Led forward Vdrop anomaly

If a blue LED without an internal boost converter glows with 1.1 volts measured across it, I would suspect much higher voltage not being indicated by your voltage measuring means. I would question your way of determining this voltage.

Blue LEDs and green ones of related chemistry can drop much lower voltages after static damage - but without glowing.

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

The only LEDs I know of that low are Infrared. Perhaps they are actually IR LEDs with some sort of coating that makes them emit blue?

Under 2V is normal for a Green LED BTW, not 3V

Dave.

I've got a breadboard with 2 AA alkaline batteries in one holder, and a third in a separate holder - power to the board is via a SPDT center off switch. Can run on 3V or 4.5 volts. Absolute max is 5V for the part I'm using but it (the pic) works from 2-5V.

The meter might be off, but the batteries have been in a year. The pair measure 1.56 volts each and the extra battery 1.71 V.

The leds work well at the 3V setting, with a 1.1 volt knee at 3 or 4.5 volt, and the pic probably eats a few tenths. I took them out of the circuit and applied the full 20 ma and the voltage drop was still ~1.1 little higher initially, then dropped as it ran. (in circuit I'm running with ~3 milliamps at 3 volts and the leds are plenty bright and except for the rogue green one, well balanced output wise - blue/red yields magenta etc.

I'm just quoting the manufacturer they say green at 3.4 typical to 3.8 maximum, so perhaps they are using a phosphor.

The final application will use a 4.5 volt 100 ma linear regulator and the common anode has a 100 ohm in series, each cathode 470 ohms to a totem pole output.

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I agree with Don, you must be measuring something wrong. With 1.1 Volts and no voltage boost, energy conservation says you can not have a photon with more than 1.1 eV of energy, that is a frequency of

2.7E14 Hz or a wavelength of 1.1 um., somewhere in the infrared. A phosphor is not going to help because they give off light at wavelength that is longer than the incident photon.

George Herold

You're saying what I always believed. Somehow or other, I have a handful of blue leds that aren't following the rules.

Measuring technique? Not too likely a 1.5 V alkaline battery that's a year old is significantly higher than 1.5. Since my meter read one at

1.7 volts that may indicate that the meter is off but reading higher not lower.

When you excite a fluorescent mineral it does output longer wavelengths of light - but I figured that Radio Shack IR card manages to turn IR into visible light (they were sold as a means of checking IR remote controls some years ago - little credit card filter of some sort) so maybe what I believed is incorrect.

Something strange here, but I don't think it is measurement inaccuracy.

Yeah, it is, unless, as previously mentioned, there's some kind of built in boost circuit. Blue Leds don't light at

1.1 volts. I don't know of _any_ leds that produce visible light at that voltage.

Also you mentioned green leds lite at around 1.7 volts, not the 3.4 the Hong Kong supplier mentioned. So you've got a tricolor that lights at voltages different than whan the supplier states, except for red.

Doesn't that SCREAM at you that it's either an incorrect measurement, or there is something boosting the voltage inside the LED?

Get a regular red, a regular green and a regular blue led and measure the voltage and current for each, using a regulated 6v and dropping resistors as a sort of baseline. Don't even use your breadboard, as there may be some defect in that.

Ed

I reckon that the OP has one of those cheap dodgy digital multimeters !

I've got one too ! As you increase the voltage the reading goes up faster than the change ! My alkaline batteries read 1.71 volts oc on it and 1.45 volts oc with my Avo 8.

At a real 5 volts it reads 7.72 volts and at 10 volts it reads 15.25 ! There are no internal adjustments that you can use to correct the reading only a few resistors that don't have any markings on them.

Its a heap of junk ! But what do you expect for 3euro (3$ approx)

--
Best Regards:
                     Baron.

Is the battery in the meter OK?

I have used some Volkmann meters where the internal reference obviously drops as the battery reaches the end of its life and so they read high.

kevin

Good idea, took a regular white LED and got 3.5 volts (in the breadboard). High flux blue still 1.1V same breadboard with just a dropping resistor. Not likely there's any circuitry the package is a clear piece of plastic and I only see the LEDs and connecting wires.

OK its a mystery. Put the white in series with the tricolor and drop on tri is 1.1 drop on white 3.5 running 4.5 volts in at 3 milliamps.

Tried second (cheap) meter and got 1.2 volts.

The tricolor is fragile and the pin spacing doesn't adapt well to the breadboard so once I get all the pins connecting I hate pulling it out again.

Do you know that is something I hadn't thought to check ! Thankyou for pointing it out...

I'll be back in a few...

--
Best Regards:
                     Baron.

I just pulled the back off and with the meter switched on the PP3 battery reads 8.6volts and 8.85 volts with the meter switched off. I would think that the battery is just fine !

The meter is branded "Gefolec CB-830B" and has warning notices in English and German on the back to remove meter leads before opening the case.

--
Best Regards:
                     Baron.

I see no plausible chance in that area, compared to blue LEDs having built-in boost converter circuits along the lines of a Lumex part or two that was available from Digi-Key in some recent years.

Borderline, closer to 2 than to 3 volts is usual for very yellowish green LEDs less efficient than top of red, orange, yellow, and non-yellowish-green. There is even "pure green" with voltage drop closer to 2 volts than to 3, though with lumens out per watt in having yet to being counted with fingers on more than one hand.

Bright and efficient green LEDs have commonly blazed for 12 years or so

- with voltage drop around 3.4 or more recently closer to 3.2 or 3 volts even, sometimes 2.7 volts or so, and not "pea-soup-contaminated- Mountain_Dew/_or_Gatorade" very yellowish green but something "much more pure green" as in slightly whitish more than anything else.

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

A 400 nm (blue) photon carries 3.1 ev of energy. I think that implies that an LED must have about 3 volts of drop to make blue photons.

John