Series Resistor or Buck Converter for a 12 V LED Sign
A workshop must power a 12 V, 1.5 A LED sign board from a spare 19 V, 3.42 A laptop-type SMPS adapter. The sign will sit 8 m from the adapter, and the only wire in stock is single-core copper of 0.5 mm² cross-section, whose resistance is about 0.034 Ω per metre. A senior technician proposes one series resistor to drop the extra voltage; a second technician wants a 12 V buck converter module rated 3 A. The resistor would be mounted inside a closed plastic box beside the adapter, where the ambient temperature reaches 45 °C. Before work starts, the trainee must show the voltage budget, the resistor value and power rating, and the current carried by the wire. The sign's driver is not a simple resistor: it behaves as a fairly fixed 1.5 A load as long as its terminal voltage stays between 11.5 V and 12.5 V. The adapter's output is specified as 19 V ± 0.5 V.
1. Calculate the voltage the series resistor must drop, account for the cable, and select a standard resistor value and power rating. Explain why the nearest E12 value above the calculated resistance may be the wrong choice.
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Cable loop length = 2 × 8 m = 16 m, so R_wire = 16 × 0.034 = 0.544 Ω and the drop at 1.5 A is 0.82 V.
The resistor must drop 19 − 12 − 0.82 = 6.18 V, giving R = 6.18 / 1.5 = 4.12 Ω.
Choosing 4.7 Ω is tempting but wrong here: at 1.5 A it drops 7.05 V, leaving 19 − 0.82 − 7.05 = 11.13 V at the sign, below the driver's 11.5 V minimum, so the sign may flicker or drop out. A 3.9 Ω resistor drops 5.85 V and leaves 12.33 V, inside the window.
Power = I²R = 2.25 × 3.9 = 8.8 W. In a closed box at 45 °C, derate heavily: use a 20 W or 25 W wirewound resistor, stood off the board, with ceramic terminal blocks and no contact with plastic.
Check extremes: at 19.5 V the sign sees 12.83 V; at 18.5 V it sees 11.83 V. Both are acceptable, but the resistor only shifts voltage - it does not regulate current if the driver's behaviour changes.
2. Compare the resistor and the buck converter for this installation and recommend one. Include efficiency, heat, cost and failure consequences in your reasoning.
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The resistor wastes about 8.8 W as heat inside a sealed box, raising the internal temperature and ageing the adapter's electrolytic capacitors. Its output also follows the adapter: any adapter replacement or adjustment changes the sign voltage, and a fault that increases load current increases the drop and reduces the sign voltage further.
The buck converter delivers 12 V at up to 3 A with roughly 88-92 % efficiency, so it dissipates only about 1.5-2 W at 18 W output and keeps the sign voltage stable over the adapter's 18.5-19.5 V range. It costs more and can generate switching noise, so it should be mounted on standoffs, fused at 3 A on the input, and kept away from the sign's signal wiring.
Recommendation: use the buck converter, with a 3 A fuse, reverse-polarity protection and a ventilated enclosure. If the resistor must be used as a temporary measure, fit a 25 W type, ventilate the box, and label the installation as temporary. Neither option should be installed live.
3. List the measurements you would take to prove the installation is correct and safe, naming the instrument settings and the expected readings.
Read the worked answer
- Adapter output, no load and at 1.5 A: set the DMM to DC 20 V; expect 18.5-19.5 V, and the voltage should not collapse under load.
- Sign terminal voltage: DC 20 V across the sign, expected 11.5-12.5 V with the sign connected.
- Current: break the circuit, put the meter in series on the 10 A DC terminal, expect about 1.5 A. Never connect the meter in parallel on a current range - that is a short circuit.
- Polarity: confirm centre-positive or the marked terminal before plugging in.
- Temperature: after 30 minutes, measure the resistor body and box ambient with a contact or IR thermometer; the resistor should be well within its rating and the box should not exceed the adapter's rating.
- Insulation and earth: with the adapter unplugged, confirm no live-to-earth path on the low-voltage side, and check that the mains side remains untouched and enclosed. Record all readings and compare with the calculated values.