Page 87 - Troubleshooting Analog Circuits

74 6. Understanding Diodes and Their Problems

curve or list any realistic typical values; the sheets list only the worst-case values.
Therefore, you may not realize that the VF of an LED in an opto-isolator is a couple
hundred millivolts smaller than that of discrete red or infrared LEDs. Conversely, the
VF of high-intensity, or high-efficiency, red LEDs tends to be 150 mV larger than
that of ordinary red LEDs. (Refer to Appendix E.) And the VF of DEADs (a DEAD is
a Darkness Emitting Arsenide Diode; that is, a defunct LED) is not even defined.
Once I was troubleshooting some intenuptor modules. In these modules, a gap
separated an infrared LED and a phototransistor. An intenuptor-say a gear tooth-
in the gap can thus block the light. I tested one module with a piece of paper and
nothing happened-the transistor stayed ON. What was that again? It turned out that
the single sheet of paper could diffuse the infrared light but not completely attenuate
it. A thin sheet of cardboard or two sheets of paper would indeed block the light.

Solar Cells

Extraneous, unwanted light impinging on the pn junction of a semiconductor is only
one of many tricky problems you can encounter when you try to design and operate
precision amplifiers-specially high-impedance amplifiers. Just like a diode’s pn
junction, a transistor’s collector-base junction makes a good photodiode, but a tran-
sistor’s plastic or epoxy or metal package normally does a very good job of blocking
out the light.
When light falls onto the pn junction of any diode, the light’s energy is converted
to electricity and the diode forward biasses itself. If you connect a load across the
diode’s terminals, you can draw useful amounts of voltage and current from it. For
example, you could stack a large number of large-area diodes in series and use them
for recharging a battery. The most unreliable part of this system is the battery. Even if
you never abuse them, batteries don’t like to be discharged a large number of cycles,
and your battery will eventually refuse to take a charge. These days one reads all
sorts of marvelous hype about battery-powered cars, but the writers always ignore the
terrible expense of replacing the batteries after just a few hundred cycles. They seem
to be pretending that if they ignore that problem, it will go away . . . .
So much for the charms of solar-recharged batteries. It’s much better to use a
solar-powered night-light. Remember that one? A solar-powered night-light doesn’t
need a battery; it simply needs a 12,000-mile extension cord. To be serious, the most
critical problem with solar cells is their packaging; most semiconductors don’t have
to sit out in the sun and the rain as solar cells do. And it’s hard to make a reliable
package when low cost is-as it is for solar cells-a major requirement.
In addition to packaging, another major trouble area with solar cells is their tem-
perature coefficients. Just like every other diode, the VF of a solar cell tends to de-
crease at 2 mV/T of temperature nse. Therefore, as more and more sunlight shines
on the solar cell, it puts out more and more current, but its voltage could eventually
drop below the battery’s voltage, whereupon charging stops. Using a reflector to get
even more light onto the cell contributes to this temperature-coefficient problem.
Cooling would help, but the attendant complications rapidly overpower the original
advantage of solar cells’ simplicity.

Assault and Battery
Lastly, I want to say a few things about batteries. The only thing that batteries have in
common with diodes is that they are both two-terminal devices. Batteries are compli-
cated electrochemical systems, and large books have been written about the charac-