I have a small bench power supply that I got from Circuit Specialists. Model: CSI1802X. I start by turning the current knob all the way down and the voltage knob gets turned up. I hook up the LED to the supply and turn the current slowly up to 20 milliamps (this is my assumption for all LEDs, but might not be the case). If there is no current I swap the leads. At twenty milliamps the forward voltage is drawn and I read the value from the display.
chris I.
Every LED I have ever seen does fine at 10 mA. All of "T1" (3 mm) size or larger in my experience do OK at any current 10-20 mA. Some are bright enough at lower currents.
If the LEDs have leads with unequal length, in my experience about 99% of the time the longer lead is positive.
In my experience about 99% of the ones with "bullet" packages (T1/3 AKA mm or T1-3/4 AKA 5 mm) with a flange and a flat spot in the flange have that flat spot being being negative.
Determining polarity by seeing which lead the chip is mounted on - I have seen enough red LEDs go either way, most other visible ones have the chip mounted on the negative lead.
I have yet to damage an LED with reverse polarity at 5 volts or less.
Voltage drop at 20 mA is usually close enough to 1.85 volts for red, 2 volts for orange and yellow, and 2.1 volts for yellowish green, and 3.5 volts for non-yellowish green, blue-green, blue, violet, UV, purple, pink and white. Give or take a few tenths of a volt.
Use 4.5 or 5 volts and a 100 to 220 ohm resistor, see what happens...
- Don Klipstein ( snipped-for-privacy@misty.com)
In my experience, most "keychain" LED flashlights have lithium "coin" cells and no resistors. It does appear to me that with fresh cells the LEDs get noticeably overpowered, but the survival rate is impressively high. I suspect the internal resistance of lithium coin cells limits current enough for the LEDs to last long enough to avoid noticed problems with cooked LEDs.
- Don Klipstein ( snipped-for-privacy@misty.com)
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.