Technical Tips for Security Power Supplies

1.	Calculating wire size for long runs
2.	Selecting lead acid Batteries for security applications
3.	Matching transformers to power supplies

1. Calculating wire size for long runs

Below are two tables for wire size needed to cover various lengths and power for 12 and 24 Volts AC or DC. The theory for these tables is based on voltage drop in a wire of given resistance, carrying a give load. When electric current flows through wire, voltage is lost. This loss is referred to as voltage drop.

Voltage Drop = Wire Resistance X Amps

For the tables below, use power in Watts or Amps and the Voltage Drop Table will show distance in feet for any size wire pair listed. The table calculations are based on the ohms of the wire at 70°F. These distances are based on a 10% voltage loss. Since this is linear, for a 20% loss double the distance, or for a 5% loss, half the distance. If your cable manufacturer makes recommendations for wire sizes, use them instead of these tables. The recommended maximum distances for AC or DC are listed in the cell below the wire size

Voltage Drop Table

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2. Selecting Lead Acid Batteries for Security Applications

Lead acid batteries are used by security installers for backup power needs because of their good charging characteristics and the memory immunity associated with repeated charging. When float charged, they typically last 5 years. Our security power supplies contain a float charger that provides a charging voltage to keep it's batteries fully charged.

Many of our power supplies are rated for constant duty. This means the supply is always delivering power according to its capacity and the load, made up of internal and external demand. Internal demand is the draw seen by the battery when AC is not present and there is no external load on the power supply output terminals. Typically our internal demand is about 100mA , which keep monitoring circuits active and LED's lit.

Battery recovery time once AC power is restored varies greatly depending on several factors; charge current, age of the battery, and temperature of the battery just to name a few. Security power supplies do not have a time requirement for full recovery like fire rated power supplies so the recharge time for a security power supply could be several days or even longer.

Back up batteries are sized by the total load and the desired duration on backup power. Battery discharge curve s provided by the battery manufacturer can be used to select an appropriate battery for a given application. Battery manufacturers recommend a 20-25% de-rating for battery age. One battery manufacturer, Power-sonic, has a discharge curve that can be found on fig 3 at http://www.batteryweb.com/manuals/techman.pdf

An example using this chart would be as follows:

Using a 5Ah battery, how long can I back up a 2.9A 12VDC load of the HPS3PM? The 2.9A load would be added to the 100mA of internal power demand for a 3A total load. Consulting the chart, we follow the 3A load line up to the 5Ah line and then move across to read off the time. It reads one hour. Discounting 25% for battery age, it is safe to assume this battery will hold up 3A for 45 minutes.

Battery Calculation Sheets

	FireForce8
	HPF24S6
	HPF24S8
	HPF602ULADA
	HPF902ULADA

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3. Matching transformers to power supplies.

Selecting a transformer for your power supply is generally an easy task, as long as the principles of transformer operation are kept in mind. Simply stated, the output AC is a function of input AC voltage and output loading. If your line voltage is on the outer edges of normal, your supply voltage may be out of range of your device being powered so be sure to check the power quality of the incoming AC. The table included with your power supply shows the proper transformer for the supply, so it's that easy.

HPP transformers have a built in auto reset feature that protects from over-voltage conditions. In addition, we have a power on LED that indicates AC is present at the transformer. Overall this is a high quality transformer. Once you have a transformer for the supply you've selected, we recommend checking the voltage at the device at maximum load. That means turn all accessories on (like heaters and other devices) before measuring the maximum voltage.

If maximum voltage is outside of specifications with the device in maximum load, with HPP supplies you can adjust the output voltage potentiometer. If you are still outside of specifications for your device then a larger power supply will be required, or the wire run will have to be shortened or rerun with larger wire. See application note on wire size for further information.

In Summary most of the time the line voltage is within range to keep your 24ac output within 10%; but always check you voltages and your specifications to be sure you are in compliance before applying power.