11 Electronics for IoT: Power and Switching

11.1 Start With the Situation

This section: Start With the Situation — The signal calculations are correct, but an IoT device must survive its load path, the route that supplies current to the part being driven.

The signal calculations are correct, but an IoT device must survive its load path, the route that supplies current to the part being driven. A power budget adds the power used by active parts; battery-life planning adds that use over time. Choose a switching component for the load, then check drive voltage and current, load current, and heat against their limits.

11.2 Overview

This section: Overview — This route turns power budgets into safe transistor, MOSFET, and load-switching decisions.

This route turns power budgets into safe transistor, MOSFET, and load-switching decisions.

This is part 2 of 2. Review Electronics for IoT: Signals and Calculators when you need the first route.

11.3 Learning Objectives

This section: Learning Objectives

By the end of this chapter, you will be able to:

• calculate an IoT power budget
• select BJT or MOSFET switching
• check logic-level, current, and thermal limits

11.4 Knowledge Check

This section: Knowledge Check

11.5 BJT Transistor as a Relay Switch

This section: BJT Transistor as a Relay Switch

An NPN bipolar junction transistor is a current-controlled low-side switch. The relay coil connects from its load supply $V_{CC}$ to the collector, the emitter returns to ground, and a GPIO drives the base through a resistor. The control supply and coil supply may be different voltages, but this simple base-drive circuit needs the GPIO ground and transistor-emitter ground to share a reference. The relay contacts remain a separate circuit and can switch an isolated load within their ratings.

Read the two switching states from current flow:

The collector voltage is therefore an inverted version of the control: a high input produces a low collector voltage. That does not mean the relay action is inverted; the coil energises because current now has a path from $V_{CC}$ through the coil and transistor to ground.

Do not size the base resistor from the transistor’s headline DC gain $h_{FE}$. Gain varies between parts and falls in saturation. Instead, choose a conservative forced gain $\beta_f$, often 10 for a small relay driver:

Walk the design. A 12 V relay coil draws $60\ \mathrm{mA}$ and is controlled by a 3.3 V GPIO. With $\beta_f=10$, require $I_B\geq6\ \mathrm{mA}$. Taking $V_{BE(sat)}\approx0.8\ \mathrm{V}$ gives

A $390\ \Omega$ resistor supplies about $6.4\ \mathrm{mA}$, provided the GPIO is rated to source it. If it is not, use a MOSFET or a driver IC rather than hoping for more transistor gain. With $V_{CE(sat)}=0.2\ \mathrm{V}$, transistor dissipation while on is only $0.2\times0.060=12\ \mathrm{mW}$, but collector-current, voltage, and safe-operating-area ratings must still cover startup and fault conditions.

The coil is inductive. When the transistor opens, $v=L\,di/dt$ produces whatever voltage is needed to keep current flowing. A flyback diode placed directly across the coil—cathode to $V_{CC}$, anode to the collector—is reverse-biased during normal operation and conducts the decaying coil current at turn-off. The diode protects the transistor, though it also lengthens relay release time; a faster release may require a deliberately higher-voltage clamp designed within the transistor rating.

Before release, verify each path separately: base current and GPIO loading in the control path; coil current and $V_{CE(sat)}$ in the load path; diode polarity and turn-off waveform in the inductive path; and contact voltage, current, isolation, and load type in the switched circuit. That four-path check is what turns the symbolic cutoff/saturation story into a safe interface between different voltage domains.

MOSFET Switch for 12V Motor

Scenario: You need to control a 12V DC motor (stall current: 2A) from an ESP32 GPIO pin (3.3V output, 12mA max). Design the transistor switching circuit.

Treat the problem as three coupled paths. The control path runs from the ESP32 GPIO through the gate resistor to the MOSFET gate; it must establish a defined OFF state during reset and charge the gate quickly enough for the chosen switching rate. The load path runs from the 12 V rail through the motor and MOSFET to ground; it must be rated for stall current, not merely the lower running current. The turn-off path is provided by the flyback diode so the motor winding’s current can decay without forcing the drain voltage beyond the switch rating. A common ground ties the 3.3 V control voltage to the MOSFET source reference.

Choose the device only after those paths are explicit. A headline drain-current rating measured under favourable thermal conditions is not proof of a safe design, and gate-threshold voltage is only where conduction begins. The useful check is the datasheet’s on-resistance at the available gate drive, followed by $I^2R$ loss and a thermal estimate. The checklist below records those decisions in the same order: switch type, electrical ratings, physical package, dissipation, gate network, protection, and finally code behaviour.

Step 1: Choose Transistor Type

BJT vs MOSFET comparison:

• BJT (2N2222): Current-controlled, max Ic ≈ 0.8A (can’t handle 2A); even for a higher-rated BJT, base current = Ic/β ≈ 2A/100 = 20mA (exceeds GPIO limit!)
• MOSFET (IRLZ44N): Voltage-controlled, gate current ≈ 0µA (perfect for GPIO)

Select: N-channel logic-level MOSFET (IRLZ44N)

Step 2: Verify MOSFET Specifications

Parameter	IRLZ44N Spec	Requirement	Status
Vds (drain-source voltage)	55V	12V motor	OK: safe
Id (continuous drain current)	47A	2A motor	OK: massive headroom
Vgs(th) (gate threshold)	1-2V	3.3V GPIO	OK: logic-level candidate; threshold alone does not prove low loss
Rds(on) @ low gate voltage	~0.035Ω near 4-5V gate drive	Low resistance	OK for this load if the datasheet curve supports the available gate voltage; do not use only the 10V headline value
Package	TO-220	Through-hole	OK: easy to prototype

Before laying out the switch, inspect Figure 11.1 to identify the package features that the electrical and thermal calculation must eventually map onto. The photograph supplies orientation, while the datasheet remains the authority for pin order and ratings.

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Read Figure 11.1 by locating the three leads first, then the insulated body, and finally the metal tab used for mechanical mounting and heat transfer. Do not infer gate, drain, and source order from appearance; verify the exact device pinout. That physical reading connects the on-resistance and junction-temperature checks to assembly, clearance, and any heatsink decision.

Step 3: Calculate Power Dissipation

At 2A, 0.050Ω would give the following illustrative loss; the IRLZ44N datasheet does not guarantee that on-resistance at Vgs=3.3V, so this is not a worst-case or sign-off calculation:

• Voltage drop: V = I × Rds(on) = 2A × 0.050Ω = 0.10V (negligible)
• Conditional power estimate: P = I² × Rds(on) = (2A)² × 0.050Ω = 0.20W

At room temperature (25°C):

• Thermal resistance: 62.5°C/W (TO-220 without heatsink)
• Temperature rise: 0.20W × 62.5°C/W = 12.5°C
• Junction temperature: 25 + 12.5 = 37.5°C (well below 175°C max)

Thermal result: This estimate does not establish junction temperature or heatsink need without a guaranteed on-resistance at the applied gate voltage and a verified duty cycle.

Step 4: Design Gate Drive Circuit

Components needed:

• R1: Gate resistor (100Ω) - limits gate charge current
• R2: Pull-down resistor (10kΩ) - ensures MOSFET off when GPIO is floating
• D1: Flyback diode (1N4007) - protects MOSFET from motor inductive kickback

Wiring map:

Node	Connects To	Purpose
ESP32 GPIO (3.3V)	100Ω resistor, then MOSFET gate	Controls the motor switch and limits gate current spikes
MOSFET gate	10kΩ pull-down to GND	Keeps the motor OFF during ESP32 boot/reset
MOSFET source	GND	Low-side switch reference
MOSFET drain	Motor negative side	Switches motor current to ground
Motor positive side	+12V supply	Provides motor power
Flyback diode D1	Across motor, cathode to +12V and anode to MOSFET drain	Protects the MOSFET when motor current is interrupted

Step 5: Calculate Component Values

Gate Resistor (R1 = 100Ω):

• Purpose: Limit inrush current when charging gate capacitance
• Gate capacitance (Ciss): 1200pF (from datasheet)
• Peak gate current: I = 3.3V / 100Ω = 33mA (brief ~120ns transient; average current negligible)
• Gate charging time: τ = R × C = 100Ω × 1200pF = 120ns (fast switching)

Pull-down Resistor (R2 = 10kΩ):

• Purpose: Ensure MOSFET is OFF when GPIO is tri-state (during boot/reset)
• Leakage current budget: 10µA maximum
• Voltage across R2 when OFF: V = 10µA × 10kΩ = 0.1V (below Vgs(th), safe)
• Power dissipation: P = V² / R = (3.3V)² / 10kΩ = 1.1mW (negligible)

Flyback Diode (D1 = 1N4007):

• Purpose: Clamp inductive voltage spike when motor turns off
• Motor inductance creates voltage spike: V = -L × (dI/dt)
• Without diode: spike can reach 100-300V, destroying MOSFET
• Diode spec: 1N4007 (1A, 1000V) handles motor’s inductive kickback
• Orientation: Cathode (stripe) to 12V, anode to motor/drain

Step 6: Arduino Code Example

const int MOTOR_PIN = 25;  // GPIO25 on ESP32

void setup() {
  pinMode(MOTOR_PIN, OUTPUT);
  digitalWrite(MOTOR_PIN, LOW);  // Motor off initially
}

void loop() {
  // Turn motor on
  digitalWrite(MOTOR_PIN, HIGH);  // 3.3V to MOSFET gate
  delay(2000);  // Run for 2 seconds

  // Turn motor off
  digitalWrite(MOTOR_PIN, LOW);   // 0V to gate, MOSFET off
  delay(1000);  // Off for 1 second
}

Step 7: Verification Checklist

Check	Expected	Measured	Status
GPIO voltage HIGH	3.3V	3.28V	OK
Gate voltage when ON	3.3V	3.27V	OK
Gate voltage when OFF	0V	0.01V	OK: pull-down working
Motor voltage when ON	12V	11.90V	Illustrative: about 0.1V drop at the assumed 0.050Ω; not a guaranteed 3.3V on-resistance
Motor current	2A max	1.8A running	OK: below stall current
MOSFET temperature	<50°C	32°C	OK: minimal heating

Step 8: Common Mistakes to Avoid

Mistake	Consequence	Fix
No gate resistor	GPIO inrush current spike damages ESP32	Add 100Ω gate resistor
No pull-down resistor	Motor runs randomly during ESP32 boot	Add 10kΩ pull-down
No flyback diode	MOSFET fails after few switching cycles	Add 1N4007 across motor
Using standard MOSFET (not logic-level)	Requires 10V gate drive for its rated Rds(on), so it may overheat from GPIO drive	Use a MOSFET with Rds(on) specified at your actual gate voltage; for ESP32, prefer 2.5-3.3V data
P-channel MOSFET for low-side switch	Requires negative gate voltage	Use N-channel for low-side

Explore: MOSFET Thermal Calculator

The power dissipation formula from Step 3 ($P = I^2 \times R_{ds(on)}$) estimates conduction loss when the on-resistance is known at the applied gate voltage. Adjust the parameters below to explore conditional scenarios; the calculator does not include PWM duty cycle or drain-transition losses.

Bill of Materials (BOM):

Component	Part Number	Quantity	Unit Cost	Total
MOSFET	Select a device with guaranteed RDS(on) at 2.5–3.3V gate drive, or add a gate driver	1	TBD	TBD
Resistor (100Ω)	1/4W	1	$0.02	$0.02
Resistor (10kΩ)	1/4W	1	$0.02	$0.02
Diode	1N4007	1	$0.05	$0.05
Total				$1.29

Key Takeaways:

1. Logic-level MOSFETs need guaranteed on-resistance at the actual GPIO gate voltage; threshold voltage alone is insufficient
2. Gate resistor (100Ω) protects GPIO from gate capacitance inrush
3. Pull-down resistor (10kΩ) prevents floating gate during boot/reset
4. Flyback diode (1N4007) is MANDATORY for inductive loads (motors, relays, solenoids)
5. Power dissipation needs duty-weighted conduction and drain-transition estimates at the approved gate voltage before thermal sign-off
6. Confirm the selected device, GPIO current, protection, and thermal limits before release; the example does not establish universal compatibility or cost

11.6 Concept Relationships

This section: Concept Relationships

Electronics fundamentals connect across multiple IoT domains:

11.7 See Also

This section: See Also — Follow these connections in order: Sensor Signal Conditioning - Amplify and filter analog signals; Motor Control Basics - Transistor H-bridge circuits; LED Drivers - Current limiting and PWM dimming.

Electronics Foundations:

Follow these connections in order: Conductors and Insulators - Material properties and component types; Semiconductors and Diodes - PN junctions and one-way current flow; Transistor Selection - Choose BJT vs MOSFET for your project.

Practical Applications:

Follow these connections in order: Sensor Signal Conditioning - Amplify and filter analog signals; Motor Control Basics - Transistor H-bridge circuits; LED Drivers - Current limiting and PWM dimming.

11.8 What’s Next

This section: What’s Next

11.9 Logic Levels and Shifting

This section: Logic Levels and Shifting — The sections above orient beginners to GPIO limits, component roles, load switching, and protection components.

The sections above orient beginners to GPIO limits, component roles, load switching, and protection components. When two real boards are connected, the next review question is whether each digital link is both logically valid and electrically survivable across its voltage domains.

For the layered engineering checks, continue to Logic Level and Level Shifting Contracts. It works through VOH/VOL and VIH/VIL thresholds, noise margin, 3.3 V and 5 V directionality, resistor-divider limits, bidirectional bus shifting, pull-up sizing, and absolute-maximum protection.

11.10 Figure Review: Logic-Controlled Load Current

This section: Figure Review: Logic-Controlled Load Current — A low-side transistor switch lets a GPIO command a higher-current load without asking the logic pin to source the coil current directly.

A low-side transistor switch lets a GPIO command a higher-current load without asking the logic pin to source the coil current directly.

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In Figure 11.2, GPIO 3V3 supplies base current through RB while the collector carries relay current; CUTOFF · LOW leaves the coil off, SATURATED · HIGH energizes it, and the flyback path safely circulates turn-off energy.

11.11 Summary

This section: Summary — This chapter introduces the electronics foundation behind IoT devices: voltage, current, resistance, GPIO limits, loads, sensors, and protection components.

This chapter introduces the electronics foundation behind IoT devices: voltage, current, resistance, GPIO limits, loads, sensors, and protection components. It frames circuit design as a balance of signal behavior, power, safety, and component limits.