Power Supplies for Security and Surveillance Systems

A security system is specified around availability. Cameras, access control and alarm hardware are expected to work during exactly the events that are most likely to interrupt the mains, so the power supply is not a supporting component — it is part of the function being bought.

Two things separate this application from general low-voltage work: the supply is often expected to manage a standby battery itself, and the loads sit at the end of cable runs long enough that voltage drop decides the design. Both are routinely discovered after installation rather than before.

Size for the night, not for the afternoon

A camera's datasheet often quotes a typical consumption that reflects daytime operation. The same camera at night, with its infrared illuminator at full output, can draw several times that figure. Add a heater in a winter housing, or a pan-tilt-zoom head that draws its peak while moving, and the difference between the quoted figure and the real worst case becomes substantial.

The failure this produces is unhelpfully intermittent. A system commissioned on a bright afternoon works perfectly. It browns out after dark, when every camera on the circuit switches its illuminator on within the same few minutes, and by the time anyone investigates the following morning the fault has cleared itself.

Size from the worst simultaneous case: all illuminators on, all heaters on, any motorised optics in motion. For a system with a standby battery, add the charging current, because a supply recovering a discharged battery is doing that on top of carrying the full load.

Voltage drop is what kills long camera runs

Voltage drop along a cable is the current multiplied by the resistance of the conductors, and the return leg counts as well as the outward one. On a thin-conductor run of any length at a camera's peak current, the drop is measured in volts rather than millivolts.

The consequence depends on the system voltage. A 12 V camera with a 10.8 V minimum has around 1.2 V of margin, and a long thin run will consume all of it. The same cable carrying the same power at 24 V carries half the current, so the drop halves, and the device has twice the absolute margin to begin with. That is the reason 24 V equipment is preferred on long runs even though 12 V cameras are more common.

Three ways out, in the order they are usually worth trying: use 24 V equipment where the choice exists, increase the conductor cross-section, or move the supply closer to the load and accept a distributed architecture. Where the run also carries video or data, Power over Ethernet moves the problem into a standard that defines its own power budget and cable limits — a cleaner answer where the camera supports it.

Supplies with the battery management built in

Backup can be assembled from a supply, a charger and a changeover arrangement, but the security-specific families exist because doing it as three components is more expensive and less reliable than buying it as one.

The security and specific category holds several families built around this pattern, each combining a regulated output, a charger for a sealed lead-acid battery and automatic transfer when the mains fails:

• AD, ADS, ADD and LAD — enclosed units combining supply, charger and backup, with the ADD variants providing a second output rail.
• DRC and DRS — the same function on DIN rail, for installations built into a distribution board rather than a dedicated cabinet.
• PSC — enclosed supplies with an integral charger across a range of output powers.

Whichever is used, the battery is the part that determines whether the system actually survives an outage, and it is the part that ages. A sealed lead-acid battery loses capacity steadily and will pass a voltage check while holding a fraction of its rated amp-hours. Backup capacity needs to be tested under load on a schedule, not inferred from a healthy-looking float voltage.

Equipment that lives outside

Perimeter cameras, gate controllers and car park hardware sit outside the heated envelope, and the supply feeding them often does too. Three specifications decide whether that works.

Ingress protection has to match the location honestly. An enclosure rated against splashing is not rated against the jet of a pressure washer, and a unit mounted where water can pond is in a different situation from one on a vertical wall. Cold start is the specification most often missed: a supply rated for operation down to a low temperature may still carry a higher minimum start-up temperature, which is what matters for a unit that loses power overnight in winter and has to restart in the cold. And condensation is a separate problem from rain, because an enclosure that seals against water also seals moist air inside it to condense on the coldest surface as the temperature cycles.

The HEP family in the same category is built for these conditions, with conformal-coated construction intended for humid, dusty and corrosive environments rather than for a clean indoor panel.

Central cabinet or local adaptors?

Small installations are frequently built with an individual plug-in adaptor at each camera, drawn from the adaptor category. It is quick, it needs no cabinet, and for a handful of devices around one building it is entirely reasonable.

It scales badly. Each adaptor is an independent point of failure in an unmonitored location, backing up twenty of them individually is impractical, and a mains socket next to every camera is not always available or secure.

Centralised supplies feeding the loads over dedicated cable reverse those trade-offs: one place to back up, one place to monitor, one place to maintain — at the cost of the cable runs and the voltage-drop calculation they force. The crossover comes earlier than most installers expect, typically once backup is a requirement rather than a nicety. For the centralised case the general-purpose units in the enclosed category, such as LRS and RS, serve where the battery management is handled separately.