Ohm's Law and Power in DC Circuits

Ohm's Law and Power in DC Circuits

Every resistive DC circuit obeys two foundational relationships: Ohm's Law and the power formulas. Mastering these lets you predict circuit behavior on paper before you ever connect a wire, and it gives you a benchmark to check your measurements against. Both skills are tested on the ETA CET and ETA FOI/FOT exams, and they are used daily on the job.

Ohm's Law

Ohm's Law states that the voltage across a resistor equals the current through it multiplied by its resistance:

V = I × R

Rearranging gives you two more useful forms:

• I = V ÷ R (find current when you know voltage and resistance)
• R = V ÷ I (find resistance when you know voltage and current)

Units: voltage in volts (V), current in amperes (A), resistance in ohms (Ω). Keep units consistent and the arithmetic stays clean.

Power in a DC Resistive Circuit

Electrical power is the rate at which energy is converted. For a resistive load the three equivalent power formulas are:

• P = V × I
• P = I² × R
• P = V² ÷ R

All three give the answer in watts (W). Choose whichever form uses the two quantities you already know.

Resistor power rating matters on the bench. A 1/4-watt (0.25 W) resistor must never dissipate more than 0.25 W continuously or it will overheat, discolor, and drift in value — or fail open. Always calculate expected power before applying voltage to confirm the resistor can handle the load.

Reading Resistor Color Codes

Standard carbon-film resistors use color bands to mark their value and tolerance. Each color represents a digit (black = 0, brown = 1, red = 2, orange = 3, yellow = 4, green = 5, blue = 6, violet = 7, gray = 8, white = 9). On a four-band resistor, bands 1 and 2 are the significant digits, band 3 is the multiplier (10 raised to that digit's power), and band 4 is the tolerance (gold = ±5%, silver = ±10%). Always verify the color code against a DMM resistance measurement before using a resistor in a precision calculation.

Resistor Tolerance and Measurement Error

A resistor marked 1 kΩ ±5% can actually be anywhere from 950 Ω to 1 050 Ω and still be within spec. That built-in spread is the most common reason a measured result differs from a calculated one. Meter loading — the small amount of current the DMM itself draws from the circuit — is a secondary source of error, usually negligible with modern high-input-impedance meters like the Fluke 115. You will quantify error using percent error:

% Error = |Measured − Calculated| ÷ Calculated × 100

A result within ±5% is considered acceptable for this lab.

Using a DMM Safely and Correctly

Before touching equipment, review these safety habits — they protect both you and the instruments:

1. De-energize before making connections. Set up the breadboard circuit with the power supply off. Apply power only when you are ready to measure.

2. Select the correct function and range first. Setting a DMM to the current (A) function while probing a voltage source can blow the meter's internal fuse and damage the circuit.

3. Voltage measurement: connect the DMM in parallel across the component. Red lead to the more-positive node, black lead to the more-negative node.

4. Current measurement: the DMM must be in series with the resistor. Break the circuit, insert the meter in the break, and confirm the red lead is in the correct current input jack (typically labeled A or mA depending on expected range).

5. Keep power supply voltage within the resistor's design limits. Calculate maximum allowable current (I = 0.25 W ÷ V_supply) before energizing.

6. Lockout/tagout (LOTO) does not apply to low-voltage bench supplies, but the habit of de-energizing before rewiring is the direct precursor to LOTO practice you will use on industrial equipment.

7. Wear safety glasses whenever working at the bench — component leads can spring free.

Setting Up the Bench

Connect your DC power supply output leads to the breadboard power rails using banana-to-alligator leads. Place resistors across the rails or in a series path as your circuit diagram requires. Double-check polarity: red rail positive, blue or black rail negative. Turn on the supply and dial to the specified voltage before measuring. Always record the actual supply voltage from the DMM — bench supplies can drift slightly from their dial setting.

Lab Flow Summary

1. Complete all pre-lab calculations before coming to the bench.

2. Build each circuit with power off.

3. Have your instructor review the circuit before you energize.

4. Measure V and I; calculate R from measurements if needed; compute measured P = V × I.

5. Enter calculated and measured values, then compute % error.

6. For any result outside ±5%, write two sentences naming the likely cause.

7. Call the instructor to initial your lab log before you disassemble your last circuit.