Power Supply Guide: 5V, 12V, 24V SMPS for Projects

Every electronics project needs a reliable power source, and an SMPS power supply (Switched Mode Power Supply) is the standard choice for powering everything from Arduino projects and LED strips to CNC machines and automation panels. Unlike older linear supplies, SMPS units are compact, efficient, and well-suited to India’s 230V 50Hz mains. This guide explains how to choose the right SMPS, understand voltage and current ratings, wire it safely, and avoid the most common mistakes.

What is an SMPS?

An SMPS (Switched Mode Power Supply) converts mains AC power to stable low-voltage DC by switching a transistor on and off at high frequency (typically 50kHz to 200kHz) through a transformer, then rectifying and filtering the output. The switching action is far more efficient than the linear regulator approach, converting 85% to 95% of input power to usable output (compared to 40% to 60% for linear supplies).

The result is a compact, cool-running supply that can deliver substantial current from a small enclosure. A 12V 10A SMPS that could fit in your hand would require a transformer several times larger and heavier if built with linear technology.

SMPS vs Linear Power Supply

For the vast majority of hobby and maker projects, SMPS is the right choice. Linear power supplies are preferred only for sensitive analog circuits (audio amplifiers, precision measurement) where switching noise is unacceptable.

Common Voltages: 5V, 12V, 24V

5V SMPS

5V is the native operating voltage of USB-based microcontrollers, Raspberry Pi boards, most Arduino boards when powered from VIN (with the onboard regulator bypassed), logic-level electronics, and many sensors. A dedicated 5V SMPS is better than using a USB charger for higher-current applications because it provides stable, regulated voltage even under load, with proper terminals for hard wiring.

12V SMPS

12V is the most versatile DC voltage for makers. LED strips (most non-addressable RGB strips run on 12V), small DC motors, solenoid valves, relay coils, CCTV cameras, and small fans all run on 12V. It is also the input for many step-down DC-DC converters that then produce 5V or 3.3V for microcontrollers, making a single 12V SMPS capable of powering an entire mixed-voltage project.

24V SMPS

24V is the industrial standard voltage used in PLCs, industrial automation, servo drivers, stepper motor drivers (like the popular A4988 and DRV8825 modules which work best at 12V to 24V), and HVAC control systems. It offers better efficiency over cable runs and is common in 3D printer heated beds and CNC spindle controllers.

Current Ratings and Wattage Calculation

Power (Watts) equals Voltage (Volts) multiplied by Current (Amps). This is the fundamental calculation for choosing an SMPS:

Power (W) = Voltage (V) x Current (A)

Example 1: 5V supply for a Raspberry Pi 4 (max 3A)
  Required power = 5V x 3A = 15W
  Choose: 5V 3A or 5V 4A SMPS

Example 2: 12V supply for 5 metres of 60 LED/m WS2812B strip
  At full white: 60 LEDs x 5m x 60mA = 18A at 5V
  For 12V strip (5050 RGB): 60 LEDs x 5m x 20mA = 6A at 12V
  Choose: 12V 10A SMPS (with derating buffer)

Example 3: 24V supply for stepper motor drivers (3 axes, 2A each)
  Required current = 3 x 2A = 6A
  Required power = 24V x 6A = 144W
  Choose: 24V 8A or 24V 10A SMPS

Always add 20% to 30% headroom above your calculated maximum load. Running an SMPS at 100% rated capacity continuously causes excessive heat and shortens its lifespan.

Open Frame vs Enclosed SMPS

Open frame SMPS (also called board-level or bare SMPS) has exposed PCB and components. It is cheaper, lighter, and easier to mount inside a custom enclosure. However, because live mains voltage is exposed, it must always be installed inside a protective enclosure. Never operate an open frame SMPS in open air on a bench.

Enclosed SMPS (also called metal box or case SMPS) comes in a metal housing with an earth terminal, AC input on one side, and DC output terminals on the other. The housing provides mechanical protection and a degree of electrical isolation. Enclosed supplies are suitable for semi-permanent installation and are the safer choice for beginners.

For most hobby projects, an enclosed 12V or 5V SMPS is the recommended starting point. Open frame units are used inside control panels and commercial enclosures.

Derating Factor Explained

SMPS manufacturers specify a maximum output current at a reference temperature (typically 25°C ambient). In Indian conditions where ambient temperatures can reach 35°C to 45°C, especially inside enclosures, the available output current decreases. Most SMPS units derate linearly above 40°C — a 10A supply might only safely deliver 7A to 8A in a hot, enclosed panel.

The standard recommendation is to select an SMPS rated for at least 125% to 150% of your maximum continuous load. For a project drawing 8A at 12V, choose a 12V 10A or 12V 12A SMPS rather than a 12V 8A unit.

Wiring and Connecting to Projects

Most enclosed SMPS units have screw terminal blocks for both AC input and DC output. The AC input terminals are labelled L (Line/Live), N (Neutral), and a ground earth symbol. The DC output is labelled +V and -V (or COM).

AC side (mains) wiring — safety critical:

• The brown wire is Line (Live) in Indian wiring — connect to L.
• The blue wire is Neutral — connect to N.
• The green and yellow wire is Earth — connect to the earth terminal.
• Use proper insulated connectors rated for mains voltage. Never use bare wire on the AC side.
• Always use a fused IEC inlet or a fused switch on the AC input.

DC side wiring: Connect +V to your project positive rail and -V to your project ground rail. Use wire gauge appropriate for the current: 1mm squared (18 AWG) for up to 10A, 1.5mm squared (16 AWG) for up to 15A, 2.5mm squared (14 AWG) for up to 25A.

Powering Arduino and ESP32

Arduino boards accept 7V to 12V on the VIN/barrel jack (the onboard regulator drops it to 5V). Using a 12V SMPS into the Arduino barrel jack is safe and reliable. Alternatively, use a 5V SMPS directly to the Arduino 5V pin for more efficient operation (skipping the onboard regulator entirely).

ESP32 and ESP8266 boards run at 3.3V. Use a dedicated 5V SMPS and connect to the module’s 5V pin (the onboard regulator handles 5V to 3.3V conversion), or use a 3.3V SMPS if you need to power multiple ESP32 modules from a common rail.

Powering LED Strips

LED strips are high-current loads. A 5-metre reel of 60 LED/m 5050 RGB strip draws up to 9A at full white. A WS2812B addressable strip at the same density draws up to 18A at full white (though typical programmed brightness is much lower in practice).

Rules for powering LED strips:

• Never power more than 2 metres of strip from one end without power injection. Voltage drop along the strip causes the far end to appear dimmer and colour-shifted.
• Always add a 1000uF electrolytic capacitor across the power rails close to the first LED to absorb startup surges.
• Select an SMPS with at least 20% headroom over your calculated strip current.
• Use appropriately rated wire (at least 18 AWG for runs over 1 metre carrying more than 5A).

Powering Motors

DC motors draw startup current (inrush current) that can be 3 to 6 times their rated running current. When selecting an SMPS for motor loads, use the stall current (not the rated running current) as your sizing basis. A motor rated for 2A running may draw 8A to 10A at stall. Use an SMPS rated for at least the stall current of all motors that could run simultaneously.

Add a freewheeling diode (1N5819 Schottky or similar) across each motor to protect the SMPS and driver circuit from back-EMF spikes when the motor switches off.

Fusing and Protection

Add a fuse on the DC output side between the SMPS and your load. Choose the fuse rating at 125% of your maximum continuous current. For example, a 12V supply powering a 5A load should use a 6.3A fuse. Use slow-blow fuses for motor loads (to handle startup inrush) and fast-blow for purely resistive or electronic loads.

Many quality SMPS units include built-in overcurrent protection (OCP) that shuts the output if current exceeds the rated limit. This protects the supply but does not protect wiring between the supply and the load — the external fuse does that.

Indian Mains Voltage and Safety Certifications

India uses 230V AC at 50Hz. Most SMPS units sold globally specify a universal input range of 100V to 240V AC, making them compatible with Indian mains without any switching or modification. Always verify the input voltage range on the label before connecting.

For compliance with Indian standards, look for the BIS (Bureau of Indian Standards) IS 13252 mark for IT equipment power supplies. Imported SMPS units without BIS marking are not technically certified for permanent installation in India, though they are widely used for hobby and prototyping purposes.

For permanent or commercial installations, use BIS-certified SMPS units and ensure mains connections are made in accordance with the Indian Electricity Rules, 1956 by a licensed electrician.

Frequently Asked Questions

Q: Can I use a 12V SMPS to power a 5V circuit?

Not directly. You need a DC-DC step-down converter (buck converter) between the 12V SMPS output and your 5V circuit. Common options are the LM7805 linear regulator (wastes heat), or an efficient buck module like the LM2596 or MP2307 which maintain 85% to 90% efficiency.

Q: What does the trim pot on an SMPS do?

Most SMPS units have a small potentiometer that lets you fine-tune the output voltage by plus or minus 10%. For example, a 12V SMPS might be adjustable between 10.8V and 13.2V. This is useful for compensating for cable voltage drop on long runs.

Q: Why does my SMPS click and shut off under load?

This is overcurrent protection activating. Your load is drawing more current than the supply’s rated output. Either reduce the load, or use a higher-rated SMPS. Check if any short circuits exist in your wiring.

Q: Is it safe to run two SMPS outputs in parallel for more current?

Generally no, unless the SMPS units are explicitly rated for parallel operation and include active current sharing. Mismatched units can fight each other with one taking all the load. Use a single higher-rated SMPS instead.

Q: How do I earth (ground) my SMPS properly?

Connect the earth terminal on the SMPS to your building’s earthing system through the power cable’s earth wire. The metal housing of the SMPS is also typically connected to earth internally. Proper earthing protects against electric shock if an internal fault occurs.