Power Supplies — Batteries, USB and Voltage Regulators

POST 12  ●  SERIES 1: ELECTRONICS BASICS  ●  13 min read

Get the power right first — everything else depends on a stable, clean supply

Introduction

The power supply is the foundation of every electronic circuit. Get it wrong and nothing works correctly — your MCU resets randomly, sensors give wrong readings, motors behave erratically. This post covers every common power source: batteries, USB, wall adapters, and voltage regulators — linear (7805, LM317, AMS1117) and switching — and how to choose the right power supply for any project.

Part 1: Batteries

Battery Type	Voltage	Rechargeable?	Capacity	Self-discharge	Best for
AA Alkaline	1.5V	No	2500-3000mAh	Low (years)	Low-power sensors, remotes
AAA Alkaline	1.5V	No	1000-1200mAh	Low	Compact devices, remotes
9V Block	9V	No	550mAh	Medium	Small prototypes, moderate current
CR2032 Coin	3V	No	225mAh	Very low	RTCs, BLE beacons, ultra-low power
NiMH AA	1.2V	Yes	2000-2800mAh	Medium	Replacement for alkaline
LiPo (1S)	3.7V nominal, 4.2V full, 3.0V min	Yes	500-10,000mAh+	Low	Portable projects, drones
18650 Li-Ion	3.7V	Yes	2000-3500mAh	Very low	DIY power banks, flashlights
Lead Acid	12V	Yes	High (Ah)	Low	Solar storage, UPS, backup

⚠️  WARNING

LiPo and Li-Ion batteries require a Battery Management System (BMS) or charge controller.

Overcharge → swelling, fire. Over-discharge → permanent damage.

Never use a LiPo below 3.0V per cell or charge above 4.2V per cell without a BMS.

Part 2: USB Power

USB is the most convenient power source for prototyping. Understanding the different standards prevents common power problems.

USB Standard	Voltage	Current available	Wattage	Notes
USB 2.0	5V	500mA = 0.5A	2.5W	Standard PC port. Enough for Arduino Uno + a few sensors.
USB 3.0	5V	900mA = 0.9A	4.5W	Slightly more current available.
USB Charging Port	5V	1.5-2.4A	7.5-12W	Charger ports. Good for Raspberry Pi 3.
USB-C (5V mode)	5V	up to 3A	15W	Most modern chargers. Raspberry Pi 4.
USB PD	5-20V	up to 5A	100W	Negotiated voltage. Powers anything.

💡  TIP

Raspberry Pi 4 needs a genuine USB-C 5V 3A charger.

Using an old phone charger (5V 1A) will cause throttling and SD card corruption.

Always check the power supply rating before connecting. The RPi OS shows a lightning bolt icon in the corner when under-voltage is detected.

Part 3: Linear Voltage Regulators

A linear regulator takes a higher input voltage and converts it to a stable, lower output voltage — the excess is dissipated as heat. They’re simple and low-noise, but inefficient, especially when the input is much higher than the output.

Figure 1 — Linear regulator block diagram: excess voltage becomes heat.

7805 — The Classic 5V Regulator

A three-pin TO-220 package regulator: Input, GND, Output. Fixed 5V output, up to 1A (with heatsink). Needs input of at least 7V (dropout ~2V). Add 100nF on input and 100nF on output. Power dissipated as heat: P = (Vin − Vout) × Iout. Example: Vin=12V, Vout=5V, Iout=500mA → P = (12−5) × 0.5 = 3.5W — very hot, needs a heatsink.

LM317 — Adjustable Regulator

Outputs any voltage from 1.25V to 37V using two resistors: Vout = 1.25 × (1 + R2/R1). Maximum output current: 1.5A. Three pins: Input, Adjust, Output. Set R1 = 240Ω (fixed), then calculate R2 for desired output. Example for 5V: R2 = 240 × (5/1.25 − 1) = 720Ω — use 680Ω + 100Ω trim pot for precision.

AMS1117 — The MCU Favourite

A low-dropout (LDO) regulator in SOT-223 or TO-252 package. Available in fixed 3.3V, 5V, 1.8V versions. Used on virtually every Arduino, Raspberry Pi, and ESP32 board to generate 3.3V from 5V. Low dropout: only 1.1-1.3V needed above output, up to 1A.

Regulator	Output	Max Current	Dropout voltage	Package	Best for
7805	5V fixed	1A (with heatsink)	~2V	TO-220	General 5V from 9-12V
7809/12	9V, 12V	1A	~2V	TO-220	Fixed voltage applications
LM317	1.25-37V adj	1.5A	~2V	TO-220	Adjustable bench supply
AMS1117-3.3	3.3V fixed	1A	1.1V	SOT-223, TO-252	MCU 3.3V rail from 5V
LD1117	3.3V, 5V fixed	0.8A	1.1V	TO-220, SOT-223	Similar to AMS1117
MCP1700	1.8-5V fixed	250mA	0.18V	SOT-23	Ultra-low power IoT nodes

Part 4: Switching Regulators

Switching regulators (SMPS) use an inductor and capacitor to store and transfer energy efficiently. Instead of wasting excess voltage as heat, they switch rapidly and convert voltage with 85-95% efficiency.

Buck Converter (Step-Down)

Converts higher input voltage to lower output, ~85-95% efficient. Common modules: LM2596 (up to 3A), MP1584 (up to 3A, very small), TPS5430 (integrated IC). Use case: 12V battery → 5V for Arduino. A 7805 would waste (12-5)×0.5A = 3.5W as heat; a buck converter wastes only ~0.25W.

Boost Converter (Step-Up)

Converts lower voltage to higher output. Example: 3.7V LiPo → 5V for USB. Common: MT3608, IP5306.

Buck-Boost

Can step up or step down — used when battery voltage changes but output must stay constant (e.g. 2-cell battery 6-8.4V → fixed 5V output).

Figure 3 — Typical efficiency ranges by regulator type.

Type	Efficiency	Noise	Complexity	Cost	Use when
Linear (7805, LM317)	30-60%	Very low (good)	Low	Very cheap	Small current, quiet needed (audio/RF)
LDO (AMS1117)	60-85%	Low	Low	Cheap	Small voltage difference, moderate current
Buck SMPS	85-95%	Medium (needs filtering)	Medium	Moderate	Efficient step-down, high current
Boost SMPS	80-93%	Medium	Medium	Moderate	Step-up needed

Part 5: Designing Your Power Supply

1. List all components and their current requirements. Sum them all.

2. Add 30% safety margin to the total current.

3. Choose a supply voltage that suits your components (usually 5V or 3.3V).

4. Choose regulator type: linear (simple, clean, low current) vs switching (efficient, high current).

5. Add decoupling capacitors: 100nF ceramic on every IC VCC pin, 10µF electrolytic nearby.

6. Add a bulk capacitor (100-1000µF) on the main supply rail.

Frequently Asked Questions

Q: Should I choose a linear regulator or a switching regulator for my project?

A: Use a linear regulator (7805, AMS1117) when the voltage drop is small and current needs are modest — they’re simpler and quieter. Use a switching regulator (buck/boost module) when the voltage drop is large or current needs are high, since it wastes far less energy as heat.

Q: Why does my 7805 get so hot even at moderate current?

A: Linear regulators dissipate the voltage difference times the current as heat: P = (Vin − Vout) × Iout. A large gap between input and output voltage, combined with a few hundred milliamps of load, quickly adds up to several watts that must be removed with a heatsink.

Q: Can I power a 5V Arduino directly from a 3.7V LiPo battery?

A: Not directly — 3.7V is below the Arduino’s minimum input requirement in most cases. You’d need a boost converter to step 3.7V up to a stable 5V, or use a board specifically designed for direct LiPo input.

Q: Is it safe to use a higher-current power supply than my device needs?

A: Yes — the device only draws the current it needs; a supply rated for more current than required simply has extra headroom and won’t force more current into the circuit. What matters is that the voltage and polarity match.

Q: Why do I need a BMS for LiPo batteries but not for alkaline?

A: LiPo cells can be permanently damaged, swell, or catch fire if overcharged, over-discharged, or short-circuited, because the chemistry is far less forgiving than the reactions in a simple alkaline cell. A BMS actively protects against these failure modes.

Q: What’s the simplest way to check if my power supply is adequate for a Raspberry Pi?

A: Use a genuine, correctly rated USB-C 5V 3A supply for a Pi 4 (or the appropriate rating for your model), and watch for the under-voltage warning icon in the Raspberry Pi OS — its appearance is a clear, built-in signal that the supply isn’t delivering enough current.