Discussion in 'LEDs and Optoelectronics' started by NuLED, Jan 14, 2012.

1. ### NuLED

294
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Jan 7, 2012
I bought a \$2 pack of assorted LEDs from Radio Shack.

None of them seem to light up, from 1.5v to 4.5v. I haven't seen LEDs that require Vf exceeding around 5v so I wonder if there are LEDs that go up to 9v or 12v forward voltage?

I might just sacrifice one of them to test on a 9v battery.

How do I figure out Vf of an unknown LED? I tried rigging up a circuit with a bunch of resistors in series but even without any resistors (max to 4.5v as mentioned above) no light from the LEDs.

And I tried both polarities.

2. ### Resqueline

2,848
2
Jul 31, 2009
Unless they're IR, or high-power LED's with multiple internal series connected chips then I'd say they're dead. Did you measure any current draw?
Vf is only measured, using a controlled If. 5V testing is safe either way. If you need to go higher you should try to establish the correct polarity first.

3. ### NuLED

294
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Jan 7, 2012
Unfortunately I also have a problem with my \$30 DMM not seeming to give accurate current. But it will give a "binary" feedback of current or no current. (I have ordered a new DMM). I will see whether there is current draw.

But if the Vf threshold is not reached, is there still current?

4. ### NuLED

294
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Jan 7, 2012
Related: Are dead LEDs a common/not uncommon thing? I am a bit surprised to get dead LEDs buying from Radio Shack (although they probably randomly source from random China suppliers also).

5. ### Resqueline

2,848
2
Jul 31, 2009
You can measure (& calculate) the current by simply measuring the voltage across the series resistor.
With red LED's you can expect them to start drawing a small current (& lighting dimly) as early as 1.5V, reaching around 2V at 20mA.
If they're IR LED's you won't see the light but they'll have a 1.1Vf drop.

Most anything coming from China is suspect, until proven good..
I've seen a "solar" calculator with an unconnected solar panel. The panel was not up to spec's either, so they probably got them cheap and used them for "show".
A cheap USB "2.0 hub" doesn't even need to contain any electronics at all. A dead green LED is all you might find in it. In essence it's just an extension cord.

I'd study those LED's with a loupe (or a microscope) to see if there's a chip in them at all. They could be fakes or just fabrication rejects.
It's very uncommon for dead LED's to reach the customer through a supply chain.

Last edited: Jan 15, 2012
6. ### NuLED

294
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Jan 7, 2012
uncommon or did you mean to say common?

anyway I will rig up the breadboard and do some Ohm's Law as you suggest and see what I get.

7. ### Resqueline

2,848
2
Jul 31, 2009
It's quite common for dead LED's to be manufactured, but they're tested and sorted out. "Anarchistic" dealers may still buy and sell them though.
It's very uncommon for dead LED's to reach the customer. I've never heard about it before, exept when buying "surprise packs" (= floor sweepings) from hobby places.

8. ### BobK

7,682
1,688
Jan 5, 2010
I would be very surprised if a pack of assorted LEDs are all bad.

What are you using for power? Is it possible IT is bad?

Bob

9. ### GonzoEngineer

321
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Dec 2, 2011
Maybe they are Darkness Emitting Diodes......DED's

10. ### NuLED

294
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Jan 7, 2012
Hi guys! Well, I rigged up a simple circuit and tested all the LEDs. Turns out SOME are bad and SOME are ok. What a hassle. More of them are OK than bad, so I will keep the pack (threw away the bad ones which did not even produce any voltage between the in-series resistor).

11. ### NuLED

294
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Jan 7, 2012
Just a random semi-related question: With the 4.5v (3 x AA) and a 10k Ohm resistor (actual 9.86 k) I get about 201 uA (micro amps?) does that seem right? Voltage across the resistor was 1.987 V. I = E/R = 201.52 x 10^-6.

12. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,491
2,833
Jan 21, 2010
Well, 4.5V and 9.86k gives you 456uA.

If there is only 1.987v across your resistor then either your batteries were very flat, or something else was in the circuit.

In that case, you'd probably see something like 201 uA (that you calculated)

13. ### NuLED

294
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Jan 7, 2012
ok thanks; actually that was part of the LED circuit and the LED took the rest of the voltage. I just needed to know if the order of magnitude sounded correct (microamps) for this kind of circuit.

14. ### NuLED

294
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Jan 7, 2012
(my DMM, you see, gives 9 milliamps or some other reading, which is totally not what you are supposed to get, thus I conclude its ammeter circuitry is screwed; it does that for pretty much any other current i try to measure)

15. ### Resqueline

2,848
2
Jul 31, 2009
Approximately 2.5V at 0.2mA sounds like a white or blue LED. Yes, that's to be expected with a 10kΩ resistor. I'd use 1kΩ (or even 100Ω) with a 4.5V battery.
Those that did not produce any resistor drop at all were most likely dead. I can't think of any working LED's that would behave like that (except multi-chip power LED's).

16. ### NuLED

294
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Jan 7, 2012
OK great; you guys are helping a lot with every increment of info as I progress through this wonderful world of tiny electrons!

17. ### BobK

7,682
1,688
Jan 5, 2010
If you are using a 10K resistor, some of the "dead" LEDs might just not be bright enough to see at the low current you are testing them with. Try them again with a 1K and then a 100R if you haven't emptied the trash yet. A blue or white LED can take as much as 3.6V at it's rated current.

Bob

Last edited: Jan 16, 2012