Circuit Components

Part of Basic Electrical Circuits

The essential building blocks of electrical circuits—what each component does, how to identify it, and how to fabricate basic versions from available materials.

Why This Matters

Every electrical circuit is assembled from a small set of fundamental components. Understanding what each component does, how to test whether it is working, and—critically—how to fabricate substitutes from available materials is foundational knowledge for rebuilding electrical technology.

In a post-collapse environment, commercial components will be scarce and eventually exhausted. The community that can make functional resistors from carbon paste, capacitors from glass and metal foil, inductors from wound wire, and switches from metal springs is not dependent on supply chains. This knowledge bridges the gap between scavenging salvaged parts and building original electrical infrastructure.

Each component described here has been manufactured using pre-industrial methods at some point in history. The same principles apply today.

Resistors

Function: Limit current flow and create predictable voltage drops.

How they work: Resistance arises from collisions between electrons and the atomic structure of a material. Longer, thinner conductors have higher resistance; shorter, thicker conductors have lower resistance.

Identifying salvaged resistors: Through-hole resistors use color bands:

Color	Value
Black	0
Brown	1
Red	2
Orange	3
Yellow	4
Green	5
Blue	6
Violet	7
Gray	8
White	9

First two bands are digits; third band is the multiplier (number of zeros); fourth band is tolerance (gold = ±5%, silver = ±10%).

Fabricating resistors:

• Carbon rod resistors: Cut pencil graphite to length. Longer = higher resistance, thinner = higher resistance. Attach copper wire leads with conductive paste (graphite powder mixed with glue) or mechanical clamping.
• Wire-wound resistors: Wind nichrome wire (from heating elements) or resistance wire around a ceramic former. Calculate turns needed using the wire’s resistance per unit length.
• Carbon paste resistors: Mix fine graphite powder with shellac or linseed oil. Paint onto ceramic or glass substrate between two copper electrodes. Resistance adjustable by varying the path length and paste thickness.

Testing: Use two points of a galvanometer-based ohmmeter. Replace any resistor showing infinite resistance (open circuit) or near-zero resistance (short circuit).

Capacitors

Function: Store charge temporarily; block DC while passing AC; filter voltage fluctuations.

How they work: Two conductors separated by an insulator create an electric field when charged. Energy is stored in the field, not as chemical energy.

Types and uses:

• Bypass/decoupling capacitors: Placed across power supply lines to absorb transient spikes
• Coupling capacitors: Pass AC signals between circuit stages while blocking DC bias
• Timing capacitors: Charge through a resistor at a rate that controls timing circuits
• Motor-start capacitors: Create phase shift to start single-phase AC motors

Fabricating capacitors: See the Capacitors deep dive for detailed construction methods.

Testing: A functional capacitor will briefly pass current when voltage is applied, then block it. A dead capacitor either always blocks (open) or always passes (shorted dielectric).

Inductors and Coils

Function: Oppose changes in current; store energy in a magnetic field; form part of filters and oscillators.

How they work: Current through a coil creates a magnetic field. When current changes, the field changes, which induces a voltage opposing the change (Lenz’s Law).

Inductance is measured in Henries (H). Practical inductors range from microhenries (radio work) to henries (audio filters, power supplies).

Fabricating inductors:

1. Choose a former: air core (no former), ferrous metal core (increases inductance), or ferrite core (salvaged from electronics)
2. Wind insulated copper wire in close turns
3. More turns = higher inductance; smaller diameter = lower inductance
4. For power-frequency work, use iron cores to keep size manageable
5. For radio frequency work, use air-core coils to avoid core losses at high frequency

Practical coil winding:

• Insulate wire by dipping in shellac or beeswax and allowing to dry before winding
• Wind in one direction only; reversing direction cancels inductance
• Count turns carefully—inductance scales with turns squared

Transformers

Function: Change AC voltage levels while transferring power magnetically; provide electrical isolation.

How they work: Two coils wound on a shared iron core. AC in the primary coil creates a changing magnetic field; the secondary coil picks up this field and generates an output voltage proportional to its turns ratio.

Turns ratio: V_out / V_in = N_secondary / N_primary

Fabricating small transformers:

1. Build or salvage a laminated iron core (thin iron or steel sheets, alternately stacked)
2. Wind primary coil with appropriate turns for input voltage
3. Wind secondary coil with turns proportional to desired output voltage
4. Insulate layers with oiled paper or waxed cloth
5. Clamp core tightly to minimize magnetic air gaps

Switches and Relays

Function: Switches manually open or close circuits; relays allow low-power circuits to control high-power ones.

Fabricating basic switches:

• Knife switch: A flat metal strip pivoting between two contact posts. Used in early electrical installations and still adequate for low-frequency switching
• Push-button switch: A spring-loaded metal plate that bridges two contacts when depressed
• Slide switch: A sliding metal conductor that connects different contact positions

Materials: Use copper, brass, or salvaged steel spring strip for contacts. Silver is the ideal contact material (low resistance, resists oxidation) but copper works if contacts are kept clean.

Contact cleaning: Oxidized contacts cause high resistance. Clean with fine abrasive (leather strop, fine sandstone) or treat with a drop of mineral oil to exclude air.

For relay construction, see Relay Construction.

Fuses and Overcurrent Protection

Function: Protect circuits from excessive current by deliberately failing first.

How they work: A thin conductor melts when current exceeds a threshold, breaking the circuit.

Fabricating fuses:

1. Determine the fuse rating needed (slightly above normal operating current)
2. Select a wire whose melting point matches: fine copper wire, tin, or lead-tin alloy
3. For a 1A fuse, use approximately 30 AWG (0.25 mm) copper wire
4. Enclose in a glass tube or ceramic holder to contain any arc when it blows

Wire fuse guide (copper):

Wire diameter (mm)	Approximate fuse rating
0.1 mm	0.5A
0.2 mm	1A
0.4 mm	3A
0.6 mm	6A
1.0 mm	15A

Never replace a blown fuse with a larger rating without finding and fixing the cause of the overcurrent.

Diodes and Rectifiers

Function: Allow current flow in one direction only; convert AC to DC.

Primitive rectification: Electrolytic rectifiers (aluminum and lead in a borax or ammonium phosphate solution) were used before semiconductor diodes. Current passes easily in one direction, is blocked in the other. Efficiency is low but adequate for battery charging.

Copper oxide rectifiers: Oxidized copper discs act as one-way valves for current. Stack multiple discs in series for higher voltage operation. These were manufactured commercially through the mid-20th century and can be fabricated with basic metalworking.

Identifying salvaged diodes: The stripe or band marks the cathode (the end current exits toward the positive terminal in normal operation). A diode conducts when the anode (+) is at higher voltage than the cathode (−).

Component Testing Summary

Component	Good test	Failed open	Failed short
Resistor	Expected resistance	Infinite resistance	Near-zero resistance
Capacitor	Brief current pulse	No current at all	Sustained current
Inductor	Low DC resistance	Infinite resistance	Same as good
Diode	Conducts one way	No conduction either way	Conducts both ways
Fuse	Zero resistance	Any resistance	—

Systematic testing of each component before installation saves significant troubleshooting time and protects functioning components from damage caused by a single defective part.