Series DC Circuits

Series DC Circuits

What Is a Series Circuit?

A series circuit has only one path for current to flow. Every component—resistors, lamps, or other loads—is connected end to end along that single path. Because there is only one path, the same current passes through every component. If you break the path anywhere, current stops flowing everywhere.

This single-path rule has a practical consequence you will see in the fault-insertion activity: one bad connection knocks out the whole circuit. Understanding why this happens—and how to find the break quickly—is a core electrician skill.

Total Resistance in a Series Circuit

When resistors are wired in series, their resistances simply add together:

R_total = R₁ + R₂ + R₃ + … + Rₙ

For example, three resistors of 100 Ω, 220 Ω, and 330 Ω in series give a total resistance of 650 Ω. Adding more resistors always increases total resistance, which reduces current.

Ohm's Law Review

Ohm's Law relates voltage (V), current (I), and resistance (R):

V = I × R

Rearranged: I = V ÷ R and R = V ÷ I

In a series circuit you use total resistance and source voltage to find the single current value shared by every component:

I_total = V_source ÷ R_total

Once you know current, multiply by each individual resistance to find the voltage drop across that resistor.

Kirchhoff's Voltage Law (KVL)

Kirchhoff's Voltage Law states: the sum of all voltage drops around a closed loop equals the source voltage.

In equation form:

V_source = V₁ + V₂ + V₃ + … + Vₙ

This law is not an approximation—it is an exact relationship derived from conservation of energy. Every volt the source pushes into the circuit must be accounted for by voltage drops across the loads. During your lab measurement you will verify KVL by summing five measured drops and comparing the result to what the bench supply reads. A match within ±5% confirms both your measurements and your wiring.

Voltage Divider Concept

Because all components carry the same current, a larger resistor drops a larger share of the source voltage. A resistor that is twice as large as another will drop exactly twice the voltage. This proportional sharing is called the voltage divider effect and is used deliberately in many control and sensing circuits.

Open-Circuit Faults in Series Circuits

An open circuit is a broken path—a loose wire, a burned-out component, or a lifted terminal. With the path broken:

• Current drops to zero throughout the entire circuit.
• The full source voltage appears across the open point. The DMM reads source voltage at the break because one probe sees the positive rail through zero resistance and the other probe sees the negative rail through zero resistance—there is no current path to create drops elsewhere.
• Every intact resistor reads 0 V because no current flows through it.

You locate an open by moving the DMM probes systematically—either node to node or component to component—until you find the one point that reads source voltage instead of zero. Document every reading and every step in your lab log. A professional leaves a written record of their diagnostic process.

Using the Digital Multimeter (DMM) Safely

Before every measurement:

1. Inspect leads for cracked insulation or bent probes. Do not use damaged leads.

2. Set the function and range before connecting probes. For voltage in this lab, set DC volts to a range above your supply voltage.

3. Turn off the supply when changing wiring. Turn it back on only to take readings.

4. Never exceed the DMM's rated input voltage. Bench supplies in this lab are low-voltage DC; still confirm the range setting before touching probes to terminals.

5. Keep your other hand at your side or in your pocket when probing—a single-hand habit limits current paths through the body.

Lockout/tagout (LOTO) procedures apply whenever de-energizing equipment for wiring changes, even on low-voltage trainers. Follow your instructor's posted LOTO procedure for the bench supply.

Personal Protective Equipment (PPE)

For low-voltage DC bench work, safety glasses are required at all times in the lab. The risk of a component failure producing a fragment or a sudden arc, though small, is real. Rings and metal watchbands must be removed or covered before working near any energized terminals.

Lab Log Requirements

Your lab log is a professional document. Record:

• The resistor values you selected and measured
• Calculated values before you build
• Measured values after you energize
• The percent difference between calculated and measured
• Every DMM reading taken during the fault-insertion activity, in the order you took them, with a one-sentence conclusion at each step

Accuracy target: calculated vs. measured within ±5%. If your results fall outside this range, check for a wiring error or a resistor that differs significantly from its marked value before rebuilding.