# How to determine LED Vf - or guess

Discussion in 'The Projects Forum' started by russpatterson, Aug 4, 2011.

1. ### russpatterson Thread Starter Member

Feb 1, 2010
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Apr 30, 2011
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Make a constant current supply using an LM317 and a resistor (62.5Ω for 20mA), then measure the voltage drop with a meter. Knowing which current is another matter. You can assume that they're made for at least 20mA. I'd be tempted to try one at 25mA and monitor the temperature.

Last edited: Aug 4, 2011
3. ### seecumulus Member

Jul 13, 2011
31
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If you have a Bench Adjustable Power Supply which has a Voltage and
Current Meter within, that would be great.
or
IF you have a variable DC power source only you could monitor the
Voltage and Current with your DVM, two DVM's would be nice one
for Voltage one for Current.

Yes - Several standard individual LED's draw 20 - 25 mA.

First I would determine the positive terminal on the LED.
Second add a current limit resistor in series, 330 ohms or close.
Attach Current Limit Resistor and LED to the DC supply.
Start with power supply at 0 and slowly increase the voltage while
monitoring the current draw.
When the LED lights relative to a normal LED brightness and
the LED is not Hot Hot in temperature that would be where
I would stay.

4. ### russpatterson Thread Starter Member

Feb 1, 2010
351
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Thanks for the replies. I have a cheap bench supply that limits current, I'll give that and the LM317 circuit a try.

5. ### THE_RB AAC Fanatic!

Feb 11, 2008
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Just a thought, but \$1.49 for one 10mm white LED sounds REALLY expensive, especially since they don't specify the mCd brightness!

If you want to use white LEDs for some lighting application you need to know or test their brightness. You can get bags of 50 or 100 super bright 5mm white LEDs on ebay, which may offer much more light per buck and per watt (depending on price and mCd brightness of course).

6. ### russpatterson Thread Starter Member

Feb 1, 2010
351
16
I actually meant to order these: http://www.goldmine-elec-products.com/prodinfo.asp?number=G17255 Which I used for a project previously. I put a 160 Ohm and two of these in series on a 12V line, every couple of feet on the underside of a patio cover structure cross beam. It's nice light and with the 40 degree focal they make noticable white, plate sized spots on the ground. I'll have to see what I wound up with regarding the 10mm ones.

7. ### iONic AAC Fanatic!

Nov 16, 2007
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It would be safe and perhaps a tad conservative to assume Vf @ 3V and If @ 20mA.
With that in mind you can create a constant current source to drive the LED's.
As mentioned above, you could try one at 25mA and see if the brightness is noticeable and worth the risk of a shorter lifespan.

8. ### Wendy Moderator

Mar 24, 2008
20,765
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BE AWARE!

Cheap bench supplies can blow LEDs using their constant current mode very easily. This is because they have large filter caps, that will discharge quickly and surge the LEDs.

The way around it is to short the power supply leads until you are wanting to route the current through the LED. This way the caps are fully discharged and don't enter the picture.

I remember another thread that had the problem. They were using a Velleman similar to mine.

9. ### Audioguru New Member

Dec 20, 2007
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All the people who are talking anbout only 20mA did not notice that the LED is a big 10mm one, not an ordinary 5mm one.

Apr 30, 2011
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Most 10mm LEDs, especially the cheap ones sold without data, are still of the 20mA variety but three of us (myself, seecumulus and iONic) mentioned the possibility of a higher current rating and it wasn't pertinent to the comments made by the other posters (THE RB and Bill Marsden).

This is probably the item: http://www.hebeiltd.com.cn/led.datasheet/1025PWC.pdf but it could be another just like it.

Last edited: Aug 6, 2011
11. ### SgtWookie Expert

Jul 17, 2007
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I would be somewhat surprised if the LEDs in question were rated for more than 25mA absolute maximum, 20mA normal; as that seems to be about the going rate for all of the 10mm LEDs I've seen. I've also seen Vf's ranging anywhere from 3.2v to 4.0v for white LEDs. Seeing that these are surplus with no specifications given, I'll suspect that their Vf's will vary a good bit, and that the average will tend to be on the high side.

If one is going to use several of them in series, the varying Vf's becomes less of a concern, as the Monte Carlo distribution will tend to distribute the high and low Vf's fairly evenly. However, if used in series pairs, it would be a good idea to verify the Vf's of the LEDs, and match up the low Vf's with the high Vf's; otherwise some strings may give less light due to low current, and some may have too high current, causing the LEDs to grow dim quickly or burn out.

You will probably find that about 75%-85% of any given batch will be pretty close in Vf, but the remaining 15%-25% will be off by varying degrees.

I suppose that if Russ didn't mind zorching a few of them, he could slowly increase the current and notice at what current the color of the LED changed, then go with about 2/3 that current.

12. ### Audioguru New Member

Dec 20, 2007
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Why do they put an ordinary LED die made for a 5mm LED in a bigger 10mm case when there is no increased current allowed?

13. ### russpatterson Thread Starter Member

Feb 1, 2010
351
16
The LED's I have bought (on sale) earlier, used, and like are rated at 30mA, they are 8mm. These 10mm ones I ordered by mistake have a much more blue tint and not so narrow a focal beam. But they illuminate pretty well. They seem as bright as the 8mm ones. So, at least in this case, the 8mm one's do handle more current than the standard 5mm.

I tested both sets of LED's at 30mA and they seem to perform fine. Whether I'm going to significantly shorten the life running at that current remains to be seen I guess.

I tried the LM317 that KJ6EAD suggested and that works pretty well. I used 4 x 10 Ohm resistors to get 40 Ohms to get 0.030 Amps out and was able to connect three LED's in series without any noticeable drop in brightness. So what's the downside to using a LM317 as a constant current source like that. My lighting is battery powered (solar charged) so efficiency is paramount. Am I better off putting 2 LED's in series with one resistor or three LED's in series with the LM317 and a resistor? It's cheaper to use the one resistor (especially since I don't need a PCB and can solder the LED's and resistor directly to the transmission wires).

@Sgt. Unfortunately I don't have a meter that is operable and accurate enough to measure current at those levels at this time. Once I get that straightened out I'll burn up a couple LED's try your 2/3'rds technique.

Thanks for all the replies.

14. ### SgtWookie Expert

Jul 17, 2007
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You should double-check your Vref with a reasonably accurate voltmeter.
Vref is the voltage on the LM317 OUT terminal, referenced to the ADJ terminal. Nominally this is 1.25v, but can range as low as 1.2v and as high as 1.3v and still be within the mfgr's specifications. However, if Vref is different from what you used to calculate the resistors, it will change your output current.
1.2v/40 Ohms = 30mA
1.25v/40 Ohms = 31.25mA
1.3v/40 Ohms = 32.5mA
Note that there is another 50uA to 80uA current that flows from the ADJ terminal that will add to the output current. Usually this isn't terribly significant; you just need to know that it exists.

The LM317 has a minimum dropout from IN to OUT of 1.7v. Then there is also the OUT to ADJ (Vref) drop of 1.2v to 1.3v, so roughly a total of 3v dropout; and you'll need to subtract that from your minimum battery voltage before you can calculate how many LEDs that you will be able to operate in series. That's a significant amount of overhead, but the current will be very well-regulated.

Well, you'd really be best off using a buck/boost regulator to keep the voltage supply to your LEDs constant at whatever voltage you choose, and then since your voltage supply will be accurate, you can use a comparatively small resistor with each string of LEDs - smaller than you would need if you were operating from unregulated battery power. For example, you could take 12v to 14v in, boost it to perhaps 15 to 24v, and use perhaps 5 or 6 LEDs in series with a fairly low-value resistor. A 33 Ohm resistor with 30mA flowing through it would have a ~1v drop across it. You always need a current limiter or active current control for LEDs.

You don't need to measure current - just voltage across an accurate resistor. 1V / 1 Ohm = 1 Ampere. 30mV / 1 Ohm = 30mA.
If your meter isn't all that accurate, you could use a 10 Ohm resistor. 1V / 10 Ohms = 100mA; 100mV/10 Ohms = 10mA.

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15. ### iONic AAC Fanatic!

Nov 16, 2007
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Given this requirement alone, staying on the conservative side aids in the efficiency.

But burn a few, please! Show us video/pics and data!

16. ### russpatterson Thread Starter Member

Feb 1, 2010
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Ok, I burned up a couple of the 10mm LED's. I used my Tektronix TDS 1002 scope to measure the voltage drop across a 10 Ohm resistor. That should be accurate right?

However the results seem like something's not right. I ran the voltage out of the bench supply, through the 10 Ohm resistor and through the LED. It lit up very brightly until I had about a 1.2V drop across the resister, so about 120mA. At the next voltage bump it turned deep blue then faded out. Not very exciting but here's the video. http://www.youtube.com/watch?v=xyOvUzLU__4

So if I went with the 2/3 rule I'd get 80mA across the LED. That sounds like way too much current. Maybe the scopes jacked up? I'm sure it would burn brightly at 80mA but I don't know how long they'd last. Maybe 80mA is right?

17. ### iONic AAC Fanatic!

Nov 16, 2007
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Excellent video! 80mA still seems high. Perhaps leaving one burning at 80mA for an hour or even for a most of the day while checking in on it periodically wound be the next step. If it runs for 10 hours. repeat the experiment the next day with the same LED and same current. See how many days the LED lasts. It's concealable that the LED could last for weeks of months, but that would be far short of a reliable device. One thing is for sure, our eyes are not a very good indicator of brightness as we see logarithmically not linearly.

Guys, when the LED is moving towards failure, what measurable(electrical) characteristics would be occurring?

18. ### SgtWookie Expert

Jul 17, 2007
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Take two LEDs, and match them for brightness at a particular current.

Then operate one of the LEDs for a week straight, 24 hours a day, at the current you're thinking of trying; and keep the other matched LED as a control LED.

At the end of a week, compare the brightness of the two LEDs again. If the one you operated for a week is even slightly less bright than the control LED, then you'll know that you have been operating it with too much current.

LEDs will gradually grow dimmer when operating at their normal rated current. If you increase the current, it will accelerate the aging process; the greater the current, the more rapid the aging occurs.

19. ### russpatterson Thread Starter Member

Feb 1, 2010
351
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I'll try the week long test and see what happens. Is there a relatively inexpensive measuring device that would be accurate enough to measure the brightness of an LED? That would be useful. I once had a harbor freight meter that measured light level. Maybe there's a Fluke that has one accurate enough for this kind of testing?

At any rate if I go with 30mA at this point I think it would be a safe bet.

20. ### SgtWookie Expert

Jul 17, 2007
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You might use a CdS photoresistive cell; their resistance decreases with an increase in light You could use an ohmmeter to get before and after readings; nothing precise, but it would be some way to tell if the light output changed.

Radio Shack sells an assortment of CdS cells for a couple bucks. The problems would be to ensure that there is the same amount of background light, and that you had the same distance and angle from the LED during both measurements. It seems that a cardboard tube from an empty paper towel roll would be a good candidate for something pretty opaque, while at the same time can maintain the distance between the LED and the CdS cell.