To determine a particular LED's Vf, you'll have to look at its datasheet. For example, Cree's XR-E datasheet is found at http://www.cree.com/products/pdf/XLamp7090XR-E.pdf . In page 6 of that PDF, you'll see a graph of Vf. This means that the Vf of a particular emitter is not fixed, but varies, with the available voltage determining the emitter's "natural" drive level. For example, if you look at where the graph reads "3V," you'll see that it corresponds to a "Forward Current" drive level of about 160mA. This means that if you solder an XR-E to the appropriate terminals in an ordinary Minimag, which operates at 3V, you'll get about 30 lumens of floody, white output.

The downside to this is that small changes in voltage can have a huge effect on forward current. For this reason, most drivers supply a constant current instead of just a constant voltage, as current is really what determines an emitter's output. Therefore, if you want to power an XR-E at 700mA from a pair of AA NiMHs (a common task), you'll need a driver that can boost the voltage from 2.4V to something closer to 3.6V as well as supply a constant 700mA. If you tried direct drive with this setup, simply connecting the emitter to the battery, the emitter would barely light up at all.

It is important to note that even emitters of the same type and "bin" (sorting system based on several characteristics) can have small variations of Vf. For example, you may have an emitter that has a Vf of 3.7V when driven at 1000mA (or 1A), while someone else has an emitter of the exact same type and bin with a Vf of only 3.5V at the same 1A current. This means that their flashlight will consume only 3.5W of power while yours consumes 3.7W, although both would have the same output given the identical drive level.

You can find the Vf of your emitters at particular drive levels with a basic DMM. If you are using a driver and you don't know the current it provides, or if you're not using a driver (a DD setup), put the DMM in the circuit next to the emitter and check the current. Next, complete the circuit without the DMM in it, and put the DMM's + and - probes on the emitter's + and - contacts, respectively. This is the Vf at the current level you measured for this particular emitter.

Most emitters can handle a bit of current at reverse voltage, so if you wire one backwards but don't activate it for more than a second or two (at most), it should be okay. Just don't make a habit of it.

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