How to Wire an SMPS: L, N, Earth, V+ and V- Explained

A switched-mode power supply (SMPS) converts AC mains electricity into regulated DC power for devices such as LED strips, CNC machines, 3D printers, and electronic projects. Correct wiring helps prevent electric shock, short circuits, and damage to connected equipment.

This guide explains how to identify SMPS terminals, connect the AC input and DC output, understand voltage adjustment, and choose suitable wiring for 12V and 24V applications.

Safety warning: AC mains voltage can cause fatal electric shock. Disconnect the mains supply before wiring. If you are not trained to work with mains electricity, have a qualified electrician complete the installation. Never touch exposed terminals while the supply is energized.

SMPS Wiring in 4 Steps

Step 1: Identify the terminals

Most enclosed AC-to-DC SMPS units have separate terminals for AC input and DC output. Always follow the labels printed on the power supply because terminal positions and markings vary between models.

Terminal marking	Function
L	AC line or live input
N	AC neutral input
⏚ or FG	Protective earth or frame ground
V+	Positive DC output
V-	Negative DC output
V-ADJ or ADJ	Output-voltage adjustment, if provided

Some power supplies have multiple V+ and V- terminals. These are often connected internally in parallel to provide convenient connection points. Check the manufacturer's documentation to confirm their function.

Step 2: Connect the AC input

With the mains supply disconnected, connect the incoming live conductor to L, neutral to N, and protective earth to the designated earth terminal.

Use appropriately rated cables, secure terminal connections, suitable strain relief, and the required electrical protection. Do not rely on wire colours alone to identify conductors, particularly in older or modified installations.

Step 3: Connect the DC output

Connect the positive input of your device to V+ and its negative input to V-. Check the device's required voltage before making the connection.

For example, a 12V LED strip needs a compatible 12V DC supply, while a 24V strip requires a compatible 24V DC supply. Verify the polarity before switching on, since reversed polarity can damage electronic equipment.

Step 4: Test the output voltage

Before connecting sensitive equipment, use a suitable multimeter to check the DC output voltage and polarity. Follow safe measurement practices and avoid touching exposed terminals.

If your power supply includes a V-ADJ control, consult its instructions before making adjustments. This control usually permits limited trimming around the rated output voltage. It does not convert a 12V supply into a 24V supply.

Connect the load only after confirming that the output voltage matches the device requirements.

SMPS Terminal Connections Explained

L: Line or live

The L terminal receives the incoming AC line conductor. It carries mains voltage and must be handled with appropriate isolation and precautions.

N: Neutral

The N terminal connects to the AC neutral conductor. Neutral is not a replacement for protective earth.

Earth or FG: Protective earth

The earth terminal connects to the installation's protective-earth conductor. For a metal-cased power supply designed to use protective earth, this connection is essential for safety.

V+: Positive DC output

V+ is the positive DC output terminal. Connect it to the positive power input of the load.

V-: Negative DC output

V- is the negative DC output return. Connect it to the negative power input of the load.

Do not assume that V- and protective earth are interchangeable. They serve different purposes and should only be connected together when the manufacturer's documentation specifically requires it for the intended installation.

V-ADJ: Output-voltage adjustment

Some enclosed SMPS units include a small adjustment control marked V-ADJ or ADJ. It allows limited adjustment of the output voltage within the range specified by the manufacturer.

Use a multimeter to verify the output after adjustment. Do not open the power supply or attempt internal modifications.

SMPS Safety: Isolation, Earthing and Enclosure

The AC input side of an SMPS presents a serious electric-shock hazard. Follow these precautions during installation and maintenance:

1. Disconnect the mains supply: Prevent accidental reconnection before working on the wiring.

2. Connect protective earth: Use the designated earth terminal on power supplies designed to require it. Never substitute neutral for earth.

3. Cover exposed terminals: Use a suitable enclosure or terminal cover to prevent accidental contact.

4. Secure the cables: Use appropriate cable glands or strain relief to prevent cables from pulling loose.

5. Allow ventilation: Follow the manufacturer's recommendations for mounting clearance and heat dissipation.

6. Use suitable electrical protection: Select fuses, circuit protection, and cable sizes according to the supply rating, installation conditions, and local electrical requirements.

7. Inspect before use: Do not energize a supply with damaged insulation, loose terminals, burn marks, or signs of overheating.

Never open the metal case or attempt internal repairs unless you are qualified to do so. If the power supply smells burnt, makes unusual noises, or shows signs of damage, disconnect it and have it inspected.

SMPS Wiring for LED Strips

LED strips are commonly powered by 12V or 24V SMPS units. Check the voltage printed on the strip or specified in its datasheet before choosing a power supply.

For example, consider a 12V LED strip rated at 60W. Its approximate current requirement is:

Current = Power ÷ Voltage

Current = 60W ÷ 12V = 5A

The power supply must support the required current and total load. Allow suitable headroom for operating conditions and any additional devices connected to the same supply.

For longer LED strip installations, keep these points in mind:

• Calculate the combined wattage of all connected strip sections.

• Check whether the manufacturer recommends power injection at multiple points to reduce voltage drop.

• Use cables, connectors, and terminals rated for the expected current.

• Maintain correct polarity throughout the circuit.

• Install the supply in a dry, ventilated location unless it is specifically rated for the environment.

Never connect a 12V LED strip to a 24V supply unless the strip is explicitly designed to support that voltage. Excess voltage can damage the LEDs.

SMPS Wiring for CNC Machines

CNC machines use power supplies to operate components such as motor drivers, controllers, fans, and other electronics. The required voltage and current depend on the machine's design.

Before connecting an SMPS to a CNC machine:

• Check the voltage and current requirements of the motor drivers and controller.

• Confirm that the power supply can handle the expected continuous load.

• Follow the machine manufacturer's protective-earth and grounding instructions.

• Keep signal wiring separated from high-current motor wiring where practical.

• Use suitable connectors, cables, and overcurrent protection.

Do not connect different power-supply outputs together simply because their terminals have the same labels. Follow the machine's wiring diagram and the power-supply manufacturer's instructions.

SMPS Wiring for 3D Printers

Many 3D printers use a DC power supply to operate the controller board, heaters, fans, and motors. Some systems use 12V, while others use 24V or another specified voltage.

Before installing or replacing a printer's power supply:

• Match the output voltage to the printer's electrical design.

• Check the current and power requirements of the connected components.

• Verify output polarity before connecting the controller board.

• Use the specified fuse, wire gauge, connectors, and terminal-tightening procedure.

• Ensure that the power supply has adequate ventilation.

• Follow the printer manufacturer's grounding and safety instructions.

Do not replace a 12V supply with a 24V unit unless the printer's complete electrical system is designed for 24V. Applying an incorrect voltage can damage the controller, heaters, fans, and other components.

Testing an SMPS With a Multimeter

A multimeter can help confirm that an SMPS is producing the expected DC output.

1. Check the power supply's label to identify its rated output voltage.

2. Set the multimeter to an appropriate DC voltage range.

3. Follow safe measurement procedures and place the probes across the DC output terminals, observing polarity.

4. Compare the measured voltage with the rating and the manufacturer's tolerance.

5. If the voltage is incorrect, disconnect the load if it is safe to do so and consult the manufacturer's troubleshooting instructions.

Do not switch a multimeter to resistance or continuity mode on an energized circuit. Never probe exposed mains terminals unless you are trained and equipped to make the measurement safely.

How to Choose the Right SMPS

Choose an SMPS based on output voltage, current capacity, total power, operating environment, and the requirements of the connected equipment.

Application requirement	What to check
12V LED strips	12V DC output and sufficient current capacity
24V LED strips	24V DC output and sufficient current capacity
CNC machines	Voltage, continuous current, motor-driver requirements, and grounding
3D printers	Compatibility with the controller, heaters, fans, and motors
Embedded electronics	Correct voltage, stable output, and suitable current capacity
Industrial installation	Enclosure, cooling, protection, and installation requirements

Calculate the expected load before selecting a power supply. For a DC load, power can be estimated using:

Power = Voltage × Current

For example, a 12V load drawing 10A requires approximately 120W. The selected power supply must support the load under the actual operating conditions.

Recommended SMPS Power Supplies

Choose the output voltage and current rating based on your equipment's requirements. The following options are intended for different 12V and 24V applications.

Product	Rated output	Suitable applications
24V 5A SMPS Power Supply	24V, up to 5A	Compatible 24V electronics and LED applications
12V 15A SMPS Power Supply	12V, up to 15A	Higher-current 12V applications
12V 10A SMPS Power Supply	12V, up to 10A	12V systems requiring a moderate current capacity

Check the current product listing for the exact specifications, input range, protection features, and availability before purchasing.

Conclusion

Correct SMPS wiring starts with identifying the AC input terminals, connecting live, neutral, and protective earth correctly, and matching the DC output to the connected device. Always verify voltage and polarity before connecting sensitive electronics.

For LED strips, CNC machines, and 3D printers, choose a supply with the correct output voltage and sufficient current capacity. Use appropriate wiring, electrical protection, ventilation, and enclosure provisions. If you are unsure about mains wiring, have a qualified electrician perform the installation.