How to Measure the Forward Voltage of an LED

66 Hacking Electronics

(a) (b)

(c) (d)

Finally, solder the resistor across the two topmost leads of the LM317 (Figure 4-14d).

Figure 4-14 Making an emergency 1-W LED light

How to Measure the Forward Voltage of an LED

If you want to power lots of LEDs at the same time, it is a good idea to test a few of the LEDs you actually intend to use and measure the forward voltage at the current you intend to use. Figure 4-15 shows how you would do this.

Figure 4-15a shows the schematic diagram. A variable resistor is used to vary the current through the LED and when the desired forward current is set, the voltage can be read.

The current and voltage do not have to be read at the same time, but if you do have two meters, this does make life easier.