LED gurus, help please

[Streak]

The attached picture shows an LED panel consisting of 24 leds.

They are connected as 12 parallel pairs of 2 in series designed to run directly off a 6v SLA battery. With a 6v SLA battery, total current draw is only about 160mA where for 20mA per led it should be nearer to 240mA to get max. brightness.

We received a new batch of 24 led panels where for a fixed voltage (bench power supply) the panel is dimmer and draws more current , 220mA.

Can I conclude that the LED's in the second panel have a lower Vf causing the higher current draw but may have a lower Mcd (6000 vs 11000 Mcd)rating causing the lower brightness?

These panels are used in an emergency light (charges battery when there is mains power and comes on when the power fails).

Would 11000Mcd leds with slightly higher Vf be the best option for this application to get best brightness with longest run time?

Performance of the 160mA panels is great with ample light and brilliant run time on a 4Ah SLA battery.

Looking forward to your comments.

Joel

 

[spencer]

This would be better suited in the LED section.

 

[Streak]

I initially I thought soo too, but it is an electronics related LED question.

Can one of the Mods. move it there or do I cross post?

 

[spencer]

Its against the rules to cross post so you have to wait for the mods to move it.

 

[Streak]

OK, but in the mean time comments would be appreciated.

 

[scott.cr]

They are connected as 12 parallel pairs of 2 in series designed to run directly off a 6v SLA battery. With a 6v SLA battery, total current draw is only about 160mA where for 20mA per led it should be nearer to 240mA to get max. brightness.

As it's connected now, the LEDs are getting 3.0V assuming no voltage sag. In reality they are probably getting a fair bit less voltage.

The new panel has twice as many LEDs but only drawing ~38% more current, if the LEDs are exactly the same then the reason why the new panel is dimmer is due to lower current per LED.

How is current to the panels limited? Resistor? If so, you can simply connect the panels to your bench supply, set the voltage to 6.0 (perhaps slightly less to factor in voltage sag) and slowly raise the current until the panel is at the brightness level you want. Then select a new limiting resistor value based on the current being delivered.

You'll notice when you ramp up the current a lot, brightness only goes up a little. It shouldn't be hard to find your happy middle-ground.

 

[Streak]

Hi Scott, thanks for the reply.
The new panel has the same no. of leds as the old one (12x2=24!!!). I was trying to find out why for a given voltage the new panel was dimmer and drew more current.

There is no current limiting resistor, as mentioned, it is a direct connection to the battery.

The 2 led's in series make a nominal total Vf of 6.6v allwing a direct drive system voltage of 6V (without the need for a current limiting resistor) x 12 lots of these in parallel to give the 24 led array.

 

[zipplet]

Hmm that's an awful way to do things... what would happen if one of the LEDs shorted out? Destruction, followed by smoke. Ever shorted out a sealed lead acid?

 

[Streak]

Zipplet, how would you do it another way?

 

[zipplet]

I've been using some 12V LED lighting strips that can be wired directly to a lead acid. They have built in current limiting resistors so that 1) the LEDs are driven at a higher current because the total vf is <12V, 2) even if an LED shorts out the maximum current is limited to a safe number.

Even so I do use a LDO linear regulator to ensure the voltage is always 12V (float charged SLA will be >12V fresh) to prolong LED life and to limit maximum current in the event of a total short. A fuse is also a good idea right next to the SLA.

Edit: An idea for your panels which does not involve changing anything would be to run them with a fuse inline, *just incase*.

Back on topic: It sounds to me like the vf is lower and the LEDs are also less efficient (lower MCD rating). I don't see how they could draw more power but be dimmer any other way.

 

[Streak]

Zipplet,
There is a DC fuse in the circuit.

The Vf in my circuit is greater than 6v per pair of series led's meaning that no current limiting resitior is needed.

I would still ike to know what conclusion I can draw about the second panel I tested that had lower output and which drew more current.

 

[zipplet]

Okay, good to know there is a fuse. I'm a little safety concious (I've seen those things shorted) so my apologies.

I edited my earlier post (but forgot to submit it ) - "It sounds to me like the vf is lower and the LEDs are also less efficient (lower MCD rating). I don't see how they could draw more power but be dimmer any other way."

The best way I think would be that which you suggested - LEDs with a vf around the same as the original (brighter) panels but the highest mcd you can find.

 

[Mr Happy]

I would still ike to know what conclusion I can draw about the second panel I tested that had lower output and which drew more current.

Different emitters have different efficiencies. For instance, there is an old model River Rock 2C light sold in Target that runs for about 7 hours at normal brightness. A new model that has replaced it runs for 21 hours on the same batteries, but is also about three times as bright. That means overall the new light is nearly ten times as efficient as the old light.

When buying LED panels you would have to specify the expected light output and the expected current or the manufacturer would feel free to vary parts and specifications and you could not be sure what you are getting.