circuit means it will spend a lot of time in the linear region, which
leads to a lot of heat and the ultimate destruction of the transistor.
COLD-CATHODE FLUORESCENT LAMPS
QWhat is a cold-cathode fluorescent lamp (CCFL)?
Is there a significant difference between it and the
normal fluorescent lamp regarding theory of start-up
and operation, efficiency, and lifetime?
■ M. John, via Internet
AExcept for size and shape, they are identical — with the
exception of the cold-cathode feature. Your standard
40-watt T- 8 lamp has a filament at both ends of the four-foot tube. When the lamp is started, the filament(s)
is turned on momentarily to vaporize a small bead of
mercury placed at the end of the tube. A CCFL lamp, on the other hand,
has no filament. Instead, it relies on a high-voltage surge — typically
1,000 volts — to “spark” the residual mercury vapor to life.
Once the gas is ignited, it turns into plasma, which changes the
lamp from an insulator to a conductor that emits ultraviolet light. A
thin layer of fluorescent material coats the inside of
the tube — a mineral that glows when activated by an ultraviolet
source. Different minerals glow different colors.
The mercury tube now becomes a negative-resistance device
that is better described as akin to a zener diode. The lamp is now a
20 NUTS & VOLTS November 2005
current operated device where the amount of current determines
the brightness; the operating voltage drops down to about 200 to 300
It used to be that CCFL tubes were most often found in laptop
computers as backlighting for the liquid-crystal display (LCD). That
role is now being filled by ever-increasingly efficient LEDs. LEDs have
a life span of 100,000 hours, as opposed to the 5,000 hours of a CCFL
lamp. The new niche for CCFL devices is as decorative lighting
inside PC cabinets and under the hoods of Toyotas.
Fortunately, the technology is well advanced and the components
are easily found on the surplus market; All Electronics (888-826-5432;
www.allelectronics.com) has a good selection at bargain prices. What
you need to play with CCFL tubes is a small DC-to-AC inverter and the
tube itself. Don’t be tempted to make the inverter yourself — it will cost
you more. They sell for less than $10, CCFL included.
NYQUIST SAID IT FIRST
QI have heard that there is a way to be able to read
higher frequencies on an oscilloscope than those for
which the oscilloscope was built. I was wondering
what circuit or devices are needed to accomplish this
■ Dustin Enns, Lehigh, KS
AWhat you are talking about is sampling. That is, you
sample small parts of a series of waves to reconstruct
them. Confused? Look at Figure 5. Most scopes have
two different sampling rates (modes) depending on
the signal being measured: real time and equivalent
time sampling — often called repetitive sampling. However, sampling
only works if
you’re measuring is stable
by building up
from successive measurements. When
sampling frequency must
be greater than
bandwidth of ■ FIGURE 5
the input signal in order to be able to perfectly reconstruct the original from the sampled version.
Otherwise, the frequencies will overlap — a condition called
aliasing — where the reconstructed waveform has nothing at all in
common with the original signal. The aliasing failure was first
formulated by Harry Nyquist in 1928 and is (duh) known as the
Nyquist theorem. For an in-depth discussion on sampling and other
oscilloscope-related facts, read Tektronix’s XYZs of Oscilloscopes
that you can find on the Nuts & Volts website ( www.nutsvolts.com)
under XYZs_OSCILLOSCOPE.PDF. ■