An Introduction To The Switch Mode Power Supply

The 5V DC power supply is one of the most common voltage rails in modern electronics. From smartphone chargers to microcontroller boards, 5V has become a de facto standard for low-voltage digital logic, USB-powered devices and a wide range of consumer electronics. This article explains the typical uses of 5V power supplies, compares USB power against dedicated 5V plug packs and outlines the current requirements for common devices.

Understanding what a 5V supply does and how to choose the right one helps avoid underpowered operation, intermittent resets and damage to sensitive circuitry.

Common Uses for 5V Power Supplies

5V DC is used across many applications because it is low enough to be safe for small electronics, yet high enough to drive most digital ICs, sensors and relays. Typical uses include:

• Single-board computers (SBCs) such as Raspberry Pi, Arduino boards and ESP32 development modules
• USB hubs and powered USB peripherals
• LED strip lighting and indicator lamps
• Small fans and cooling modules
• Battery chargers for Li-ion and NiMH cells (via charging circuitry)
• Sensors and modules in hobbyist and industrial projects
• Modems, routers and network switches
• LCD and OLED display panels
• Security cameras and IP cameras

Because 5V is so widely used, there are many plug packs and open-frame power supplies available to suit different current draws and form factors.

USB Power vs Dedicated 5V Plug Packs

USB Power

USB is the most familiar way to deliver 5V. A standard USB-A port provides 5V at up to 500mA (USB 2.0) or 900mA (USB 3.0). USB-C ports can provide more, depending on the power delivery (PD) negotiation, but many basic USB-C supplies still output 5V.

USB power is convenient because it is ubiquitous and interchangeable. However, it has limitations:

• Current capacity is often lower than a dedicated plug pack
• Voltage drop can occur over long or poor-quality cables
• Some USB chargers are designed primarily for battery charging and may not provide clean, regulated 5V under load
• USB ports on computers may limit current or shut down entirely when overloaded

Dedicated 5V Plug Packs

A dedicated 5V switchmode plug pack, such as the 5V 1A or 5V 2A units available from Campad Electronics, provides a regulated, continuous 5V output through a barrel jack (typically 2.1mm or 2.5mm centre-positive), see the article Selecting the Right Plug Size for Your DC Power Supply for more information. These are better suited to fixed installations, bench testing and equipment that needs a reliable supply without relying on a USB port.

Advantages of dedicated 5V plug packs include:

• Consistent, regulated output voltage under load
• Higher current ratings available (typically 1A to 5A or more)
• No negotiation required, power is available immediately
• Suitable for devices that do not have USB input connectors
• Often include short-circuit and overvoltage protection

The choice between USB and a plug pack comes down to the device interface and the current demand. A sensor node drawing 200mA might run fine from a USB port, but a Raspberry Pi under load will need a dedicated 5V supply with at least 2A.

Current Requirements for Small Electronics and Single-Board Computers

Different devices draw different amounts of current from a 5V rail. The current rating of a power supply must be equal to or greater than the total current demanded by the load.

Typical current draws for common devices:

• Arduino Uno (bare board): 50–100mA
• ESP32 DevKit (with Wi-Fi active): 200–350mA, peaks above 500mA
• Raspberry Pi 3/4 (under load): 1.2A to 2.5A depending on peripherals
• 5V LED strip (1 metre, 60 LEDs): 1.2A to 3.6A depending on brightness and colour
• USB hard drive: 0.5A to 1A during read/write
• Small 5V fan (40mm): 0.1A to 0.2A
• PIR motion sensor: 10–20mA
• HC-SR04 ultrasonic sensor: 15mA

It is important to check the peak current, not just the average. Many microcontrollers and radios draw brief current surges that can cause a marginal supply to drop voltage, leading to resets or corrupted data.

For example, a Raspberry Pi 4 can draw over 2A when multiple USB peripherals are attached and the CPU is active. A 5V 1A supply will not suffice, even though the board may boot. Campad Electronics offers a 5V 2A switchmode power supply that is better suited to such loads. This article Power Adapters From Campad Electronics has more information about the range of 5V power adapters from Campad Electronics

Real-World Examples

Example 1: Raspberry Pi Media Centre

A Raspberry Pi 4 running as a media centre with an external USB drive can draw 1.8A to 2.2A. A dedicated 5V 2A plug pack provides stable power without the brownouts seen with undersized USB chargers. Using a quality 2.1mm plug pack is essential for reliable operation.

Example 2: ESP32 Weather Station

An ESP32 board reading temperature, humidity and sending data over Wi-Fi may average 150mA but burst to 400mA during radio transmission. A 5V 1A plug pack gives sufficient headroom. A USB port on a computer may work but could be less reliable, especially if the compter is in sleep mode or the USB port is shared.

Example 3: Bench Testing with an Arduino

When prototyping on a bench, a 5V 2A switchmode supply connected via a barrel jack is far more robust than powering the board from a laptop USB port. It isolates the testing setup from the computer and provides consistent power for sensors, motors and display modules.

Safety and Wiring Notes

• Always connect the positive and negative terminals correctly. Most plug packs use centre-positive polarity, but check the device label before connecting.
• Do not overload the supply beyond its rated current. This can cause overheating and failure.
• Use a supply with adequate protection features, such as short-circuit and overvoltage protection, which are common on switchmode power supplies.
• For permanent installations, ensure wiring is rated for the current and secured properly.
• While Australian electrical standards apply to mains-connected equipment, it is a general practice to use certified supplies and avoid modifying plug packs.

Summary

5V power supplies are essential for powering microcontrollers, single-board computers, LED lighting and a wide range of USB and low-voltage devices. While USB power is convenient for small loads, dedicated 5V plug packs offer better regulation, higher current capacity and more reliable operation for demanding applications. 

By checking the current draw of the device and selecting a supply with appropriate headroom, users can avoid common power-related faults and extend the life of their electronics. 

The range of DC power supplies from Campad Electronics includes multiple 5V options suitable for hobby, education and light commercial use.

Frequently Asked Questions

What is a 5V power supply used for?

A 5V power supply provides a regulated 5-volt DC output, commonly used to power single-board computers like Raspberry Pi and Arduino, USB hubs, LED strips, sensors, small fans, modems, routers and various low-voltage digital electronics.

Can I use a USB charger instead of a 5V plug pack?

You can use a USB charger for low-current devices, but USB ports often have lower current limits (500mA to 900mA) and may not deliver clean, regulated voltage under load. Dedicated 5V plug packs provide higher current ratings and more stable output, making them better for demanding devices like Raspberry Pi or bench setups.

What happens if my 5V supply has too low a current rating?

If the supply’s current rating is lower than the device’s peak demand, the voltage can drop, causing resets, corrupted data or failure to boot. Always choose a supply with headroom above the device’s peak current draw.

How do I know what current rating I need?

Check the device’s datasheet or label for average and peak current draw. Add up the current demands of all connected peripherals. Select a supply rated at least 20–30% higher than the total peak current to ensure stability.

Is centre-positive polarity standard for 5V plug packs?

Most 5V plug packs use centre-positive polarity, but always check the device label or manual before connecting. Incorrect polarity can damage sensitive electronics.

Can I power multiple devices from one 5V supply?

Yes, provided the total current draw of all devices does not exceed the supply’s rated current. Use proper distribution and ensure wiring is rated for the total current.

Are 5V plug packs safe for continuous operation?

Yes, quality switchmode 5V plug packs are designed for continuous operation and typically include short-circuit and overvoltage protection. Ensure the supply is certified and not overloaded.

Why does my device reset when using a USB port?

This is often due to voltage drop or current limits on the USB port, especially under load. A dedicated 5V plug pack with adequate current rating eliminates this issue by providing stable power.

Can I use a 5V supply to charge batteries?

Yes, but only with appropriate charging circuitry between the supply and the battery. Do not connect a 5V supply directly to a battery without a charge controller, as this can cause overcharging and damage.

What cable should I use with a 5V plug pack?

Use a cable rated for the current draw, with correct connectors e.g. 2.1mm or 2.5mm barrel jack. Long or thin cables can introduce voltage drop, so keep cable length short and gauge adequate.

How do I know if my 5V supply is failing?

Symptoms include excessive heat, buzzing, voltage sag under load or device resets. Test with a multimeter under load to verify the output stays near 5V. Replace the supply if unstable.