measure forward voltage...

does anyone know how to find the Vf of a diode that is unknown. i have an IR diode from the LPC sled that im using as an IR illuminator for a B/W camera. im regulating current with a lm317. i know that i need to comp. for the voltage across the LM317 (~3.0V) but i dont want to feed it more voltage than it needs (extra voltage will be converted to heat to be dissipated by Vreg). can i measure the Vf at the diode pins while running or does anyone know what the Vf is for this diode. all i could find on the forum is

nvmextc said:

I have one running at 150ma for 90mW. It doesn't get hot really so can propably be pushed a bit.
I'm not sure how the maths works for v=ir but the IR only has a voltage drop of about 2.5V, less even.

Click to expand...

any ideas?

You can directly measure the Vf of the diode (that varies at every current - see PIV curve). Just turn it on and measure (beware with short circuits that WILL kill the diode.

The other way.
If I would do it, I think I would need 2 multimeters for doing it (or a PSU with a GOOD voltmeter). Run an ammeter in series with the laser diode, and use a variable supply, measuring its output voltage.
If you setted your driver at 150mA, set your supply at around 5V, and run the LM317 driver. The ammeter should read 150mA (what you set the diode at...)

So you start lowering the voltage 'till the current on the diode starts to drop. That moment it start to drop, you just lowered the voltage below the minimum, so if 150mA dropped to 140mA @ 4.3V, you will need 4.4V at least (example).

Lets hope you understand that... Sorry for my english. :beer:

As stated above, lowering the voltage to find the regulation point is the simplest way to do this.

Here's another application where a bench power supply would come in handy. Everything you need to test this is in one device.