Video security isn’t a gamble
20 October 2010
Even before the heightened awareness brought on by threats of terrorism, many public and private institutions had already began to incorporate video security into critical infrastructure.
Video security provides the visual images necessary to improve situation awareness, deter vandalism, theft or other crime, accelerate response and management decisions, and increase overall staff and public safety.
One of the most demanding video security installations is a casino, where sleight of hand and the quickest of movements must be captured, reviewed and archived to meet stringent regulations. The strategic placement of eye in the sky cameras supplements the resources of security staff, helping them to better manage crowds and deploy personnel in a timely and effective manner.
High-quality video of gaming areas provides security professionals with the detailed information needed to prevent cheating. Surveillance cameras can also help deter crime while aiding authorities in identifying criminals during their investigations.
Retail stores can also benefit from video surveillance systems by serving as a visual deterrent and providing managers and security professionals with tools for dealing with liability claims or employee theft. As a result, crime can be dealt with more easily when the perpetrator is caught in the act on video. Furthermore, slips, falls and liability claims can be accurately understood and processed when captured on video.
Contraband, violence, inmate and officer safety are just a few of the issues that must be dealt with on a routine basis in the management of correctional facilities. Video security has never played a more important role in maintaining order and ensuring a safe working environment. These systems provide continuos monitoring of remote locations, and as a result, some of the monitoring cameras need to be several hundred feet away from the base control system or power outlet.
To achieve complete surveillance coverage at certain sites, somewhere between 200 to 300 cameras may be needed. Each camera requires its own power for operation and typically needs 24V DC at 20W unless it is mounted in an open environment, where it is subjected to the elements and needs to be heated to prevent ice from forming. In this case, the power demand can approach 70W.
Therefore, the system integrator needs to either have an electrician install an AC outlet box close to the camera or power the camera through a long run of wires. However, long wire runs can produce a large voltage drop causing considerable load regulation errors, and as the load current increases, the voltage drop in the wiring increases due to the line impedance, and the voltage delivered to the load drops. System integrators have routinely increased the wire size from 22 AWG to 16 AWG to maintain regulation even though the cost delta between these sizes can be as high as $250 for a 1000ft wire run.
Alternatively, the traditional approach to solving this problem has been to sense the output voltage with a pair of sense wires that run from the regulator’s feedback network to the load. While this approach works well, the additional pair of low gauge wires, typically 26 AWG, may not be practical due to the cost, weight or logistic considerations. Figure 1 shows a remote sense block diagram with one wire sensing the voltage drop in the positive voltage wire and the other wire sensing the drop in the negative voltage wire.
Linear Technology has developed the LT4180 Virtual Remote Sense (VRS) controller, eliminating the need for remote sense wires to compensate for the IR losses in cables and wiring.
This device continuously interrogates the line impedance and corrects the power supply output voltage to maintain a steady voltage at the load regardless of current changes. The block diagram in figure 2 shows the LT4180 VRS controller’s basic operation.
VRS works with a small modulation on the output of the regulator to compute the correction. This modulation is filtered by an output capacitor at the load that is always present. The LT4180 works with nearly any conversion topology, isolated or non-isolated power supplies, DC/DC converters including bricks, modules and adjustable linear regulators. VRS can open up opportunities previously unavailable in surveillance equipment installation and works with nearly any type of power supply or regulator; switching or linear, isolated or non-isolated, providing maximum flexibility.
The LT4180 operates in conjunction with a power supply or DC/DC converter and forces the output current to change from 95% and 105% of the nominal output current at a pre-determined dither frequency. Effectively, the LT4180 forces a square wave current with a peak-to-peak amplitude equal to 10% of the DC current on top of the regulators output current (IL). A decoupling capacitor at the load filters out the dither current from the VRS square wave. This capacitor is sized to produce an AC short at the current square wave frequency, resulting in a voltage square wave at the power supply with peak-to-peak amplitude equal to one-tenth of the line impedance. The LT4180 continuously monitors the line impedance and corrects the regulators output voltage to provide accurate load regulation.
The simplified circuit in figure 3 shows how the LT4180 can be designed into an off-line power supply to deliver 24V at 0.7A with a 3A peak output through a CAT5E cable to power a surveillance camera. The LT4180 continuously monitors the line impedance of the cable and its DRAIN pin drives the diode in an optocoupler to change the duty cycle of the power supply in order to maintain tight regulation at the load, regardless of load current changes.
The curve in Figure 4 shows the load regulation of an LT4180 line loss compensation circuit for a 5V output voltage. The load current is increased from zero until it produces a 2.5V drop in a cable. With the LT4180 correcting the power supplies, duty cycle results in only a 73mV change in voltage at the load over the entire load range.
The 3V to 50V input voltage range covers a variety of applications. Its output driver has a 5mA sink capability to drive an opto-coupler for isolated designs. A programmable fixed dither frequency or spread spectrum frequency determines the sampling rate of the line impedance, and the soft-correct function insures an orderly start-up. When the run pin rising threshold is first exceeded (indicating VIN has crossed its undervoltage lockout threshold), the power supply output voltage is set to a value corresponding to zero wiring voltage drop (no correction for wiring). Over a period of time (determined by the CHOLD4 pin on the LT4180), the power supply output increases to account for the wiring voltage drop, providing best load-end voltage regulation. A new soft-correct cycle is also initiated whenever an overvoltage condition occurs.
LT4180 is available in an SSOP-24 package with three temperature grades; an extended grade version from -40°C to 85°C, an industrial grade version from -40°C to 125°C, and a military grade from -55°C to 125°C.
Bruce Haug is Product Marketing Engineer of Power Products at Linear Technology
Contact Details and Archive...
Related Articles...
Most Viewed Articles...