Practical case: Selection of transformer for LED

Materials

• 1x T1 — 12 VDC transformer-based adapter (wall adapter), 1 A minimum
• 1x F1 — Fuse, 1 A fast-blow, inline holder
• 1x LED1 — 12 V LED strip (e.g., 5 W, ~0.42 A)
• 1x Breadboard or terminal block (for low-voltage side)
• 2x Multimeter probes; 2x oscilloscope probes (optional for ripple check)
• Wire, insulated, for low-voltage DC

Notes:
– If your LED strip draws more than 0.8 A, size T1 and F1 accordingly. Aim for ≥ 20–50% margin in current.

Wiring guide

• Ensure the adapter is an isolated, transformer-based 12 VDC unit; do not open it. Work only on its low-voltage output.
• Identify T1 output: +12 V (red or marked +) and 0 V (black or marked −).
• Connect F1 in series with the +12 V line from T1.
• Connect LED1 between +12 V (downstream of F1) and 0 V (GND).
• Keep wire lengths short; observe LED strip polarity (+12 V and GND).
• Do not exceed the strip’s rated power for the chosen T1 current.

Schematic

                                +V ────────────────● V+─────────────┬──────────────┐
                                                                  ┌─┴─┐            │  R1 470 Ω 1/4 W
                                                                  │   │            │
                                                                  │   │            │
                                                                  └─┬─┘            │
                                                                    │              │
                                             BR1 Puente 1A/100V     │            ┌─┴─┐  D1 LED blanco
                     T1 230:9 VAC ≥1 VA        ┌───────┐            │            │   │
                 ┌─────────┐                  ┌┤       ├┐           │            │   │
   Secundario ~──┤         ├─────────────┬────┤       ├┤─────┬──────┴───────● V_LED │
                 │         │             │    └┬──────┬┘     │                     │
                 └─────────┘             │     │      │      │                     │
                                         │     │      │      │                     │
                                         │     │     ┌┴┐     │                     │
                                         │     │     │ │ C1  │                     │
                                         │     │     │ │ 470µF                     │
                                         │     │     └┬┘ 16V │                     │
                                         │     │      │      │                     │
                                         │     └──────┴──────┴───────────────┬─────┘
                                         │                                 ┌─┴─┐
                                         │                                 │   │
                                         └─────────────────────────────────┴───┴─── GND

                                                 ● I_LED

Measurements and tests

• • Power-up checks:
  • With LED1 disconnected, verify T1 polarity using a multimeter: +12 V on +V relative to GND.
• • VDC:
  • Measure DC voltage at the dot labeled VDC (to GND). Expect ~12.0 VDC (±10% typical).
• • I_LED:
  • I_LED = LED current. Open the wire at the I_LED dot and insert the multimeter in series (A-range). Expect I_LED ≈ P_strip / 12 V. Example: 5 W strip → ~0.42 A.
• • V_LED:
  • V_LED = voltage across LED1. Place the voltmeter across LED1 (top lead to +V, bottom lead to GND). Expect ≈ 12 V while on.
• • Thermal:
  • After 10 minutes, the adapter should be warm but not hot; if too hot to touch or shuts down, upsize T1.
• • Optional ripple (scope):
  • If you use a non-regulated adapter, check ripple on +V with respect to GND (AC-coupled). Vpp ≤ 0.5 V is fine for most 12 V strips.

How to select the transformer (practical rule of thumb)

• Determine LED strip power and current: I_strip ≈ P_strip / 12 V.
• Choose T1 current ≥ 1.5 × I_strip for cool operation and startup margin.
• Example:
• P_strip = 5 W → I_strip ≈ 0.42 A.
• Pick T1 ≥ 0.63 A → choose a common 12 VDC, 1 A adapter.
• If using a non-regulated (older, heavy) adapter, expect no-load voltage > 12 V and some ripple; verify under load that VDC is within 11–13 V.

Common mistakes

• Reversing LED strip polarity (won’t light, may damage strip).
• Underrated adapter (voltage droops, flicker, overheating).
• Skipping the series fuse; a short can overheat wiring or the strip.
• Mixing AC adapters with DC LED strips; ensure the output is 12 VDC.

Safety

• Use only isolated, certified wall adapters for the low-voltage supply.
• Never open the adapter or work on mains; all work here is on the 12 V side.
• Fuse the branch feeding the strip (F1) to limit fault current.

Possible improvements

• Add a switch on the +V line for convenient control.
• Add a DC jack and proper strain relief for robust connections.
• If brightness regulation is desired, use a 12 V PWM dimmer rated above strip current.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

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