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Overdriving LEDs beyond their specs shortens their lifespan?

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R2-D2

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I used a 12v DC wall adapter to power 3 LEDs as a light source for my project.
I measured the output of this adapter power supply plugged into the wall and its output was measured at 17.4v DC! :eek: I believe it's because there's much more than 120v AC coming out of the wall socket for it have that high of a DC output - or maybe it's just very cheap. (Wall adapter input is rated at 120v AC.) I plan to use a regulated 12v DC supply instead.

Well, I powered three of these LEDs with this juiced up wall adapter and may have overdriven them in the circuit. (A 100Ω resistor is connected in series.) LEDs are rated at 3.0 - 3.2 volts at 24mA max (75mA peak) With the 17.4 volts from this wall adapter that powered these LEDs, I measured about a 3.35 voltage drop across each one at about 56mA - beyond their rated specs. My question is, could I have done some damage to them or drastically shorten their lifespan? :confused: So far they work fine but was just curious to know.
 

MikeMl

Well-Known Member
Most Helpful Member
If they still seem as bright as new ones you haven't hurt them. There is nothing wrong with using the 17V wall-wart as long as you recalculate the correct current-limiting resistor. Most wall-warts are not intrinsically well voltage-regulated; i.e. they put out a much higher voltage when unloaded or lightly loaded compared to when they are delivering their rated load current. Also, check what type of rectification your wall wart has. Some have half-wave rectification; some have full-wave rectification; some have a built-in filter capacitor and most dont.
 

Sceadwian

Banned
17.4V from an unregulated DC wall adapter is not that unusual. 56ma is pushing it for any length of time but you probably haven't permanently damaged them. Generally you're okay if they still put out light. Compare them with a brand new LED at the same current if they're substantially dimmer then it's probably damaged.
 

R2-D2

Member
They still seem to work just fine - just as bright as before. I might just substitute a higher value resistor instead. It makes sense that I'm getting a higher voltage output from the wall adapter because obviously powering only three LEDs with it is a very light load.
Thanks for the info!
 

Thunderchild

New Member
any unregulated wall wart will output 1.5 X the nominal voltage, this is because the mains supply is a sinusoidal waveform and 120 volts is the equivalent voltage if it was a DC supply in reality it will peak at 1.414 X 120 V and so reflect this on the lower side of the transformer also, with no or little load the capacitor in the adapter will charge to the peak voltage of 12 X 1.414 - diode bridge voltage drop, you really need a regulated supply
 

Sceadwian

Banned
If you calculate the resistor for the voltage you measure on the power supply when the LED's are running at their current power and just round up you should be fine. With good air flow and low duty use 56ma might not even be too much.
 

Willbe

New Member
Lifetime halves for each ~10 C rise so if the LEDs would have lasted 40,000 hrs at 40C they'll last 20,000 at 50C and 10,000 at 60C.
I suppose the LED unpowered [shelf life] is ~infinite.
 
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audioguru

Well-Known Member
Most Helpful Member
any unregulated wall wart will output 1.5 X the nominal voltage, this is because the mains supply is a sinusoidal waveform and 120 volts is the equivalent voltage if it was a DC supply in reality it will peak at 1.414 X 120 V and so reflect this on the lower side of the transformer also, with no or little load the capacitor in the adapter will charge to the peak voltage of 12 X 1.414 - diode bridge voltage drop, you really need a regulated supply
You are correct and you are very wrong.
The filter capacitor charges to the peak voltage (x1.414 times) minus the rectifiers voltage drop, loaded or unloaded.
But the cheap transformer uses cheap thin wire that has a high resistance that reduces the voltage when there is a load, so they use a transformer with a high enough voltage that the voltage is correct at the rated load current. Then the voltage is too high without a load.
 

Thunderchild

New Member
you mean I am correct AND there is more bad news with cheap unregulated power supplies, I stated the predictable theory
 

stuartiannaylor

New Member
Running LEDs Hot

I glanced at what everybody else posted as one thing i have come across is that Hi-Power leds can be very sensitive to thermal runaway.

The package can seem warm but instant core temperatures without thermal consideration can kill them.

I don't really know your setup but if the LED dimmed and died is characteristic of thermal runaway.

Stuart
 
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