As the holiday season approaches, we begin to see houses and trees decorated with strings of lights. An engineer's mind, though, never stops thinking. So what about those lights? This EE-Quiz deals with series and parallel light strings.

Questions:

1. A string of lights can be in series or parallel. The greatest advantage of a parallel string is that a bulb can burn out and the others remain lit. But the majority of inexpensive light strings are wired in series. Why? How many reasons can you come up with?

2. Higher wattage lights are generally in parallel, but what about really high-powered strings? Really high-powered lights?

Airport runways are lit by a row of bulbs on each side. They are wired in series, fed by a constant-current supply that pushes (typically) 20 amps. High intensity systems may use 300W bulbs and there are typically 330 per side. If a bulb fails, a shunt device shorts the bulb. For a 330-lamp string, they're feeding about 5kVAC!

Why are they using a series string? (There are several reasons, but one reason is especially compelling.)

Figure 1

3. In trying to save component costs, a junior engineer wires six LEDs in parallel, using a single current-limiting resistor (Figure 1). Why is this a bad idea?

— Question 2 and inspiration for the series lighting theme came from EE-Quiz reader Dave Wiegand.

Answer