How do you determine Vf on SCC P4

One of our member vendors has a beautiful website and sells SCC P4 Stars and lists the Vf like this:
3.7V typical driving voltage (see specs section for more info)
The specifications sheet is on this page:
http://www.seoulsemicon.co.kr/_homepage/home_kor/product/spec/W42180.pdf
The method SCC list the Vf in the spec sheet is that it states the wa41280 has a Vf 3.00-3.8, 3.25 typical, or something like that, they identify all the bins ranges and not the individual bins. The vendor does not realize that this specifiation is not a voltage bin but is the description of the range of all the bins. Bins are in quarter volt increments.
The individual Voltage bins are listed here:
http://www.essc.co.kr/_HOMEPAGE/home_kor/product/spec/P4_BL.pdf

So the vendor are providing, shipping, the the LED without any Vf information. The LED is a P4 UWSO? so my gut feeling is it is an H bin, 3.2 Vf and will go away if I drive it 3.7Vf.

How can I determine the Vf if I do not get a response from the vendor? I doubt that I will since they did not have this basic understanding of the product specificaiton in ther first place.

I also would like to know this.
Having read members saying things like: " i replaced my emitter with a low Vf cree " etc.
Would like to know what that really means, and also how to identify low Vf emitters.

Mash said:

I also would like to know this.
Having read members saying things like: " i replaced my emitter with a low Vf cree " etc.
Would like to know what that really means, and also how to identify low Vf emitters.

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What you are asking and what I am asking are really two different things. I need to know how to test it.
To answer your question. Each LED requires a limited range of voltage to operate with. Below and no glow too high and it burns out. Each LED is therefore tested and labeled by the manufacture and the labeling is refered to as a voltage bin. So LEDs of color, voltage and lux out put are grouped into closely related ranges of performance called bins. LEDs are not coming out of production with a given specification, like computer chips they vary and must be grouped.
The votage at the LED where it is rated to work is the forward voltage Vf. For the P4 SCC pure white the voltage range is 2.9-3.8 with the typical production or average piece measuring 3.2 volts. SCC breaks the voltage ranges up into quarter volt ranges starting at 2.00 volts and ending at 4.50 volts. This is the master list of how voltages are labled, each type of LED has its own operating range. The ranges are labeled by letters, D is 2.00~2.25 and so on. So a low Vf for this P4 would be the lower side of the range 2.9-3.8, that would be a G bin as G is 2.75~3.00 volts. The lower Vf allows you more design leeway and the ability to reach maximum lumens easier.
Normally, a legitimate and knowledgeable vendor buys a real of the LEDs, reads what is says on the label for forward voltage, Vf and passes it on to you.
Now follow the two links above, look at voltage bins in the binning spec and the overall performance spec of the individual LED and you should see what I was saying. In all it means the voltage you deliver to the LED must be specific, you cannot always drop one in it must be matched to its own requirement.

One way to test the Vf is to connect the LED to a DC power supply with a constant current mode. Set the current to 350mA and the voltage reading across the LED will be the Vf. In this pic is a sample USVOH driven at 350mA and the Vf is 3.08v.


You can change the current to a level that matches the level you will be driving the LED with, that will give you the Vf at that level.
The stated Vf is usually at the LEDs rated drive level. (I think Seoul's are speced at 350mA but I could be wrong)

A cheaper option would be to use a variable voltage power supply and 2 DMMs (one for voltage and one for current). The accuracy would depend on the DMMs though.

Thanks all for the explanations!
Northern Lights, I knew about LED binning, but always thought it only applied to colour variations. So thanks for the info, it is appreciated!

ya..all's getting too complicated..

turn on light, place voltmeter leads across emitter. reading is your forward voltage.

Note that when testing, the temperature must be at according to the manufacturer spec sheet, which is 25C.
It's easier to use a know vf bin LED to compare.

Morelite said:

One way to test the Vf is to connect the LED to a DC power supply with a constant current mode. Set the current to 350mA and the voltage reading across the LED will be the Vf. In this pic is a sample USVOH driven at 350mA and the Vf is 3.08v.

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You got very nice low vf SSC there!!!

cy said:

ya..all's getting too complicated..

turn on light, place voltmeter leads across emitter. reading is your forward voltage.

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But that only works if you've already got it connected to a driver that is providing the appropriate current through the LED. My impression of the purpose of this thread is to determine the Vf before building the circuit.

When I was experimenting and only had one meter, I used a 25 ohm rheostat to dial in the appropriate current on the meter. Then I removed the meter from the circuit, switched it over from current to voltage, and then measured Vf. But it's much easier to use the two meter method mentioned earlier.

cy said:

ya..all's getting too complicated..

turn on light, place voltmeter leads across emitter. reading is your forward voltage.

Click to expand...

Great point for a light already built. Problem here is I got stars and a project and no specs.
Don is correct.
Thanks, guys, I got two meters and can run some measurements between them to check their calibrations then off to the LEDs.

if you have two meters great. set your voltage from power supply slightly over rating of emitter at rated output. (350 milliamps or what ever)

micro clamp leads makes job easier. clamped to 3/4in thick aluminum

has anyone experimented or tested the 'newer' led's(ssc, cree) if they also exhibit a vf shift similar to that of the luxeons?