figuring unknown LED milliamps

[ruger9]

I bought some LEDs from Small Bear:
LED T-1 3/4 5 mm Water-clear High-brightness

But they came completely unmarked. I need to know what the ma rating is, so I can figure out which resistor to use.  SmallBear has not answered by email.  Is there a way to tell, with a DMM or something, what an LED is rated for?

[R.G.]

There is no simple way to tell without the manufacturer's number and data what they were rated for.

However, you can make very good guesses. The current rating will be based on the power that the LED can dissipate. That's going to be a relative constant for a given physical package. The T 1 3/4 packages are usually limited to 20ma dc, 100ma pulse by this, so if you have the little round standard LED packages, assume it's 20ma until you get more info.

Given enough time and money, you could measure the LED voltage versus current curve with pulse techniques, and then construct a power dissipation versus current curve, and also measure temperature rise per watt for the package. That would let you get your LED as hot as it can be in that package before frying.

However, with modern high-brightness LEDs, it's unlikely that you will want it up to full current. Start at 20ma and go down.

[stm]

Usually 5mm LEDs are rated for 20mA and 3mm for 10mA max (even though some units may allow currents in excess of 30 to 50mA).

I would recommend a LED current between 2mA and 5mA in your case. See if the amount of light is satisfactory, and settle for the least current which produces decent results, as this will maximize battery life.

Regards,

STM

[amz-fx]

I bought some LEDs from Small Bear:
LED T-1 3/4 5 mm Water-clear High-brightness

Since they are high-brightness,  you should start with a 4.7k resistor and see how that looks to you.  If you need it brighter, go to 2.2k...  if you want to dim it down a little, go to 10k.

regards, Jack

[ruger9]

Thanks, guys!