How do I test Vf?

Hi everyone, I just got a few TW0K stars that are going to be connected in parallel and I want to determine the exact Vf of each one so I can find the appropriate resistance for 3xNiMH batteries. I have a multimeter and know I need to connect the multimeter across the +ve and -ve areas of the LED but how can I supply a power source to do that without risking frying the LED? Right now I've got one lighting up by connecting it to a 3xAAA adapter from Radio Shack with some alkaline batteries very briefly as I know their resistance probably wouldn't allow too much current, but I'd like to know if there is a safer way. Thanks!

Simple. Equipment needed:

1 variable voltage power supply
1 voltmeter
1 ammeter.

Proceedure. Connect the voltmeter across the LED terminals, and the ammeter in series with your supply.

The Vf is the voltage required to get an appreciable amount of current to flow through the LED. If dealing with an LS, 70mA is around the figure I'd go for without a heatsink to be safe. Note, once you pass the Vf, the current the LED can pass will increase VERY rapidly, so you'll know when you've got to that point.

An easier way, is if you have a current regulated supply. Then just dial in your current, connect up your LED, and turn on. Then measure the voltage across the LED.

The former method is the one I've always used, as I don't have a current regulated supply (Would like one though), and it's always worked well, never fried anything yet.

As usual with my advice though, I'd probably wait for someone to second it first. Those stars are a LOT more expensive than anything I've been playing...er...experimenting with.

Hope this helps.

If you're going to use a variable voltage power supply, put like 2 ohms of resistance in series with the LED for a luxeon, or like 50 for a 5mm LED, so you don't push too much current by turning up the voltage too much.

Note, Vf is not the voltage at which appreicable currnent flows - it's the voltage across the LED when a very specific current flows through it - the rated current.

For a Luxeon I, the Vf should be measured at 350mA. For a Luxeon III and Luxeon V, 700mA. For regular 5mm LEDs, 20 or 30mA.

If you plan on running the LED at a lower current, the Vf will be lower than at the current measured above. If you plan on running it at higher current, the Vf will be higher.

Here are some pretty average measurements for a Luxeon I/III

<font class="small">Code:</font><hr /><pre>
Current Vf
100mA 3.0V
350mA 3.3V
500mA 3.5V
700mA 3.6V
1000mA 3.9V
</pre><hr />

Thanks for that correction regarding the current being measured, Evan.

Worth noting as well, if using a current regulated supply, turn everything off, connect load, then start increasing it - otherwise there will be a far higher voltage present on the output terminals as the supply will increase the voltage there as much as it can to attempt to get current to flow (Unless of course you're lucky enough to have a voltage limiting control as well) - this is a good way of frying LEDs - trust me on that one! Zero controls, connect load, switch on. Not the other way around!

Even better is the usage of a powre supply such as this that features both constant voltage and constant current.

Let'z say the current regulator is set low, but not zero. Voltage button is turned down. Connect LED. Slowly raise the voltage. The power supply works in constant voltage mode and supplies the current defined by the load (in this case, the LED). Raising the voltage further will raise the current. Once the current reaches the setpoint defined by the current button, the supply switches to constant current mode and supplies only the current set as limit. Further raising the voltage button has no effect. If you now raise the current limit, the opposite happens until the voltage limit is reached. Successively raising the voltage and current limits until you reach the specified driving current gives you the forward voltage.