External power supplies are used when an alarm system needs additional power beyond what is supplied from a plug-in transformer. This usually refers to sensors and accessories rather than an alarm control panel. Normally, a panel can get all the power it needs from a plug-in transformer. However, the same cannot always be said for the additional accessories and other equipment. This is often true for larger hardwired system, where every device feeds power off the panel. If you use a large number of devices, you may need additional power from an external power supply.
When it comes to hardwired alarm panels, there is only so much power current that the panel can supply to all the accessories. Each accessory will draw a certain amount of current from the panel. This applies to security and life-safety sensors, as well as accessories. Sensors include devices like motion sensors and glass break detectors. Accessories include devices like keypads and sirens. In all of these cases, some power must be applied for the device to work properly.
In many cases, the device may draw a smaller amount of system power before it is activated. An example is a keypad. When the keypad is left idle it is drawing a much smaller amount of power from the panel than when it is activated and in use. You need to consider both the idle current draw to keep the device powered on and the maximum current draw that the device will utilize when it is activated and in regular use. Make sure to consider the maximum calculated current draw for all your equipment, or you could run into problems.
If you find that your equipment is going to exceed the maximum available current for your alarm panel, then you will need to obtain an external power supply. These devices also receive primary power from a plug-in transformer. A backup battery is also usually used with an external power supply to keep the device running in the event of an electrical outage. You will then use the power supply with any equipment that requires supplementary power beyond what the alarm panel can normally provide.
In particular, hardwired sirens often exceed the maximum current draw that the panel is able to provide on its own. Even if the panel has a trigger output, you will still need additional power from an external power supply to get the siren to activate during alarm events. You will also normally need a relay. This relay will tell the power supply to start supplying current to the siren when an alarm event occurs. The relay will also tell the power supply to cut power to the siren when the alarm is cleared. This is necessary for getting the siren to stop sounded after you have disarmed the system.