-up transistors had, to make a nice-sounding "Pop!".

 

[DerStrom8]

Carbonzit, NOBODY is saying it can't be done. That is not the issue. The issue is that it really shouldn't be done. No one is arguing that it

violates all the laws of electronics, even physics

Those are your own words, and nobody else has said anything like that. They are only saying that it is definitely not the best way to drive an LED from mains. There are safer and more effective methods out there that should be used instead.

And in case anyone is wondering where the heck this came from, here is the original thread.

Der Strom

 

[MrAl]

Hi,

The circuit is bad for at least two reasons.

1. The current surge during turn on could blow out the LED on the first run.
2. The reverse voltage across the LED could blow the LED on the first run. Many LEDs are only rated for 5v reverse voltage, and this circuit puts a lot of reverse voltage across the LED on every half cycle.

I would have predicted that the LED would burn out right away. It sounds like you got lucky with a few that may have some sort of internal protection.

 

[carbonzit]

"Internal protection"? Where in the world do you get that?

It's just a plain, ordinary LED, yanked out of some phone-answering machine, VCR or something. No "protection". (You can see right through the diode and see plainly that there's only the usual LED die in there.)

Sheesh; everyone's trying to come up with explanations of why this can't possibly work. All of which makes me all the more want to do it!

As I said, I'll be reporting back periodically to say "it's still working: I told you so!".

 

[DerStrom8]

"Internal protection"? Where in the world do you get that?

It's just a plain, ordinary LED, yanked out of some phone-answering machine, VCR or something. No "protection". (You can see right through the diode and see plainly that there's only the usual LED die in there.)

Sheesh; everyone's trying to come up with explanations of why this can't possibly work. All of which makes me all the more want to do it!

As I said, I'll be reporting back periodically to say "it's still working: I told you so!".

CZ, will you stop being so stubborn! Nobody is saying it can't work! They're just saying it shouldn't be done!

 

[ericgibbs]

cz,
You did say that you destroyed a number of LED's while carrying out this trial.

Lets say 4 'died' and 1 is working at the moment, thats only a 20% 'success' rate, which could hardly be called proof of your challenge.

As pointed out by a number of experienced members, "some LED's may survive being operated in the reverse voltage mode, other will not".

The reliable way to connect LED's to a capacitor/mains drive, is to connect either a diode or another LED in reversed parallel with the first LED. I would suggest a LED of the same colour, as it will double the apparent brightness and also reduce the 'flicker' due to pulsing the LED at 25Hz. The overall efficiency of the circuit will also be improved, instead of converting the reverse half cycle into heating the LED, it will be converted to light.

Its also recommended that a series resistor be included in the LED circuit in order to reduce the 'inrush' current thru the LED on the occasions when the circuit is switched on at the peak of the mains cycle.

For the sake of a couple of extra components you would have a 'reliable' project that others could copy.

BTW:
connecting the circuit the correct way, you would still have a number of usable LED's in your 'bits box' rather than 'dead' ones in the bin.

 

[misterT]

I should note that in testing this circuit, I caused the death of several poor li'l LEDs, of various colors, that just couldn't take the stress.

Yes, I would agree that the circuit is a bad design.
Here is a challenge for you: Take all your LEDs and test all of them one by one with the circuit.. are you willing to do that. If not, why?

 

[carbonzit]

I certainly could do that, and with a fair degree of success, if I properly sized the capacitor to provide enough reactance to limit the current through the LED.

I happened to grab a likely-looking size capacito that was in my parts boxr. The LEDs I sacrificed cost me nothing (except time; they were salvaged from electronics left on the curb). (The only one I really missed was a kewl-looking bluish-white one that came from an iPod dock, but I've got more of those.)

The experiment was worth it, if only to say "I told you so!". We'll see how long my red LED burns.

 

[misterT]

We'll see how long my red LED burns.

That is not a proper test for the circuit. Trying 100 leds and counting how many fails is a proper test.
Nice word selection though "burns".. that is exactly what the LED in that circuit is doing.

I certainly could do that

Do it. Then we have a genuinely interesting test.

 

[MrAl]

"Internal protection"? Where in the world do you get that?

It's just a plain, ordinary LED, yanked out of some phone-answering machine, VCR or something. No "protection". (You can see right through the diode and see plainly that there's only the usual LED die in there.)

Sheesh; everyone's trying to come up with explanations of why this can't possibly work. All of which makes me all the more want to do it!

As I said, I'll be reporting back periodically to say "it's still working: I told you so!".

We're both in the world right? So it must have came from somewhere in the world.

"Sheesh", you dont want to listen to the most experienced people here so go ahead and blow out some more LEDs.

Nobody said it will never work, they just said it should not work. There's no transient current limiting and nothing to limit the reverse voltage which kills LEDs.

If the LED does survive, then it probably has something special about it like internal protection of some sort, which could be by design or by coincidence because most LEDs will burn up. I suspect that it has a built in reverse diode.

Also, it doesnt matter how long it runs for. Tomorrow the power could go out, then come back on, and that's the end of your "long life" LED.
If you are UNLUCKY (note i said 'unlucky' not 'lucky') it will burn for several months and you'll get the wrong idea about experiments like this.

Good design techniques do not come up by inventing your own specs for devices (like LEDs). Good design techniques involve knowing what is good practice as well as what the manufacture specs are for a given device. I think you understand the specs, but i dont think you understand what good design practice is.

Of course it's nice to experiment, and i think you will learn something from this if this LED burns out too.

Also, it doesnt help as much to experiment with an unknown part number because we can not know the manufacturers specs. Like i said, they may have made it special for some reason.

After all is said and done, what if this LED burns for hours on end, maybe weeks or months. How will you apply the results of this experiment to future designs?
Are you going to practice hooking up LEDs and blowing them out until you find one that doesnt blow out? (little chuckle there sorry)

 

[alertlink]

If you connect a bridge rectifier in between capacitor and LED, it will lght more brightly and operation will also be flicker free because of doubled frequency. In addition reverse voltage problem automatically be solved. Adding a series resistor with capacitor will limit inrush current which can also damage LED.
Can you randomly connect and disconnect like a loose connection, and see your LED survived. Or connect it in parallell with power transformer and repeat the test to see if back EMF kills the LED?

 

[audioguru]

Most Instructables are designed by people who know NOTHING about electronics. They build only one circuit and I betcha that most copies do not work or do not last long.

 

[Mr RB]

Wait until your fridge clicks on in the middle of a mains cycle, and it makes a 2kv 10uS spike on your AC mains supply...

Capacitive reactance supplies generally include a series resistor along with the reactance cap, to limit the tendency for high power short duration mains spikes to blow the crap out of everything. And my personal preference with LEDs on mains is to always include a cap across the LED itself, for similar reasons.

 

[carbonzit]

It's been illuminated for two whole days now. My 'fridge has clicked on many, many times in that period.

I'll report back periodically.

 

[DerStrom8]

It's been illuminated for two whole days now... I'll report back periodically.

Don't you remember?

That is not a proper test for the circuit. Trying 100 leds and counting how many fails is a proper test.
Nice word selection though "burns".. that is exactly what the LED in that circuit is doing.


Do it. Then we have a genuinely interesting test.

Nobody cares how long it stays lit. It is not convincing anyone that it is any better of a circuit. It's still not a good way to drive an LED. Period.

 

[MrAl]

Hello again,


Little story here...
I rarely connect an LED up backwards in a circuit, but hey it happens

One time i connected a high intensity red LED up backwards to only a 9v battery and resistor. That means it got a reverse voltage of 9v for about three seconds. Most LEDs are rated for only 5v reverse voltage, but when i connected it up normally it did light up to what looked like full brightness.
Putting the LED away after i was done with my tests, i didnt use it again for a few months or so. Some 3 to 6 months later when i went to use the LED for something more permanent, it would not light.

The only conclusion that i could reach was that the reverse voltage damaged the LED but there was some other mechanism that took time to damage it to the point where it did not work at all anymore. I've also seen this behavior in high power bipolar transistors, where an over voltage or over current is considered an age reducer so in a bridge where one transistor has failed often all four transistors are replaced so as to make sure to get rid of any that are what we might call "partially damaged", even though they seem to test good.

One thing Carbon has seemed to prove is that there may be some LEDs out there that can take the stress, at least for a little while. Would be nice to see some photos of the real life circuit and the LED in real life operation. Would be nice to monitor the current surges too when the cap is discharged and the circuit is turned on at the peak of the voltage.
What else we dont know is the ESR of the cap being used, and we also dont know what turns the fridge on and off--electronic circuit or simple relay? An electronic circuit could be detecting the zero cross of the input line.

 

[carbonzit]

Nobody cares how long it stays lit. It is not convincing anyone that it is any better of a circuit. It's still not a good way to drive an LED. Period.

Why do you seem offended by this? Is it really that egregious an infraction against the Electronics Police?

 

[carbonzit]

What else we dont know is the ESR of the cap being used, and we also dont know what turns the fridge on and off--electronic circuit or simple relay? An electronic circuit could be detecting the zero cross of the input line.

Nope; older fridge, mechanical thermal switch. (I know, as I had to work on the damn thing, fan stopped working.)