1210 smd white led vf

[large_ghostman]

i have some normal 1210 size Chinese bog standard white smd Led's, i cant seem to measure the vf on them, i have tried my normal 2 ways 1 using diode thingy on DMM but that just reads OL and lights the led very very dimly, then i tried the setting psu to max I 20mA 5V and then i normaly subtract the voltage reading from 5V to get the vf, but that isnt working!! all the sheets i have seen say they should be about 3V but these must be under 1V ????
any ideas? as i need to run these at around 25mA

 

[ronsimpson]

White LEDs probably are 3 volt. Most DMM are made to measure 0.7V so they probably don't make enough voltage to measure white LEDs.
With a bench power supply, you can set the current limit to (small) or 25mA and set the voltage so some thing over 3 volts. The voltage across the LED is vf.
If your supply does not have current limit then use a resistor to limit the current.
Example: If all you have is a 5V supply start out with a resistor that will only allow 25mA (more or less) when all 5 volts are across the resistor. This will give you a very safe way to see the forward voltage. Say vf=3V. Now that you know it is a 3VLED change the resistor so that 25mA flows with 2V across the resistor. Now measure the vf at 25mA.

 

[large_ghostman]

White LEDs probably are 3 volt. Most DMM are made to measure 0.7V so they probably don't make enough voltage to measure white LEDs.
With a bench power supply, you can set the current limit to (small) or 25mA and set the voltage so some thing over 3 volts. The voltage across the LED is vf.
If your supply does not have current limit then use a resistor to limit the current.
Example: If all you have is a 5V supply start out with a resistor that will only allow 25mA (more or less) when all 5 volts are across the resistor. This will give you a very safe way to see the forward voltage. Say vf=3V. Now that you know it is a 3VLED change the resistor so that 25mA flows with 2V across the resistor. Now measure the vf at 25mA.

i tried this way with psu. i set I max 20 mA V=5V and the ocp cut in! its really odd! maybe it's a iffy Led i will try another , normally my DMM measures Leds no problem but not this time!

 

[ronsimpson]

A red LED might have a vf of 1.5V while a white is more like 3V. Some have two dies inside so the vf is 6v.
The OCP should cut in at about 3 volts and 20mA. It is probably working. Was there light?
LEDs are current devices not voltage devices. So setting the OCP at 20mA is a good plan.

 

[large_ghostman]

i will try again later, these are the first smd's to give me trouble!