# How to measure forward voltage?



## Dodge (Jan 14, 2008)

Novice question - how do I measure the forward voltage of my LED?

Is it as simple as turning up the voltage until I get the expected current (350mA, 750mA, 1A or whatever) then reading off the voltage at that point? Or is there anything more clever about it?

(Yes, I have tried the search feature, but if anyone can suggest search terms which will lead me directly to the answer that will be a useful reply).


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## SteveDavis (Jan 14, 2008)

If you're doing a one-off project, that is a good way to measure forward voltage. If you're doing something that you want to be repeatable, the best method is to not measure it at all, but to design so that the entire range of possible forward voltages, as indicated by your datasheet, will be supported at a current within the range of acceptability. The best thing is always to simply drive your device with a constant current driver and not worry about the Vf. Remember, a single part can creep in forward voltage over time anyway, so if you need consistency, always use a topology that will drive at the right current no matter what the Vf is (within reason, of course).


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## Oznog (Jan 14, 2008)

Well you probably want a current-limiting resistor in there if it's a common voltage source. Otherwise, there's only a fairly small voltage difference between 700mA and... smoke. Also the forward voltage drops as the die and heatsink heat up, so even if it was tweaked to 700mA at first it could be 1A a short time later at the same voltage. However a properly sized ballast resistor will buffer these radical changes in current as the supply and/or forward voltage change.

Be sure to use proper heatsinking. A 3W emitter not mounted on a Star will overheat VERY quickly when powered up at 3W. Faster than you can take any useful measurements on it.


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## Dodge (Jan 15, 2008)

OK - but if I put a resistor in there, surely that's going to throw the calculations right out? Certainly it means I can't rely on the supply voltage to tell me anything, but I guess I can directly measure the voltage across the LED.

What sort of value resistor would make sense when testing a Cree or SSC?


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## MatajumotorS (Jan 15, 2008)

Dodge said:


> Novice question - how do I measure the forward voltage of my LED?


Yo need constant current source, for example Lab PSU, or some AMC7135 drived from DX.
For example, you would like to know forward voltage @ 350mA:
take driver with one AMC (or cut traces to others if you have 2 3 or pieces on board) apply power 4.. 4,5V, and measure the voltage across the LED.
Same for other currents.
The by the Ohm's law calculate the limiting resistor.


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## SteveDavis (Jan 15, 2008)

Dodge:

To do the Vf measurement, if you have an adjustable voltage supply, you do not need a current limiting resistor. But that's only while you're doing the measurement. Once you're at the point that your ready to put the LED into it's permanent circuit, you need to make sure that the current is somehow regulated. As Oznog pointed out, the Vf will change over time, so a precise voltage regulator in the circuit will not be adequate. The resistor is there to take up excess voltage and react linearly with changes in voltage, to mitigate the effects that lowering the Vf can have on the current. 

The larger the voltage gap you leave for the resistor initially, the more consistent your current will be as the Vf changes. However, P=IV, so for the same current, your circuit will get less efficient as you make the gap larger.

Going with a linear regulator can greatly help to reduce this issue. A linear current regulator will adjust it's own resistance to maintain constant current. This way, you can keep a lower gap from your supply voltage to your initial Vf, and still maintain current. A buck switching regulator will do even better for efficiency, but will have a higher cost and take up more board space.


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## Dodge (Jan 15, 2008)

Thanks for the reply. Unfortunately, for the application I have in mind (4 Crees in an 18V DeWalt Snakelight, fan cooled) it's pretty hard to find a suitable regulated circuit. It needs to take in up to 20.5V (freshly charged) and supply all four LEDs - or maybe two pairs. If anyone can suggest a suitable board I'd be very grateful!

Kai has a board which will take up to 18V and supply multiple emitters - would it work to connect two of those boards in series?

As it's a one-off project I'm not too concerned about differing Vfs in different emitters, but I guess I should worry about it changing over time. Even more reason to be able to measure it accurately!


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