Project system
12 V LED strip lighting: choose the supply, wire it and dim it
By Bulan Sarkar, founder of ElectronicsInfoline (since 2000) · About a 11-minute read
A plain 12 V LED strip is one of the simplest festival lights to build safely. It runs from a certified plug-in adapter, so you never open anything on the mains side.
The planning comes down to three numbers: the strip's watts, the adapter's amps and the resistance of the wire between them. This page works out each one, then shows how to cut, join, feed and dim the strip.
Multiply the strip's watts per metre by its length, add 25 % and choose a 12 V adapter rated above that. A 5 m strip at 4.8 W/m draws 24 W, or 2.0 A, so it needs at least 30 W: a 60 W, 5 A adapter fits and a 25 W one does not. Run 0.75 mm² twin wire or thicker from the adapter. Cut the strip only on its marked pads, every 3 LEDs (25 mm). To dim it, switch the minus side with an IRLZ44N MOSFET from an Arduino PWM pin.
Build it, buy it, or get help
Build it yourself
One or two strips on a balcony, a shelf, a puja corner or a cabinet, up to about 60 W per adapter, plugged into a socket you already have. An evening of cutting and soldering.
Buy a module instead
Cove lighting for a whole ceiling, or more than about 10 m of strip: a ready-made, certified LED luminaire or strip kit with its own driver is simpler and tidier than several adapters.
Stop and get help
Anything that needs a new socket, an outdoor power point or wiring inside a switchboard. That is mains work, and a licensed electrician does it. Never cut or rewire a mains-powered rope light.
The steps, in order
1. Read the strip's label
Have ready: The strip reel or its product listing
Find the voltage (12 V), the watts per metre and the length. A typical 60 LED/m warm-white strip says about 4.8 W/m. High-output 2835 strips run 16.5 to 19.2 W/m (YUJILEDS and LuxaLight datasheets). Strip watts = W/m × metres.
Check: Strip watts written on the card
2. Size the adapter
Have ready: The strip watts from step 1
Add 25 % and pick a 12 V adapter rated above that; amps = watts ÷ 12. A 5 m strip at 4.8 W/m takes 24 W and 2.0 A, so with the margin you need 30 W and 2.5 A. The Mean Well GST25A12 (25 W, 2.08 A) is too small; the GST60A12 (60 W, 5 A) has room. A high-output strip at 16.5 W/m reaches the 60 W limit at 2.9 m: 3 m needs 61.9 W, which is too much, and 2.5 m needs 51.6 W, which fits.
Check: Adapter amps above strip amps × 1.25
Figure 1. The whole setup: the adapter plugs into an existing socket, and everything after its plug is 12 V DC. The dimmer is optional. Figure 2. Adapter sizing with the 25 % margin: a 5 m plain strip needs 30 W, so it overloads a 25 W adapter and fits a 60 W one; 3 m of high-output strip needs 61.9 W. 3. Choose the wire from adapter to strip
Have ready: The distance from the socket to the strip
Current flows out and back, so double the distance: 3 m away means 6 m of copper. At 2 A, 0.5 mm² wire (39.0 Ω/km) loses 0.47 V and 0.75 mm² (26.0 Ω/km) loses 0.31 V. At 5 A the 0.5 mm² wire loses 1.17 V, nearly a tenth of the 12 V, and the strip turns dim and yellow. Keep the loss under about 0.5 V: 0.75 mm² up to about 2.5 A, 1.5 mm² for 5 A.
Check: Wire loss from the table under 0.5 V
Figure 3. Voltage lost in 3 m of twin wire (6 m of copper) at 2 A and 5 A, from IEC 60228 resistances. Keep it under 0.5 V. 4. Cut the strip only on the marked pads
Have ready: Sharp scissors and the strip
A 12 V strip is built from segments of 3 LEDs and a resistor, 25 mm long (YUJILEDS, LuxaLight). Cut through the middle of the copper pads marked with a scissor symbol or a line. Cut anywhere else and that whole segment stays dark.
Check: Both cut ends show half a copper pad
Figure 4. Where to cut: each 25 mm segment carries 3 LEDs and a resistor, and the cut line runs through the middle of the copper pads. 5. Join the wires
Have ready: Soldering iron, red/black twin wire, heat-shrink, or a clip-on connector for your strip width
Tin the pads and the wire, solder red to + and black to −, and shrink a sleeve over the joint. A solderless clip connector works too if it matches the strip width. Then fit a DC jack pigtail to the other end of the wire: the GST60A12 plug is 5.5 × 2.1 mm, centre positive.
Check: About 12 V at the far end with the strip lit
6. Feed long runs from both ends
Have ready: A second pair of wires
The strip's own copper is thin, so the far end of a long run gets less voltage and glows dimmer. For a full 5 m reel, run a second pair of wires from the adapter to the far end. If the strip needs more than one adapter can give, split it into runs and give each its own adapter. Never chain a second reel onto the end of the first.
Check: Near and far ends look equally bright
Figure 5. Three ways to feed a run: one end only (the far end goes dim), both ends from one adapter, or two runs on two adapters. 7. Dim it, with code or without
Have ready: IRLZ44N, 330 Ω, 10 kΩ and an Arduino, or an in-line 12 V PWM dimmer
With an Arduino, follow step 6 of the festival LED lights system: strip − to the MOSFET drain, source to ground, a PWM pin to the gate through 330 Ω, and 10 kΩ from gate to ground. Infineon specifies the IRLZ44N at 0.025 Ω with 5 V on the gate. The IRFZ44N is specified only at 10 V, so it may not turn fully on from an Arduino. For no code at all, put a ready-made 12 V in-line PWM dimmer between the adapter and the strip.
Check: Strip off at power-up, full at 255, dim at 64
Figure 6. Dimming with an Arduino: the same IRLZ44N circuit as step 6 of the festival system, 2.0 A for the 5 m example strip. 8. Check the heat, then fix it in place
Have ready: The strip's adhesive or an aluminium channel, cable clips
At 2 A the IRLZ44N runs only about 6 °C above room temperature. At 5 A it runs about 39 °C above, so fit a small clip-on heatsink above about 4 A. Wipe the surface with alcohol before you stick the strip down, and mount high-output strips on aluminium channel.
Check: After 30 min, MOSFET and adapter warm, not hot
Figure 7. IRLZ44N temperature rise with 5 V on the gate, from 0.025 Ω and 62 °C/W in free air: about 6 °C at 2 A and 39 °C at 5 A.
What goes wrong, and how to find it
| Symptom | Likely cause | Check and fix |
|---|---|---|
| The far end is dimmer or yellower | The voltage falls along the strip and the lead wire. | Measure at both ends with the strip lit. Feed the far end with a second pair of wires, or use thicker wire from the adapter (steps 3 and 6). |
| The adapter clicks, or the strip blinks on and off | The adapter is overloaded and has gone into hiccup mode (Mean Well lists 105 to 150 % overload protection). | Work out the strip watts again from the label. Use a bigger adapter or a shorter run. |
| One 25 mm segment stays dark | It was cut off the pads, or a solder joint is dry. | Re-cut on the pads and re-solder; the joint should be shiny and cover the whole pad. |
| The strip stays faintly on, or flickers when the Arduino resets | There is no 10 kΩ pull-down, or the MOSFET is an IRFZ44N. | Fit 10 kΩ from gate to ground and check the part number: use the IRLZ44N. |
| The Arduino resets when the strip turns on | The grounds are joined by a long, thin wire. | Run one short, thick ground from the adapter to the MOSFET source and a separate wire to the Arduino's GND. |
| The adhesive lets go after a week | The surface was dusty, or the strip runs hot. | Clean with alcohol and press the strip down firmly. Use aluminium channel for high-output strips. |
Variants
5 V USB strip vs 12 V strip
A short USB strip (under 1 m) runs from a phone charger with no adapter sizing. At the same watts, though, 5 V draws 2.4 times the current of 12 V, so 5 V only suits short runs.
12 V vs 24 V strip
At the same watts a 24 V strip draws half the current, so the same wire wastes a quarter of the power (0.25 of it). Runs over 5 m are easier at 24 V. Use a 24 V adapter and never mix the two.
Arduino PWM vs an in-line dimmer
The Arduino gives you patterns, fades and timers; the in-line dimmer only needs plugging in. The power arithmetic in steps 1 to 3 is the same for both.
Indoors vs a balcony
Outdoors, use an IP-rated (coated) strip such as an IP64 type, seal the cut end with silicone, and keep the adapter indoors so only the 12 V wire goes outside.
What we would do
For a balcony or shelf this Diwali, we would use one 5 m warm-white strip of about 5 W/m, a BIS-registered 12 V, 5 A adapter and 0.75 mm² twin wire, with a second feed to the far end if the run is the full 5 m. If you want it to fade or come on at dusk, add the IRLZ44N and an Arduino from the festival system. If you only want it on, leave the electronics out. Any new socket is a job for an electrician.
Tips and tricks
- Write the strip's W/m on the back of the reel with a marker the day you buy it. Listings change or vanish, and you will want the figure when you add a second strip.
- Measure the voltage at the far end of the strip with it lit, not at the adapter. The far end tells you whether the wire is too thin.
- Cut the strip one segment longer than you think you need and trim it in place. You cannot add a segment back.
- Put a small label on the DC jack: "12 V, strip only". Then nobody plugs a laptop charger into it.
- Run the strip for an hour on the bench before you stick it down. Dark segments and weak joints show up then, not after the tape is on the wall.
- Warm white (2700 to 3000 K) sits well next to diyas in photos. Cool white looks blue beside them.
Print the working copy
One A4 page for the bench: the parts list as a checklist, every connection with a tick box, and the tests with the readings to expect. It prints in black and white.
Source and assumptions
Infineon IRLZ44NPbF datasheet (PD-94831). p. 1: VDSS 55 V, ID 47 A at 25 °C, RθJA 62 °C/W, TJ max 175 °C, VGS ±16 V; p. 2: RDS(on) 0.022 Ω at VGS 10 V, 0.025 Ω at 5.0 V, 0.035 Ω at 4.0 V, VGS(th) 1.0 to 2.0 V. Checked 7 October 2026.
Our own choices: the 4.8 W/m example strip, the 3 m lead, and the 0.5 V limit for the wire loss. Also cited: Infineon IRFZ44NPbF p. 2 (RDS(on) only at 10 V); Mean Well GST25A and GST60A specs p. 2 (2.08 A and 5 A at 12 V, 105 to 150 % hiccup protection) and p. 3 (5.5 × 2.1 mm plug, centre positive); YUJILEDS 2835L 12 V strip YJWJ029 p. 1 (16.5 W/m, cut every 3 LEDs) and p. 5 (25 % supply margin); LuxaLight LS12W120X2835PLX p. 2 (19.2 W/m); Lapp table T11 for IEC 60228 class 5 copper at 20 °C; the BIS Compulsory Registration Scheme list (adapters and SMPS). MOSFET heat is conduction loss at 25 °C; RDS(on) rises when the part is hot, so treat the figures as "about". All checked 7 October 2026.
Questions people ask
What size adapter do I need for a 5 m LED strip? Multiply the watts per metre by 5, add 25 % and divide by 12 for amps. A 4.8 W/m strip needs 30 W or 2.5 A, so pick a 12 V, 5 A adapter. A 16.5 W/m strip needs 103 W, more than one 60 W adapter gives, so split it into two runs.
Where can I cut a 12 V LED strip? Only through the copper pads marked between segments. On 12 V strips a segment is 3 LEDs and 25 mm long (YUJILEDS and LuxaLight datasheets).
Why is one end of my LED strip dimmer? The voltage drops along the strip's thin copper and along the lead wire. Feed the far end with a second pair of wires from the adapter, or use thicker wire from the socket.
Can I use an IRFZ44N instead of an IRLZ44N? Not from a 5 V Arduino pin. Infineon specifies the IRFZ44N's on-resistance only with 10 V on the gate. The IRLZ44N is specified at 0.025 Ω with 5 V and 0.035 Ω with 4 V.
Tools and guides this system uses
- Festival LED lights systemStep 6 has the full MOSFET wiring and the Arduino fade sketch.
- LED resistor calculatorFor single LEDs added beside the strip.
- LM317 calculatorTo take a 5 V or 9 V rail off the 12 V adapter.
- Battery backup calculatorHow long a battery-powered strip will run.
- Duty cycle and PWMWhy a switching MOSFET looks like a dimmer.
- 555 LED flasher systemBlinking lights with no code at all.
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