Choosing between an unregulated adapter, an SMPS and a battery for a 12 V load
A neighbourhood repair shop is asked to sort out a customer's 12 V lighting setup. The customer uses a small black '12 V 1 A' adapter, a 5 m LED strip rated 12 V 0.6 A, and a 12 V router rated 0.5 A through a DC splitter. The strip flickers and the adapter case is too hot to touch after 20 minutes. Bench measurements: adapter no-load output 16.8 V DC; at 0.6 A it is 13.2 V; at 1.1 A it is 11.2 V; ripple at 0.6 A is 2.4 V peak-to-peak. The adapter label says '12 V 1 A' but does not say regulated. The shop has a 12 V 2 A regulated SMPS, a 12 V 7 Ah sealed lead-acid battery, a 1 A fuse, and a 3 A fuse. The LED strip has no series resistor and is marked '12 V, do not exceed 12.6 V'. The customer wants to keep the router running for about 4 hours during power cuts, using the battery. The router draws 0.5 A at 12 V.
1. Interpret the measurements and explain why the adapter is unsuitable for this load, referring to regulation, ripple and rating.
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The no-load voltage of 16.8 V and the drop to 13.2 V at 0.6 A show poor regulation, typical of a small transformer-rectifier adapter with only a smoothing capacitor. At 0.6 A the strip is already above its 12.6 V limit, and the 2.4 V peak-to-peak ripple means the instantaneous voltage swings widely, which explains the flicker. Adding the router brings the total to 1.1 A, above the 1 A label rating. The measured 11.2 V at 1.1 A shows the adapter is overloaded and its output is collapsing; the heat is a warning of transformer and regulator stress. A 12 V 1 A label is a maximum, not a guarantee that the adapter will hold 12 V at that current. The correct judgement is to stop using this adapter for this load, not to add a capacitor and continue.
2. Recommend a replacement supply and fuse selection. Explain why a regulated SMPS is preferable and calculate the current headroom.
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Choose the 12 V 2 A regulated SMPS. The total normal load is 0.6 A + 0.5 A = 1.1 A, leaving 0.9 A headroom, about 45 percent. Its regulated output should stay near 12 V and its ripple should be far below the 2.4 V peak-to-peak measured, so the strip will not flicker and the router will see a safe voltage. A 2 A time-lag fuse would be appropriate for a 1.1 A load, but the shop only has 1 A and 3 A. The 1 A fuse will nuisance-blow at 1.1 A; the 3 A fuse will not protect the 1 A adapter or small wiring. The correct action is to obtain a 2 A fuse or a fused distribution box. If a 3 A fuse is used temporarily, it protects only against a short circuit, and the wiring and connectors must be rated for it. Do not rely on the adapter's thermal cutout as the only protection.
3. Calculate the battery backup time for the router at 0.5 A, using 50 percent depth of discharge, and recommend how the battery should be connected and charged.
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A 12 V 7 Ah battery has 7 Ah of capacity, but a sealed lead-acid battery should not be discharged below about 50 percent regularly. Usable capacity is 7 x 0.5 = 3.5 Ah. At 0.5 A, time = 3.5 / 0.5 = 7 hours. The customer needs about 4 hours, which requires 2 Ah, or about 29 percent depth of discharge, so the battery is adequately sized. However, the battery must not simply be paralleled with the SMPS output: that can overcharge the battery and back-feed the supply. Use a proper changeover arrangement or a small UPS/charger with a charge controller, a fuse at the battery positive terminal, and a low-voltage cut-off. The router's input tolerance should be checked; a 12 V lead-acid battery varies from about 12.6 V full to 11.5 V under load, which is usually acceptable for a 12 V router but not guaranteed.