Other small sources

A solar cell joins P-type semiconductor (electron deficiency) and N-type (electron excess). Photons from light hit the junction; energy knocks electrons across → voltage appears at terminals.

Typical silicon cell in direct sun: ~0.5 V open-circuit per cell; current depends on surface area (bigger cell → more amps).

Schematic symbol: battery cell plus light arrows.

Because one cell is low voltage, arrays connect:

• Series → raise voltage (28 × 0.5 V ≈ 14 V to trickle-charge a 12 V lead-acid bank)
• Parallel → raise current (second 14 V string paralleled doubles charge current)

No light → no output. Storage battery often sits behind the panel for night loads.

Target: 14 V at 0.5 A charge. Cell: 0.5 V, 0.25 A short-circuit per unit.

1. Series count: 14 ÷ 0.5 = 28 cells → 14 V at 0.25 A
2. Need 0.5 A → duplicate 28-cell string in parallel → 14 V at 0.5 A

Real installs add charge controller, blocking diode, fuse, and temperature compensation — but the series adds V / parallel adds I rule is the same as batteries.

When two dissimilar metals join at one end and that junction is heated while the other ends stay cooler, a voltage appears — Seebeck effect. Device = thermocouple.

Voltage depends on:

1. Metal pair (Type J, K, T, etc.)
2. Temperature difference between hot junction and reference (cold) ends

Output is tiny — millivolts — not volts. Example: Type J might produce ~7.9 mV at 300 °F.

Polarity can flip below a reference temperature for some types — know the curve if you interpret sign.

Thermopile = several thermocouples in series to sum millivolts into enough EMF to drive a control.

Classic use: gas pilot flame heats the junction → current holds a solenoid valve open. Flame out → no EMF → valve closes → gas stops. That is why you hold “pilot” until the thermocouple proves heat — you are charging the magnetic hold circuit, not “warming the pipe.”

Same physics powers high-temperature measurement transmitters when paired with cold-junction compensation.

Piezo (Greek: pressure): certain crystals generate voltage when compressed, bent, twisted, or stretched. Reverse also holds — apply voltage → mechanical motion ( buzzers, ultrasonic transducers).

Historical example: phonograph stylus of Rochelle salt riding grooves → audio signal.

Field examples today:

• Piezo igniters on grills and lighters (sharp strike → kilovolt-scale pulse across gap)
• Vibration sensors and ultrasonic devices
• Energy harvesting on micro scales

Output is pulse or AC-like — not a steady DC bus without conditioning.

All differ from chemical cells (fixed chemistry EMF) and from generators (magnetic induction at scale — next lessons).

• Solar: use charge controller; prevent reverse current into dark panels at night.
• Thermocouple: run correct alloy wire all the way to instrument — copper extension wrong for precision Type R/S/B.
• Piezo: high impedance and spike — don’t expect to run a motor directly.

Treat each as a sensor or trickle source unless engineered as a power system.

Situation. Rooftop telecom cabinet keeps dropping offline at night. Daytime solar charges; single 12 V gel looks fine at noon. Tech adds a second panel in parallel but wires only positive buses — negatives still one path through a thin jumper.

Result. Strings fight; fuse blows at dusk; battery never reaches float.

Applied lesson. Solar parallels need matched Vmp strings and equal cabling — same discipline as battery parallel banks. Verify charge controller sizing for combined Isc.