Led forward Vdrop anomaly

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I got some tricolor leds for a project from "superbrite" in Hong Kong.
The general specifications are what you'd expect typical values: RED
2V, GREEN and BLUE 3.4

The leds I got are at full current (20 ma superflux style package)
with Red 2 Volts, Green 1.7 volts, and Blue 1.1 volts (with some
variability in the green ones - one tested at 3.4 volts but it was the
exception the rest were much lower.

I don't think I've ever seen a blue led work down that low before. Is
there some new technology afoot, or does the normal variability extend
down that low?
--

 

[Don Klipstein]

I got some tricolor leds for a project from "superbrite" in Hong Kong.
The general specifications are what you'd expect typical values: RED
2V, GREEN and BLUE 3.4

The leds I got are at full current (20 ma superflux style package)
with Red 2 Volts, Green 1.7 volts, and Blue 1.1 volts (with some
variability in the green ones - one tested at 3.4 volts but it was the
exception the rest were much lower.

I don't think I've ever seen a blue led work down that low before. Is
there some new technology afoot, or does the normal variability extend
down that low?

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 ([email protected])

 

[David L. Jones]

I got some tricolor leds for a project from "superbrite" in Hong Kong.
The general specifications are what you'd expect typical values: RED
2V, GREEN and BLUE 3.4

The leds I got are at full current (20 ma superflux style package)
with Red 2 Volts, Green 1.7 volts, and Blue 1.1 volts (with some
variability in the green ones - one tested at 3.4 volts but it was the
exception the rest were much lower.

I don't think I've ever seen a blue led work down that low before. Is
there some new technology afoot, or does the normal variability extend
down that low?

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.

 

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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 ([email protected])

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.
--

 

[[email protected]]

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.  
--- Hide quoted text -

- Show quoted text -

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

 

[ehsjr]

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

 

[Baron]

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)

 

[kevin93]

56:14 +0000 (UTC), [email protected] (Don
Klipstein) wrote:







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)

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

 

[default]

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

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.
--

 

[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

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

I'll be back in a few...

 

[Baron]

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

I'll be back in a few...

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.

 

[Don Klipstein]

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?

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.

Under 2V is normal for a Green LED BTW,

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.

not 3V

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.

Dave.

- Don Klipstein ([email protected])