What actually kills LEDs .... ?

[Nickhlx]

People say "15 volts kill LEDs" and "LEDs must have a regulated supply not exceeding 14 volts" or whatever...

 

Whenever I have used a "standalone" LED, the generally max continuous current is around 25 mA, so a series resistor has to be chosen to limit the current to no more than this - say 15 mA for this example. I imagine this rating of 25 mA is the max current it can pass to dissipate the heat generated and limit its temperature to under a safe level. ( is that correct ?)

 

If the working voltage was say 14 volts, the LED took say 2 volts, the the resistor would need to drop the other 12 volts, which at 15 mA gives 800 ohms.

 

If the supply volts went up to say 16 volts, then the LED would still be dropping its 2 volts and the 800 ohm resistor would have 14 volts across it and around 17.5mA would be flowing.. Even at 18 volts, the current flowing is only 20 mA and not until the supply voltage is up to 22 volts is the rated maximum current reached.

 

Even then it is a heat / time thing, so a spike of microseconds or even milliseconds is not going to heat the device up much ...

 

With the devices that have a number of LEDs and a controlling chip within, I can see that the electronics will have a limit above which, even for the short duration of the spike, will be killed by over voltage.

 

So, am I correct in assuming that it is these "electronically controlled" LEDs that are the ones susceptible to spikes ?

 

And is it the heating effect in individual LEDs that needs to be controlled / limited rather than the voltage of a spike ?

 

Nick

[blueprince]

This is complicated, essentially, a higher voltage will create a conductive bridge which destroys the light emitting properties of the diode, you will weld the diode shut. see http://www.chemistry.wustl.edu/~edudev/Lab...talBonding.html

[Keeping Up]

So, am I correct in assuming that it is these "electronically controlled" LEDs that are the ones susceptible to spikes ?

 

And is it the heating effect in individual LEDs that needs to be controlled / limited rather than the voltage of a spike ?

 

Nick

 

I would have said this analysis was fundamentally correct. However I would add that a "reverse voltage" spike could also destroy an individual diode, even one that has no electronics.

 

This is complicated, essentially, a higher voltage will create a conductive bridge which destroys the light emitting properties of the diode, you will weld the diode shut. see http://www.chemistry.wustl.edu/~edudev/Lab...talBonding.html

 

I would guess that this "conductive bridge" would only be created by a reverse voltage?

[Nickhlx]

I would have said this analysis was fundamentally correct. However I would add that a "reverse voltage" spike could also destroy an individual diode, even one that has no electronics.

 

 

 

I would guess that this "conductive bridge" would only be created by a reverse voltage?

 

Can a "reverse voltage" spike be catered for by a simple ( meaty) diode acroos the supply so it shunts the spike safely past the LED ?

 

Nick

Edited April 11, 2009 by Nickhlx

[Ex- Member]

I was told by a small british manufacturer of LED's that anything over 13 volts would be detrimental and 15 volts would almost certainly cause single LED's to blow. I was also told this by a U.K ebay seller.

 

The first batch of 20 we bought 4 years ago started blowing when we started cruising, ie engine running. The company who made the bulbs said they wouldn't have sold them if they knew they were for a boat or RV, unfortunately we bought through a dealer not direct from this munufacturer so wasn't given this info that they needed a regulated supply or had to be run through transformers.

 

I've now regulated all supply to the led's and had none blow. I would regulate them as a matter of course IMO unless of course they state somewhere they can handle excess voltages.

 

I used 3 of these regulators encased in plastic enclosure, very easy to wire in. You can do it cheaper if you want to solder.

 

http://www.reuk.co.uk/buy-12-VOLT-REGULATOR.htm

[Nickhlx]

I was told by a small british manufacturer of LED's that anything over 13 volts would be detrimental and 15 volts would almost certainly cause single LED's to blow. I was also told this by a U.K ebay seller.

 

The first batch of 20 we bought 4 years ago started blowing when we started cruising, ie engine running. The company who made the bulbs said they wouldn't have sold them if they knew they were for a boat or RV, unfortunately we bought through a dealer not direct from this munufacturer so wasn't given this info that they needed a regulated supply or had to be run through transformers.

 

I've now regulated all supply to the led's and had none blow. I would regulate them as a matter of course IMO unless of course they state somewhere they can handle excess voltages.

 

I used 3 of these regulators encased in plastic enclosure, very easy to wire in. You can do it cheaper if you want to solder.

 

http://www.reuk.co.uk/buy-12-VOLT-REGULATOR.htm

 

 

Hi,

 

Thanks for the input - This company has some really useful components and circuit ideas

 

I am beginning to believe that suppliers are running the single LEDs at their maximum rating and a small over-voltage, with no margin of sagety, is tipping them over the edge...

 

I have built a couple of LM338 based regulators which give a minimum of 5 amps output current, and possibly 7 amps... and use this to run e.g. the TV ( 3 amps) other electronics ( radio etc ) - they seem to have quite a low minimum input/output differential voltage drop, and give about 3 amps regulated at 12.0 volts with as little as 13.8 volts in... the 12V items can genearally work down to about 11 volts or a touch less, so they seem a good bit of extra protection.

Heat dissipation is minimal as the in/out differential is usually less than 2 volts...

 

Nick

[blueprince]

All leds should be on a regulated circuit, they are far less tolerant of abuse than regular 'resistive' bulbs, a good source for components is PSU's off old computers, as these often supply a variety of voltages,the circuits are already built, and they are really cheap/free. I get around the voltage drop along the length of the boat by having a dedicated 110v supply to the PSU transformers,(you can get the kind that run on 240v/110v.) This seems to work well so far.

[Canalwatcher]

All leds should be on a regulated circuit, they are far less tolerant of abuse than regular 'resistive' bulbs, a good source for components is PSU's off old computers, as these often supply a variety of voltages,the circuits are already built, and they are really cheap/free. I get around the voltage drop along the length of the boat by having a dedicated 110v supply to the PSU transformers,(you can get the kind that run on 240v/110v.) This seems to work well so far.

 

Our G4 fitting LEDs are individually regulated and were sold as suitable for 8-30v. So far none have blown

 

Mick

[ditchcrawler]

All leds should be on a regulated circuit, they are far less tolerant of abuse than regular 'resistive' bulbs, a good source for components is PSU's off old computers, as these often supply a variety of voltages,the circuits are already built, and they are really cheap/free. I get around the voltage drop along the length of the boat by having a dedicated 110v supply to the PSU transformers,(you can get the kind that run on 240v/110v.) This seems to work well so far.

Well the ones we have run on our 24 volt system, the charging is controlled by an Adverc so gets up to 28+ volts and today we had all the lights on going through Blisworth tunnel again. The are plugged straight into the lighting circuit and have been for some time.

[kayDee]

NEVER feed them Levi Roots' Reggae Reggae sauce, it kills them instantly...

I've also heard that it then becomes untraceable in their systems so a ruling of 'death due to undetermined cause' would no doubt be issued...

Kay

x

[Nickhlx]

NEVER feed them Levi Roots' Reggae Reggae sauce, it kills them instantly...

I've also heard that it then becomes untraceable in their systems so a ruling of 'death due to undetermined cause' would no doubt be issued...

Kay

x

 

 

Go on then, what did you do... ?

 

Nick

Edited April 12, 2009 by Nickhlx