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[on:]

I've used some generic blue led modules to build a big clock with 7 segment displays.
This modules have 5 piranha leds and resistors in a weatherproof enclosure (a brick of molten plastic).
Modules look like this:

They are rated for 12V, and a real datasheet is not available.

After a year and a half of constant use, I've noticed that some of the leds are burnt, and most are not as bright as the new ones.

Modules are wired to 12V, switched with darlingtons in a ULN2003.

The display uses a sheet of white hight impact polyestyrene as a diffuser, and I've noticed that it turned light yellow where the led light it.

So my first suspicion is that the modules are dissipating heat, and this is shorting their life. Or is it the UV light that yellowed the plastic?

Modules are not warm to the touch. And there is no sign of heat on the MDF piece they are glued with hot glue. The sign is place inside, and summer temperatures can go up to 32 degrees celcius.

Does anyone have experience with this kind of modules an their expected life? Would SMD LED strips last longer?

[Reply #1 on:]

I've used some generic blue led modules to build a big clock with 7 segment displays.
This modules have 5 piranha leds and resistors in a weatherproof enclosure (a brick of molten plastic).
Modules look like this:

They are rated for 12V, and a real datasheet is not available.

After a year and a half of constant use, I've noticed that some of the leds are burnt, and most are not as bright as the new ones.

Without a circuit diagram it's hard to say what's happening.

Usually dead/dim LEDs are the result of people not using the right resistors to regulate the current. They might be overdriven to make cheap LEDs look brighter than they really are.

Some designers seem to think it doesn't matter if LEDs are abused this way, that if an LED survives for a week it will survive forever. Even some members of this hallowed forum seem to think this. The reality is that damage from overcurrent can be very gradual resulting in products like this.

The display uses a sheet of white hight impact polyestyrene as a diffuser, and I've noticed that it turned light yellow where the led light it.

So my first suspicion is that the modules are dissipating heat, and this is shorting their life. Or is it the UV light that yellowed the plastic?

Modules are not warm to the touch. And there is no sign of heat on the MDF piece they are glued with hot glue. The sign is place inside, and summer temperatures can go up to 32 degrees celcius.

If the yellowing is only where the LEDs are then the LEDs are probably producing a small amount of UV light. "Blue" isn't very far from UV in the electromagnetic spectrum. Even a small amount of UV adds up over time.

Would SMD LED strips last longer?

SMD LEDs are no more resistant to overcurrent than regular ones are. What you need is better current regulation, not a different type of LED.

[Reply #2 on:]

This modules include the resistors inside (you can see them inside the melted plastic brick).

The circuit is easy:
- led module anode to +12V
- led module cathode to one of the ULN2003 darlingtons, that switches the path to ground.

Thats it, no added resistors.

First experiment will add PWM to the +12V, because adding resistors would be a pain.

[Reply #3 on:]

This modules include the resistors inside (you can see them inside the melted plastic brick).

My crystal ball is broken so I can't see any brick from here. What 'brick' are we referring to, and, much more importantly, why is it melted?

Can you see any resistor values? Can you measure the current flowing through a module?

It's quite hard to figure out the problem with no information.

[Reply #4 on:]

It could be that your 12V is a bit high.

[Reply #5 on:]

Pardon my awful description that led to misunderstandings.

The led modules look like a brick of plastic, where the leds and resistors are placed, and all is covered with more melted plastic. You can see inside the plastic some conventional THT resistors. All this info was in the effort to describe the product. Here is a better picture of some similar modules that have only 3 leds:

The modules look ok from the outside, though the leds are no longer bright enough and some of them do no longer work.

12V come directly from a commercial switching power source with 2.5 amps capacity (Can this be blamed?). Each led module consumes 40mA, and I have 42 modules.

Thanks for your help guys. looks like I shoudn't have trusted the dealer who said that these where 12V modules, and lower it a little bit. I'll try dimming with PWM on the high side.

[Reply #6 on:]

12V come directly from a commercial switching power source with 2.5 amps capacity (Can this be blamed?).

Measure the voltage under load...?

Each led module consumes 40mA, and I have 42 modules.

Five LEDs could be arranged as a string of 2 and a string of 3 in parallel to work from 12V. 20mA for each string adds up to 40mA. That's just a guess though.

Do the dead LEDs form any kind of pattern in the modules? Do they die in similar groups of two or three LEDs?

[Reply #7 on:]

Those LED's are not genuine Pirahna. I'm pretty sure those "Pirahna" LED's are very cheap, and that's what you should expect from a no-name product form china. The LED dies inside are under-rated and will not last very long. Nothing to do about that.

// Per.

[Reply #8 on:]

Thanks fungus. In most of the cases a pair of leds dies, and the other 3 survive.
Zapro: I wasn't even aware piranha was a brand 
You are right, I was a victim of "EL CHEAPO" Leds Made in China.
Problem is....everything I can buy arround here are from the same origin.

[Reply #9 on:]

Thanks fungus. In most of the cases a pair of leds dies, and the other 3 survive.

I think we're getting closer to the truth.

Can you see the resistor color codes through the plastic? Are they the same value or different?

You are right, I was a victim of "EL CHEAPO" Leds Made in China.
Problem is....everything I can buy arround here are from the same origin.

Does your power supply have a voltage adjustment screw? Try turning it down a bit

[Reply #10 on:]

Resistors looks like these:
One is brown, yellow, black, black, brown (140 ohm)
Other is orange, black, black, black, brown (300 ohm)


They are not big.