Many of us have assorted LEDs in bags, boxes, drawers and old circuit boards. With clear packaged devices we can’t even tell what colour they are! Without markings a simple means of testing the LEDs is required. Here is a safe test method for most LED types.
LEDs require a current limited power supply. A 5 V supply is adequate for most LEDs and is low enough not to destroy the LED if connected in reverse. A series resistor is required to limit the current to a safe value – but more on this point later.
LEDs are diodes so polarity of supply voltage is important. If the LED doesn’t light reverse the polarity and try again.
Drawing a resistor loadline on the LED curves allows us to quickly estimate the current through each LED colour. e.g., For a yellow LED, find where the yellow line crosses the loadline and draw a line to the current axis on the left. On this we see that it will draw 15 mA.
A higher value resistor can be used but will result in less current. Most LEDs – other than infrared and real UV LEDs – will be light reasonably brightly at 5 mA. You can calculate a suitable resistance for your supply voltage with the information above.
LED forward voltage is dependent on color. Measuring as shown above should give you a forward voltage reading close to that anticipated on the loadline chart. Note that many “white” LEDs are really blue or UV LEDs with phosphors that re-emit in the visible spectrum. You can use this technique to figure out what method the LED is using to generate the white light.
Most small 3 mm and 5 mm LEDs can handle 20 mA without difficulty. There is no simple method to determine the maximum current from an unkown LED other than to test some samples to destruction.
