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White LED light output vs current

I just picked up a set of four "puck" lights. Each has 24 white LEDs wired in series. (Cheap, under $20 for 96 LEDs, and in nice cases.) They run from 120VAC/60Hz, through a 0.1uF capacitor into a 4-diode bridge, with the LEDs as a load directly on the bridge, so the reactance of the 0.1uF cap limits the current. I got them thinking that I'd modify them for use in my trailer: more battery-friendly than the 1.5A incandescent lights provided by the trailer manufacturer. So I applied DC to just the LED string of one, and discovered that the light output (vs current) increases nicely up to about 7mA, and then--starts to decrease. I was a bit surprised to find LEDs that don't increase in brightness up to at least a few tens of mA. Anyone know what kind of LEDs these might be? They're for sure bright enough to be useful, and at around 2 watts total for the four pucks to get about the same light at the work surface as the incandescent 18 watt bulb provides, they're a big win. But the behaviour vs current is a bit of a puzzle to me at the moment.

Cheers, Tom

Reply to
Tom Bruhns
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Do you suppose the chip is extremely tiny and prone to overheating?

Jim

-- "It is the mark of an educated mind to be able to entertain a thought without accepting it." --Aristotle

So I applied DC to just the LED string of one, and

Reply to
RST Engineering (jw)

Well, it doesn't _look_ all that tiny, but then it's behind the curved front surface of the LED case plastic, so that may be deceptive. 7mA is only 20mW or so...not what I'd expect to cause much heating, but...it's perhaps possible. I may be able to test that theory: monitor the LED drop with a tiny current, and pulse a "large" current (e.g. 15mA) thru it for a second or so, and see if the drop at low current has changed.

Cheers, Tom

Reply to
Tom Bruhns

I consider that very unusual.

Possibly the LEDs are overheating, and they do easily in cluster lamps, but I find it surprising for them to level off at 7 mA.

Possibly they are having a spectral shift to wavelengths that don't excite the phosphor as well. Blue LED chips often have wavelength varying with current. But I am still awfully surprised.

Usually, white LEDs (of 5 mm and similar "low power" sizes) have efficiency peaking at a few mA. Increasing current past that point results in more lighht - just less than proportionately more.

===============================

One more thing: If you replace that capacitor, make sure it has an actual AC rating. Merely having voltage rating exceeding the peak voltage and having the capacitor not being an electrolytic type is not sufficient. I have found out the hard way!

If the capacitor has an AC rating, it can be 40% or less of the DC rating. Some that I recently purchased have 630 VDC and 250 VAC ratings.

- Don Klipstein ( snipped-for-privacy@misty.com)

Reply to
Don Klipstein

Any chance one of the 96 LED's looks a little different from the others? Perhaps a diode or resistor buried under the forest of LED's? My guess(tm) is that there's a current regulator, one LED with a built in current source, or some kind of current limiter in there somewhere.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558
Reply to
Jeff Liebermann

Well, no, they all emit very close to the same intensity at any particular current. And note that I'm measuring current: I have no trouble getting more current if I increase the voltage; it's just that the emitted light intensity drops as I increase the current beyond about 7mA. For fear of permanent damage to the LEDs, I'm avoiding running the current very high or for very long above ~10mA. But I also have to admit, this has fallen below the top of my rather long list, so I'm not sure when I'll get back to running better instrumented transient testing.

Cheers, Tom

Reply to
Tom Bruhns

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Well, there are 24 LEDs in series, each with a bit of inherent resistance in addition to their exponential diode characteristic. I'd guess there's at least an ohm of series resistance in each one, probably a few times that. At one ohm each, the peak current (for a very short time) would be no more than a couple amps. A couple amps charges 0.1uF at 20V/usec. There is a fuse, and there is a couple hundred kohms in parallel with the cap, but I didn't see any specific series resistor--and there isn't much of any place for one to hide on the board. -- Note also: there are different "levels" of AC rating for caps. Some are simply rated for operation at AC, and some are certified at different levels for use in places where safety is an issue.

Cheers, Tom

Reply to
Tom Bruhns

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