Nuts and Volts - February 2018

rating as can be see in Figure 10, where the 2.5W bulb is barely glowing relative to the 1W bulb.

Should we ask the manufacturer for a refund? No, sorry. Bulb wattage ratings assume parallel connections. The same holds for incandescent light bulbs we use in our homes at 110V AC. A 100W bulb, for example, would glow dimmer than a 60W bulb in a series AC circuit.

The LEDs

We’re now well equipped to handle the LED circuit.

We’d still like to know what will happen if the circuit in

Figure 2 was wired up (as shown in Figure 11). Let’s approach it as a research scientist and look straight to the I-V curves for a red and a blue LED, shown in Figures 12 and 13. Here, the black dots are the actual I-V data points, and the straight lines are trend lines to aid the eye.

Since LEDs are diodes, we see a typical “diode” I-V curve: no significant current for a few volts, then suddenly a rather rapid “turn on” and surge of current that gets comparatively very large for small increases in the driving voltage. Looking carefully, we see that the red LED turns on (starts to draw current) around 1.6V, and the blue at around 3V. To figure out the LED circuit in Figure 2, we need to study the I-V curves carefully, and keep in mind that the two LEDs are wired in parallel; meaning the voltage across each will always be the same.

We know the blue LED won’t turn on until 3V develops across it, and if we look at the red LED’s I-V curve, by 3V. Not only would it be on, it would be consuming around 0.1A. This is quite a large potentially damaging current for a miniature LED.

When running this circuit, the red LED was quite bright already by 2.5V, and I could only begin to see the faintest light emerging from the blue LED at about 2.7V. Owing to the steep slope of the red LED’s “on” curve, however, small increases in driving voltage led to rather large increases in current. For the sake of the red LED, continually upping the driving voltage trying to reach 3V would not be a good idea. Thus, the circuit behavior in Figure 2 is best described as a case where — within reasonable operating specifications — the red LED will light, but the blue will not.

I hope you see the advantage of the I-V curves for something “exotic” like an LED. The steep slope of the I-V curve is readily revealed, helping to explain why the blue LED in Figure 11 stays off unless we make the red LED really sweat.