Voltage, current, and resistance
Relate electrical push, flow, and opposition to choose a current-limiting resistor and predict node behavior in a low-voltage build.
- Apply Ohm's law and power equations with explicit polarity and current direction to size a resistor for a stated operating point.
Understand
The working idea
Voltage is energy difference per unit charge between two nodes. It is always measured relative to a reference: saying a node is “5 V” usually means 5 V above the named ground. Current is the rate of charge flow through a path. Resistance describes how strongly a component opposes current in a regime where the relationship is approximately linear. A useful water analogy is pressure and flow, but wires do not “use up current” and voltage is not a substance stored at a point.
For an ohmic resistor, V = IR. Any form—V = IR, I = V/R, or R = V/I—describes the same relationship with a declared voltage polarity and current direction. Electrical power is P = VI; substituting Ohm's law gives P = I²R or P = V²/R. Power becomes heat, light, motion, sound, or stored energy. A resistor must meet both resistance and power requirements.
Consider an indicator LED on a 5 V controller. If its forward drop is approximately 2 V at the desired 10 mA, the resistor must drop about 3 V. R = 3 V / 0.010 A = 300 Ω, so a nearby 330 Ω standard value gives slightly less current. Its modeled power is I²R, only about 0.03 W, so a common 0.25 W resistor has margin. This resistor choice protects both LED and output pin.
A frequent misconception is that a resistor forces a fixed current regardless of supply or load. It sets current only through the surrounding voltage relationship. Another failure is placing an LED straight across a supply because its “voltage is 2 V”; the LED's nonlinear current can rise destructively. State polarities, estimate component power, and remember that real sources, LEDs, motors, and screens do not all behave as constant resistances.
Follow the reasoning
Worked example
Choose a series resistor for a red LED modeled with a 1.9 V forward drop, powered from 3.3 V at a target of 6 mA, and calculate resistor power.
- 1
Resistor voltage is 3.3 V − 1.9 V = 1.4 V.
- 2
Required resistance is 1.4 V / 0.006 A = 233 Ω.
- 3
Choose the next common value 240 Ω; modeled current is 1.4 V / 240 Ω = 5.83 mA.
- 4
Resistor power is (0.00583 A)^2 × 240 Ω = 0.0082 W.
A 240 Ω resistor gives about 5.8 mA and dissipates roughly 8 mW in this model.
Sanity checkThe current is slightly below target because the selected resistance is slightly higher, and 8 mW is far below a 0.25 W resistor rating.
Try it yourself
Independent check
Move to the bench
Predict and measure how series resistance changes LED current and brightness.
- 2×AA battery holder
- red LED
- 220 Ω resistor
- 1 kΩ resistor
- breadboard
- multimeter
- jumper wires
Experiment
- Measure the unpowered resistor values and record actual readings before assembly.
- Build the LED with 1 kΩ in series, then power it and measure supply and resistor voltage in parallel.
- Calculate current from measured resistor voltage divided by measured resistance; compare with the expected order of milliamperes.
- Remove power, substitute 220 Ω, predict the direction and approximate factor of current change, then measure again.
- Deliberately reverse the LED with power removed; repower briefly, diagnose the dark LED from polarity rather than bypassing the resistor, then restore it.
Observe
- Expected: 220 Ω produces more current and usually greater brightness than 1 kΩ; actual voltages and calculated currents must be reported.
- Expected fault clue: a reversed LED remains dark while supply voltage is present, distinguishing polarity from an absent supply.
For a 5 V camera status LED with a stated 2.2 V drop and 4 mA target, select a resistor and check its modeled power.
- Use only the battery holder and keep a resistor in series with the LED at all times.
- Remove batteries before changing wiring or using resistance mode.
Keep evidence
Bench note
Record what you expected, what you actually measured or observed, and one difference worth investigating. This is self-reported evidence, not a certification.