High Vf & If... how do you tell?

As the title says, is there any way to tell if your emitter has an unusually high Vf or If?

:thinking:

Measuring VF of LEDs??? - CandlePowerForums
Norm

... so you just set the DMM to V-DC and measure voltage across the LED +/- terminals?

Not that I'm complaining, but it sounds almost too easy...:thinking:

This is how I do it. You need a bench power supply that has constant current limiting feature. First set the current limit to whatever you want - say 700mA. With the voltage at 0, connect the LED observing correct polarity. Raise the voltage until the constant current limit kicks in. Note the voltage on the power supply - or for better accuracy, measure the voltage across the LED with a DMM.

kramer5150 said:

... so you just set the DMM to V-DC and measure voltage across the LED +/- terminals?

Not that I'm complaining, but it sounds almost too easy...:thinking:

Click to expand...

The trick is to know what the associated If is for the Vf that you just measured. Just because the driver is advertised as delivering 750mA or whatever doesn't make it so. Thus, you need to measure If as well, and ensure that this measurement for If is the same If for the Vf measurement that you made.

You might think that you can measure Vf by putting the DMM in parallel with the LED (i.e., probes across the LED terminals), and then separately measure If by putting the DMM in series with the LED (e.g., one probe on the wire coming from the LED+ side of the driver, the other probe on a wire soldered to the + side of the LED). The problem with measuring Vf separately from If using one DMM is that the the DMM can produce its own slight voltage drop when measuring If. Thus, your If measurement will be different than the actual If associated with the Vf that you had measured.

Another issue is that you have only one V-I data point, which is essentially dictated by the output current of your driver board. A bench power supply, as described by rmteo, allows you to explore the LED's complete V-I curve. It's also a good idea to heat sink the LED while making these V-I measurements. LEDs can get very unhappy otherwise.

If you are making a single data point V-I measurement for an LED that is already hooked up to a boost driver, make 100% certain that your DMM connections are secure when measuring If. If your DMM gets disconnected while the boost driver is being powered because your alligator clip slipped off some wire, you'll probably instantly fry the driver. Boost drivers don't like to be powered up without a load across them. You can avoid this problem by using a sense resistor soldered in series with the LED. Then measure the voltage drop across the sense resistor and use Ohm's Law to calculate current.

I think the easiest thing to do is to use a bench power supply if all you want to do is characterize the LED's V-I behavior. You can then indirectly confirm the output current from your driver board by measuring Vf across the LED terminals when the LED is being powered by the driver. Since you have the V-I curve for the LED from the bench supply tests, you can just look up the current associated with the measured Vf. Hopefully, that current matches the advertised output of the driver.

You can then estimate driver efficiency by combining current measurements at the tail with battery voltage sag data from Silverfox. Now you have Power In and Power Out, giving you driver efficiency.

hot dawg... you guys are sharp!!

thanks so much.

Another way to measure If is to use some extra wire that you can attach a clamp-on meter. The meter has to be able to read DC current. If the bench supply can run in constant voltage, you can also probe the driver's performance in terms of where it goes in or out of regulation. And also get a better handle on driver efficiency.

A bench power supply with CV/CC is very handy to have - just as much as the need for a good DMM. You can get a 0-15V, 0-3A one starting at about $60. The one I use is a 0-40V, 0-30A switching supply. You can get it from here:
http://www.valuetronics.com/Details.aspx?Prodid=53

rmteo said:

You can get a 0-15V, 0-3A one starting at about $60.

Click to expand...

Where?

Here is a 0-18V, 0-3A PSU for $60:
http://www.mpja.com/prodinfo.asp?number=14600+PS

Justin Case said:

You might think that you can measure Vf by putting the DMM in parallel with the LED (i.e., probes across the LED terminals), and then separately measure If by putting the DMM in series with the LED (e.g., one probe on the wire coming from the LED+ side of the driver, the other probe on a wire soldered to the + side of the LED). The problem with measuring Vf separately from If using one DMM is that the the DMM can produce its own slight voltage drop when measuring If. Thus, your If measurement will be different than the actual If associated with the Vf that you had measured.

Click to expand...

The thing is, if you have a constant current regulator, this won't matter unless you exceed its limits with the small voltage drop from the multimeter. On the other hand, maybe you're using a constant voltage regulator, in which case, never mind.