Electronics & Circuits · Study deck

Ohm's Law and Power

Picture an indicator light that works at first and then damages its controller pin.

Voltage Vera is your guide for this deck.

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Voltage Vera, the module guide, in a scene from this chapter.
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After studying this chapter

Learning objectives

You will be able to:

  • Explain: With 100 ppm/°C it reaches 261.56 ohms at 85°C, only +0.600%, while a copper conductor with 3,930 ppm/°C would move +23.6%.
  • Explain: In the worked LED case, the resistor receives the supply voltage left after the diode drop: 3.3 V−2.0 V=1.3 V3.3\ \text{V}-2.0\ \text{V}=1.3\ \text{V}3.3 V−2.0 V=1.3 V.
  • Explain: Converting the 5 mA target to 0.005 A and applying R=V/IR=V/IR=V/I gives 1.3 V/0.005 A=260 Ω1.3\ \text{V}/0.005\ \text{A}=260\ \Omega1.3 V/0.005 A=260 Ω.
  • Explain: A microcontroller means the small computer that reads inputs and controls a device.
Six axial-lead resistors with coloured value and tolerance bands. Photo: Evan-Amos, public domain.
Six axial-lead resistors with coloured value and tolerance bands. Photo: Evan-Amos, public domain.
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Major section

Start Simple

The circuit needed a current limit and a power check before it was connected.

  • A microcontroller means the small computer that reads inputs and controls a device.
  • GPIO means general purpose input/output.
  • A GPIO pin is one of the controller's small digital connections.
  • This runway does not prove that Ohm's Law describes every component by itself.
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Major section

Phoebe's Field Notes: Why "Constant Resistance" Is Only Approximately True

With 100 ppm/°C it reaches 261.56 ohms at 85°C, only +0.600%, while a copper conductor with 3,930 ppm/°C would move +23.6%.

  • The same temperature-aware resistance then sets LED current and resistor heat through I=(3.3−2.0)/RI=(3.3-2.0)/RI=(3.3−2.0)/R and P=I2RP=I^2RP=I2R.

Let Eddie connect temperature coefficient, resistance drift, LED current, and resistor power.

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Major section

Overview: Ohm's Law Is A Safety Check

In the worked LED case, the resistor receives the supply voltage left after the diode drop: 3.3 V−2.0 V=1.3 V3.3\ \text{V}-2.0\ \text{V}=1.3\ \text{V}3.3 V−2.0 V=1.3 V.

  • Converting the 5 mA target to 0.005 A and applying R=V/IR=V/IR=V/I gives 1.3 V/0.005 A=260 Ω1.3\ \text{V}/0.005\ \text{A}=260\ \Omega1.3 V/0.005 A=260 Ω.
Ohm's Law, power, and Kirchhoff equations used as a circuit-review sequence
Ohm's Law, power, and Kirchhoff equations used as a circuit-review sequence
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Major section

Size an LED Resistor and Its Heat

A 3.3 V output drives a red LED whose forward drop is 2.0 V.

  • The resistor must take the remaining 3.3−2.0=1.3 V3.3-2.0=1.3\ \text{V}3.3−2.0=1.3 V.
  • For a 5 mA target, Ohm’s Law gives R=V/I=1.3/0.005=260 ΩR=V/I=1.3/0.005=260\ \OmegaR=V/I=1.3/0.005=260 Ω.

Key terms

Resistor power
Resistor power is P=VI=1.3×0.005=0.0065 WP=VI=1.3\times0.005=0.0065\ \text{W}P=VI=1.3×0.005=0.0065 W, or 6.5 mW.
Ohm's Law, power, and Kirchhoff equations used as a circuit-review sequence
Ohm's Law, power, and Kirchhoff equations used as a circuit-review sequence
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Deck summary

Key takeaways

The circuit needed a current limit and a power check before it was connected.

  • With 100 ppm/°C it reaches 261.56 ohms at 85°C, only +0.600%, while a copper conductor with 3,930 ppm/°C would move +23.6%.
  • In the worked LED case, the resistor receives the supply voltage left after the diode drop: 3.3 V−2.0 V=1.3 V3.3\ \text{V}-2.0\ \text{V}=1.3\ \text{V}3.3 V−2.0 V=1.3 V.
  • A 3.3 V output drives a red LED whose forward drop is 2.0 V.
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Retrieval practice

Recall check 1 of 4

Voltage Vera says: answer from memory, then check your reasoning.

Q1A 3.3 V GPIO drives a red LED through a resistor. The LED forward voltage is about 2.0 V. What voltage should be used across the resistor when estimating LED current?

AUse the full 3.3 V supply in the calculation because the LED's forward drop does not reduce resistor voltage.
BAbout 2.0 V, using the LED drop as if it were the resistor's voltage
CBattery capacity in mAh as if it were a voltage across the series resistor
DAbout 1.3 V, the 3.3 V supply minus the LED's 2.0 V forward drop
Show answer

Answer: D The resistor voltage is approximately 3.3 V - 2.0 V, so Ohm's Law should use about 1.3 V for the resistor path.

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Retrieval practice

Recall check 2 of 4

Voltage Vera says: answer from memory, then check your reasoning.

Q2A resistor path carries current. What completes its power safety check?

ACompare resistance with supply voltage as the complete power safety check for this load
BCheck that the circuit powers on once as proof the resistor stays safe
CCompute path power from voltage and current; compare component and source ratings
DUse the GPIO current limit as the resistor's dissipation rating
Show answer

Answer: C The calculation is useful only against actual limits.

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Retrieval practice

Recall check 3 of 4

Voltage Vera says: answer from memory, then check your reasoning.

Q3A prototype uses a resistor divider to scale a 5 V sensor output for a 3.3 V ADC. The calculated output is safe, but the measured ADC value is noisy and shifts when the sensor cable is connected. What should the review check next?

AReview sensor documentation for ADC input constraints before measuring the live board.
BDivider loading, total divider resistance, ADC input behavior, grounding, and measurement setup.
CRecheck the nominal sensor voltage and divider ratio before considering the absolute resistor values.
DAccept the calculated divider voltage as proof of stable ADC readings.
Show answer

Answer: B The voltage-divider equation is only the first check; practical loading, impedance, noise, and grounding decide whether the divider works in the real circuit.

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Retrieval practice

Recall check 4 of 4

Voltage Vera says: answer from memory, then check your reasoning.

Q4A small DC motor normally runs at a modest measured current, but the microcontroller resets whenever the motor starts. What is the best next review step?

AUse the measured running current to size the supply, since it reflects the motor under its normal mechanical load.
BMeasure startup and stall current, supply sag, driver margin, wiring, and flyback or suppression behavior.
CUse the resistor design calculation to rule out a supply problem.
DTreat the resets as a firmware logging issue and revise the motor-status display.
Show answer

Answer: B The reset is likely tied to transient load behavior or supply disturbance that a steady running-current calculation does not capture.

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Print reference

Answers

Answer key.

  1. D · The resistor voltage is approximately 3.3 V - 2.0 V, so Ohm's Law should use about 1.3 V for the resistor path.
  2. C · The calculation is useful only against actual limits.
  3. B · The voltage-divider equation is only the first check; practical loading, impedance, noise, and grounding decide whether the divider works in the real circuit.
  4. B · The reset is likely tied to transient load behavior or supply disturbance that a steady running-current calculation does not capture.
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