Powering Up Security Devices

Power supplies come in two basic designs – linear and switching – both with advantages and disadvantages. Which power supply design is best? There is a time and place for both types.

Many types of power supplies are available to the security industry because the requirements for each type and size of project present a different set of requirements. But as far as the actual creation of power, there are two basic power supply designs: linear and switching.

Some installation instructions for keypads and other electronic devices may clearly specify that the device may be used with one or the other type power supply. The instructions may state that using a switching mode power supply is not recommended, or the device might state a warning that the input voltage to the equipment must be within a certain percentage of the level specified. For a 24VDC device, that means the voltage must remain between 21.6 and 26.4 volts.

When you are dealing with long wire runs; multiple devices; marginal line voltage power quality and fluctuating loads, keeping your voltage within spec can be challenging, and disregarding the manufacturer’s specifications can result in unexpected consequences.

Security electronics typically operate on low voltage. Security equipment requires AC (Alternating Current) or DC (Direct Current).

AC requires the use of either a transformer which steps down line voltage (110-120VAC) from the receptacle to usually 12 or 24 Volts Alternating Current (12VAC or 24VAC).

DC requires a power supply which typically performs the step down from line voltage, then does the additional steps required to produce an adequate quality Direct Current.

The first question that tech support will ask is: “What is the voltage reading at the input of the equipment?” Make that measurement before you make that phone call. You might be surprised.

Linear and switching mode power supplies each have their own characteristics which you should keep in mind so that you can make an informed choice for future products.

Linear power supplies work by first lowering line voltage to a lower value (e.g. 12VAC) using a transformer. This lower voltage is still AC. Then rectification is done by a set of diodes transforming this AC voltage into pulsating voltage. The next step is filtering by an electrolytic capacitor.

The DC obtained after the capacitor contains ripple. Ripple is a small amount of AC voltage which rides on the DC voltage. Unregulated linear power supplies have ripple which is not critical for many applications. Excessive ripple is manifested as hum bars in video equipment, and a low frequency audible noise on audio based equipment.

For applications where the amount of ripple is an issue, or a steady voltage output is important, a regulated power supply is required. This is done by installing a zener diode or a voltage regulator integrated circuit.

The size of the transformer and the capacitance (and thus the size) of the electrolytic capacitor are inversely proportional to the frequency of the input AC voltage. The lower the AC voltage frequency, the bigger the size of those components and vice-versa.

Since linear power supplies use the low frequency 60 Hz, the transformer and the capacitor are very big. The higher the current rating of the linear power supply, the larger its transformer will be. You can tell the power rating of a linear power supply by how much it weighs.

By comparison, switching mode power supplies (SMPS) offer different features, and advantages over linear versions. SMPS technology tends to be smaller that the corresponding linear model and their output remains constant. Very frequently a linear power supply’s output will fluctuate with the line voltage level, and also the output voltage may fluctuate with the load. For example, an electric locking device may draw an inrush current when power is first applied, and the lock actuates, while the holding current will be substantially lower.

Because much state-of-the art security electronics have very specific voltage requirements, and electric locks may cause fluctuations of voltage level with linear power supplies, SMPS (switching mode power supplies) are growing in use in security.

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