EE-Quiz Answer

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EE-QUIZ QUESTION 21
OUR ANSWER!

Question 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?

Answer:

• Low wattage bulbs for high voltages require very long, thin filaments which are hard to manufacture and fragile. They need multiple anchor points, making the bulb larger and more expensive. A series string permits the use of low voltage bulbs. For instance: 50 2.5-volt bulbs in series can run on 125 volts.
• With a series string, all bulbs pass the same current and their brightness depends only on bulb-to-bulb characteristics.
• The current through the wire is much higher for a parallel string, and changes along the length of the string. For instance, the wire between the power source and the first bulb must carry all the current for all the bulbs, while the far end of the string carries just the last bulb's current. This means fatter wire and higher costs.
• A series string is somewhat more fault tolerant. If the wires are shorted, the bulbs will limit the current.
• But note that burned out bulbs must be replaced soon. Bulbs have a shunt that kicks in when the filament snaps, so the string stays on, but the total resistance is lowered, meaning current is higher, meaning each bulb is stressed. Soon a second one goes, then a third and a fourth-fifth-sixth- and pffft!

Question 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.)

Answer:

• The compelling reason is that, as with the holiday lights, brightness depends only on bulb-to-bulb characteristics. A parallel string has an especially dangerous attribute: the voltage is a function of distance from the power source, meaning the brightness falls off monotonically along the length of the string. Relative brightness is an important visual cue to distance. So depending on the airplane's direction, the runway will appear either longer or shorter than it actually is. A runway that ends earlier than the pilot expects is not a good thing at all!
• Dave Weigand (who contributed this part of the EE-Quiz question) also mentioned that it is easier to precisely control the brightness of a series string. Air traffic controllers vary the runway lights depending on ambient lighting. As you might imagine, using the same brightness at night as you did at high noon would make the pilot's job difficult.
• And Dave also mentioned that halogen bulbs are used, primarily for their constant brightness throughout their life.

Question 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?

Answer:

• The I-V curve of a diode is quite steep, meaning that a small difference in voltage cause large differences in current. Manufacturing variances in the three LEDs will cause slight differences in their forward voltages but these slight differences are magnified by the steepness of the I-V curve and the brightness of the LEDs will vary considerably.
• But worse is the likelihood of thermal runaway. LEDs have a negative temperature coefficient. If one carries more current it gets hotter. If it gets hotter it carries more current. Soon one LED is hogging all the current and depending on the values involved, it could even burn out.
• It's also more efficient to wire them in series, dropping less of the voltage across the resistor.
• Note that Maxim's LED-drivers use DC-DC converters which push constant current into a LED, or a series string of LEDs, with no need for an energy-wasting series resistor. Examples: The MAX1910 and MAX1912 are white LED power supplies which drive the LEDs by regulating the current.

See:

/max1910

or the whole family of white LED drivers:

http://para.maxim-ic.com/results.mvp?q=whiteled&an_1=Family&av_1=White%20LED%20Drivers

Learn more about driving LEDs in this app note:

Driving LEDs in Battery-Operated Applications: Controlling Brightness and Using Power Efficiently