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E6F

CIRCUIT COMPONENTS

Electro-optical technology: photoconductivity; photovoltaic devices; optical sensors and encoders; optically isolated switching

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E6F011 of 11

What absorbs the energy from light falling on a photovoltaic cell?

Why In a photovoltaic cell, incoming light arrives as photons, and each photon's energy is transferred to an electron in the semiconductor's valence band. If the photon energy exceeds the material's bandgap, the electron absorbs it and jumps to the conduction band, leaving behind a hole. The junction's built-in field then separates that electron-hole pair, and the moving electrons form the cell's output current.
Watch out Photons are tempting because they are the light itself, but they are the carriers that deliver the energy, not the thing that absorbs it; holes are created as a byproduct of the absorption, and protons are locked in the nuclei and play no part.
Photons deliver, electrons receive. The particle that moves the current is the one that soaked up the light.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E6F022 of 11

What happens to photoconductive material when light shines on it?

Why Photoconductive materials are semiconductors whose valence electrons can be knocked into the conduction band by incoming photons. More light means more free electron-hole pairs, which means more charge carriers available to carry current, so conductivity rises and resistance falls. That is exactly how a CdS photocell (light-dependent resistor) in a streetlight controller works: dark means high resistance, bright means low resistance.
Watch out The choice saying resistance increases has it backwards, and reflectivity is a surface optical property, not what photoconductivity describes.
Photo-CONDUCTIVE: light raises conductivity, so resistance goes down. More light, less ohms.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E6F033 of 11

What is the most common configuration of an optoisolator or optocoupler?

Why An optoisolator is just a light source and a light detector sealed in one opaque package. The input signal drives an LED, its light crosses a small transparent gap, and a phototransistor on the other side conducts in response. Because the only coupling is light, the input and output circuits share no electrical connection, so the device can pass a control signal while blocking hundreds or thousands of volts of potential difference and breaking ground loops.
Watch out The helium-neon laser choices describe lab-grade optical sources, far too bulky, fragile and expensive for a part costing a few cents; photomultipliers are high-gain detectors for extremely faint light, not for signal isolation.
Optoisolator = LED in, phototransistor out: light crosses the gap, voltage does not.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E6F044 of 11

What is the photovoltaic effect?

Why The photovoltaic effect is the direct generation of electrical energy (a voltage and current) when light strikes a material, typically a semiconductor PN junction. Photons with enough energy free electron-hole pairs, and the junction's built-in field separates them, producing a potential across the terminals. This is the principle behind solar cells, which need no external bias to produce power.
Watch out The choice about emitting light when voltage is applied describes an LED, which is the reverse process (electroluminescence). The voltage-to-current conversion wording is just electrical, with no energy source created from the light.
Photo = light, voltaic = voltage: light in, voltage out. Solar cell, not LED.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E6F055 of 11

Which of the following describes an optical shaft encoder?

Why An optical shaft encoder converts mechanical rotation into electrical pulses. A slotted or patterned disc is mounted on the shaft and spins between an LED and a phototransistor, so the light beam is alternately passed and blocked as the shaft turns. Counting those pulses gives rotation amount, and a second offset track (quadrature) tells the circuit which direction the shaft is turning. This is how VFO tuning knobs on modern radios work, with no electrical contact to wear out.
Watch out Measuring light strength with an A/D converter describes an optical power meter or light sensor, not an encoder, which cares about interruption of the beam over time rather than its intensity.
Encoder = slotted wheel chopping a light beam: chop, chop, chop = count the turns.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E6F066 of 11

Which of these materials is most commonly used to create photoconductive devices?

Why Photoconductivity happens when incoming photons carry more energy than a material's bandgap, knocking electrons from the valence band into the conduction band and creating electron-hole pairs that lower the material's resistance. Only semiconductors have a bandgap in the right range for visible and near-infrared light, so crystalline semiconductors such as cadmium sulfide, silicon and germanium are what photocells, photodiodes and phototransistors are made from. Metals already have free electrons, and insulators have gaps too wide for light to bridge.
Watch out Argon is a noble gas used in discharge lamps and welding shielding, not a solid-state light sensor, and the plastic-sounding compound is not a photoconductor at all.
Light in, resistance down - that only works in a semiconductor with a bandgap. Think CdS photocell.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E6F077 of 11

What is a solid-state relay?

Why A solid-state relay does the job of an electromechanical relay, switching a load on and off under control of a separate input signal, but with no moving parts. Inside, the control input typically drives an LED that optically couples to a phototransistor or photo-triac, which in turn gates a power transistor, MOSFET or TRIAC to switch the load. The optical coupling gives the same input-to-output isolation that a relay coil and contacts provide mechanically, which is why this topic sits with optoelectronics.
Watch out The choice about transistors driving a relay coil describes an ordinary relay driver circuit; there is still a real coil and real contacts, so it is not solid state. The latching mechanical relay is just a different style of electromechanical relay.
Solid state = no moving parts. Same relay function, semiconductors and an optocoupler instead of coil and contacts.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E6F088 of 11

Why are optoisolators often used in conjunction with solid-state circuits that control 120 VAC circuits?

Why An optoisolator couples a signal using light: an LED shines onto a phototransistor or photo-triac inside a sealed package, so the only path between input and output is optical. That means the low-voltage logic or microcontroller side shares no conductive path with the 120 VAC mains side, and isolation ratings of several kilovolts protect the control circuitry and the operator from line voltage and surges.
Watch out The impedance-related choices get it backwards: the whole point is a very high impedance (essentially infinite DC resistance) between the two sides, not a low-impedance link or impedance matching, which is a job for transformers or matching networks.
Opto = light, and light carries signal but not current. No copper path means no shock path.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E6F099 of 11

What is the efficiency of a photovoltaic cell?

Why Efficiency for any energy converter is useful output power divided by input power, and for a photovoltaic cell the input is the optical power falling on it and the output is electrical. So efficiency is the fraction of the incident light energy that ends up as electrical current and voltage in the load. Typical silicon cells run roughly 10 to 20 percent, meaning most of the sunlight is lost as heat or passes through unabsorbed.
Watch out Open-circuit voltage divided by short-circuit current is just a resistance in ohms, not an efficiency, and it uses two conditions where the cell delivers no power at all. Lumens per watt describes a light source's luminous efficacy, the opposite direction of energy conversion.
Efficiency is always power out over power in. For a solar cell, in is light and out is electricity.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E6F1010 of 11

What is the most common material used in power-generating photovoltaic cells?

Why Commercial solar panels are overwhelmingly built from crystalline silicon, either single-crystal or polycrystalline wafers doped to form a large-area PN junction. Silicon is cheap, abundant, non-toxic, and its bandgap of about 1.1 eV is well matched to the solar spectrum, giving a typical cell output around 0.5 V in sunlight. The same semiconductor technology used for transistors and ICs scales up nicely to wafer-sized power cells.
Watch out Selenium and cadmium sulfide were used in early photocells and light meters, and cadmium telluride and indium compounds appear in specialty or thin-film cells, but none of them dominate power generation the way silicon does.
Solar panels are just big silicon junctions: about 0.5 V per cell, 1.1 eV bandgap.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E6F1111 of 11

What is the approximate open-circuit voltage produced by a fully illuminated silicon photovoltaic cell?

Why A silicon solar cell is essentially a large-area PN junction; when light frees electron-hole pairs, the junction field separates them and builds a forward voltage that self-limits at roughly half a volt. Typical open-circuit voltage for a fully illuminated silicon cell is about 0.5 to 0.6 volts, which is why panels string many cells in series to reach 12 or 24 volt levels.
Watch out The 0.7 volt choice is the forward voltage drop of a silicon diode when it is conducting current, not the open-circuit output of an illuminated cell, and 1.1 volts is really the silicon bandgap energy in electron volts, while 1.5 volts is an alkaline dry cell.
Silicon cell: half a volt each, so ~36 cells make a 12 V panel. 0.7 V is a diode drop, 1.1 is the bandgap.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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