CCTV power distribution, voltage drop and battery backup
A shop installs a four-camera CCTV system in a hotel corridor. Each fixed dome draws 500 mA at 12 V in day mode and 700 mA with IR on; the DVR draws 2 A at 12 V. A single 12 V / 5 A SMPS at the reception desk feeds everything through 25 m of 0.75 mm² copper two-core cable to a junction box, then short 2 m tails to each camera. Video goes on separate coax. The measured voltage at the farthest camera is 8.1 V in day mode, and the SMPS output reads 12.6 V at its terminals. At night, when the IR cuts in, two cameras reboot every few minutes. The owner wants to buy a 12 V / 10 A SMPS 'to push more current'. A 12 V / 7 Ah SLA battery on a 13.8 V charger is fitted for backup, and the site suffers power cuts of two to three hours.
1. Calculate the voltage drop on the feed and show whether a 12 V / 10 A SMPS will fix it. What would you do instead?
Read the worked answer
Loads. Day: 4 × 0.5 = 2.0 A for cameras plus 2 A for the DVR = 4.0 A. Night: 4 × 0.7 = 2.8 A plus 2 A = 4.8 A.
Cable resistance (copper, ρ = 0.0175 Ω·mm²/m, 0.75 mm², 25 m)
- R per conductor = 0.0175 × 25 / 0.75 = 0.583 Ω
- Loop (out and return) = 1.17 Ω
- Day drop at 4.0 A = 4.7 V → 12.6 − 4.7 = 7.9 V at the far end, close to the 8.1 V measured (the current is slightly below 4 A because the cameras are already under-volted and drawing less).
- Night drop at 4.8 A = 5.6 V → about 7.0 V, far below the 9–10 V minimum most domes need. That is why the IR switch-on, which is the largest current step the camera ever sees, causes reboots.
The 10 A SMPS does not help. Drop = I × R; the resistance is the problem, and a bigger supply only adds current capacity, not voltage at the far end. The extra headroom might delay failure but will not fix it, and the site would then be running a 10 A unit at under half load for no benefit.
Fixes, in order of preference
- Move to a local 12 V / 2 A supply per camera, or one 4-channel PSU at the junction box, fed with 230 V – this eliminates the long 12 V run entirely.
- If a single supply must stay, use 2.5 mm²: R = 0.175 Ω per conductor, loop 0.35 Ω, drop at 4.8 A = 1.7 V, which is acceptable.
- Use PoE if the domes support it: 48 V over the same cable loses far less for the same power. Also size the supply at about 80% load: 4.8 A needs a 6 A unit, not 5 A, and not 10 A for the wrong reason.
2. Size the backup battery for the stated outages and comment on the existing charger and protection.
Read the worked answer
Energy required. Use the night load, because the outages are the worst case: 4.8 A at 12 V for 3 hours = 14.4 Ah delivered to the load.
Battery sizing. A lead-acid SLA should not be taken below about 50% depth of discharge if it is to survive more than a few hundred cycles, and the Peukert effect at a 0.4 C rate costs a little more. So:
- nominal capacity ≈ 14.4 / 0.5 = 28.8 Ah;
- choose 12 V / 42 Ah for margin (or 12 V / 26 Ah if only two hours must be covered).
A 12 V / 7 Ah battery gives roughly 7 × 0.5 / 4.8 ≈ 0.73 h, about 44 minutes – nowhere near the requirement, and it will be ruined within months by repeated deep discharge.
Charger. A fixed 13.8 V supply is a float voltage, not a charger. For a 42 Ah VRLA, a proper three-stage charger is needed: boost/cycle at 14.4–14.7 V, float at 13.6–13.8 V, current limited to about C/10 (roughly 4 A), preferably temperature compensated. At 13.8 V only, the battery will sit partially charged and lose capacity over time. If LiFePO4 is chosen instead, use 14.2–14.6 V charging with a BMS-rated charger and never float it at lead-acid voltages.
Protection. Fit a fuse at the battery positive terminal (within 150 mm), a low-voltage disconnect set at 10.5–11 V, and a changeover that isolates the load from the SMPS when mains fails so the battery is not back-fed.
3. At night the video shows horizontal bands and one camera has a faint hum. What power-quality checks would you make, and how do they relate to the voltage drop?
Read the worked answer
First, separate power from signal. Horizontal rolling bands are usually 50 Hz or 100 Hz ripple on the 12 V rail or a ground loop; the hum on one camera points to a ground potential difference between that camera and the DVR.
Checks in order
- Ripple on the 12 V output. Measure with a true-RMS DMM on AC volts, and better, a scope on DC coupling with the probe at the SMPS terminals. A healthy 12 V SMPS shows well under 120 mV peak-to-peak ripple; several hundred millivolts, or a waveform with a sawtooth, means dried output capacitors. Replace the unit rather than adding filters to a dying supply.
- Load against rating. 4.8 A on a 5 A supply is 96% loading, and ripple rises sharply near the limit. This alone justifies a 6–7 A unit or per-camera supplies.
- Voltage at the camera under load. Re-measure at night with IR on. If the camera sits at 7 V, its internal regulator is dropping out and sync can be lost; the bands and the reboots share a cause.
- Ground loop. With the camera and DVR on different earth points, current flows on the coax screen. Test by powering the camera from a battery or an isolated supply; if the hum disappears, fit a video balun with isolation or a ground-loop isolator, and earth the system at one point only.
- Routing. Keep the 12 V cable away from mains and from the coax where possible, and add a ferrite clamp at the camera tail if switching noise persists.
Link to the drop. Low voltage makes the camera's own regulator work harder, increases current draw and makes it more sensitive to ripple, so fixing the distribution (2.5 mm², local supplies or PoE) often removes the interference symptom at the same time.