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.

electricityohms
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: The same temperature-aware resistance then sets LED current and resistor heat through I=(3.3-2.0)/R and P=I^2R.
  • Explain: The visual is a route through the checks, not a substitute for naming the two voltage points and the current path.
  • Explain: For a 5 mA target, Ohm’s Law gives \(R=V/I=1.3/0.005=260\ \Omega\).
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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)/R and P=I^2R.

Numbers to remember

85°CWith 100 ppm/°C it reaches 261.56 ohms at 85°C
+0.600%With 100 ppm/°C it reaches 261.56 ohms at 85°C, only +0.600%
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Major section

Overview: Ohm's Law Is A Safety Check

The visual is a route through the checks, not a substitute for naming the two voltage points and the current path.

  • The calculated resistance still has to become a physical part.
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 V.
  • For a 5 mA target, Ohm’s Law gives \(R=V/I=1.3/0.005=260\ \Omega\).

Key terms

Resistor power
Resistor power is P=VI=1.3 × 0.005=0.0065 W, or 6.5 mW.

Numbers to remember

1.3 VThe resistor must take the remaining 3.3-2.0=1.3 V.
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%.
  • The visual is a route through the checks, not a substitute for naming the two voltage points and the current path.
  • 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 3

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?

AAbout 1.3 V, because the resistor sees the supply voltage minus the LED forward voltage.
BThe full 3.3 V, because the LED forward voltage does not matter.
CAbout 2.0 V, because the LED forward voltage sets the voltage used to calculate the series current.
DThe battery capacity in mAh, because capacity is the same as voltage.
Show answer

Answer: A 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 3

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

Q2A 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?

ADivider loading, total divider resistance, ADC input behavior, grounding, and measurement setup.
BOnly the page title, because a noisy ADC reading is a documentation problem.
CRecheck the nominal sensor voltage and divider ratio before considering the absolute resistor values.
DNo follow-up, because one correct equation proves the deployed circuit.
Show answer

Answer: A 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 3 of 3

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

Q3A 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?

AMeasure startup and stall current, supply sag, driver margin, wiring, and flyback or suppression behavior.
BUse the measured running current to size the supply, since it reflects the motor under its normal mechanical load.
CIgnore the supply because Ohm's Law removes the need to measure voltage.
DOnly change the quiz answer, because resets cannot be caused by circuit behavior.
Show answer

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