Transformers 
Transformers are simple devices that, in low voltage lighting applications, step down or reduce 120 volt A.C. house current to 12 volt A.C. current.  You may remember from elementary science class that this is accomplished by induction. There are two coils around a core of iron plates. The house or line current passes through the primary coil creating a magnetic field. The second coil, called the secondary coil is energized by the magnetic force created by the primary coil. Depending on the number of turns of the secondary coil relative to the primary coil, the voltage will be reduced at the output terminals of the secondary coil. This is true when the number of turns on the secondary coil is less than the turns on the primary coil. If there were more, the voltage would be higher and you would have a step up transformer. To reduce 120 volts to 12 volts the primary coil has 10 times the turns of the secondary coil.

Typical low voltage transformers are designed to reduce 120 volt line voltage to a secondary, or output voltage of 12 volts. However the transformer has no way of knowing the line voltage. It simply reduces the current by a factor of 10. So if the line voltage was 115 volts, the secondary voltage would be only 11.5 volts. Conversely, if the line voltage was 130 volts, the transformer would produce 13 volts. Line voltage may vary with time of day and when other electrical loads are on the system. For this reason it is important to measure the line voltage at the transformer at the same times that the transformer will be in use. Remember from the lamp section, a small change in voltage can make a significant difference in lamp life and output.

As discussed earlier, voltage will decrease the further the current has to travel. In order to compensate for this loss and the variations in line voltage, some transformers offer several output voltages. These transformers are generally known as multi-tap transformers and may offer voltage outputs ranging from 10 volts to 14 volts. This is accomplished by having leads coming off the primary or secondary coil at various points near the end of the coil to produce several different voltages. This is a valuable feature for a system designer wanting to have better control over light output and lamp life.

There are also buck (reduce the voltage) and boost (increase the voltage) transformers that can also be used to change the line voltage a small amount. If you had a situation where the line voltage was typically 110 volts, the landscape lighting transformer would have a normal output of only 11 volts. This reduced output could be offset by using a multi-tap transformer (the 14 volt lug would output 12.8 volts and the 13 volt lug would have an output of about 11.9 volts.) or you could have an electrician install a 10% boost transformer (110 volts + 10% = 121 volts) .You can correct higher than 120 volt situations in the same way by using a buck transformer to reduce the current.

Some companies also offer these transformers to boost voltage on the secondary side. This type of transformer is designed to be buried in the ground as part of the circuit. The typical boost is 20% to 25%. Therefore if the voltage drop is down to 10 volts at the fixture, installing a booster transformer ahead of the fixture could increase the voltage to 12 or more volts. 

It is important to understand that transformers can not stabilize voltage variations. They simply transfer the current at a fixed ratio and do not sense changes. There is equipment to handle this problem, but it is typically too expensive and unnecessary for landscape lighting applications.

Transformers are offered with and without a cabinet. Those without an enclosure are attached to an electrical box and wired directly to the line voltage source. This type of transformer should be installed by a licensed electrician.

Transformers built into an enclosure are connected to the line voltage source with a standard 6’ plug and cord set. When the transformer is mounted outside, the outlet used to power it should be protected by a Ground Fault Circuit Interrupter (GFCI) device. This device will shut off the current instantly when it senses an over current situation. They are designed to protect people from electrical shock while the breaker protects the building and electrical system. GFCIs are available for individual outlets or as part of the circuit breaker that protects the entire electrical circuit. This type of GFCI is located in the electrical service panel and should only be serviced by a licensed electrician.

The transformer enclosure may be made of plastic, composites, painted steel, or stainless steel. The least expensive are plastic models that are frequently completely sealed with a single output cable used to connect to the lighting system. Steel cabinets generally open to allow for the use of controls and have electrical screw down lugs to attach the wires from the lighting circuits. If the transformer is multi-tap, there will be several lugs, each marked with the output voltage.

Composites cases are used for direct burial type transformers. These units can be used when the lighting system is at a distance so far from the power source that there is a significant voltage drop that cannot be compensated for with larger wire. Because the location of the transformer in this situation is typically away from a structure, it is often convenient to bury the transformer. By running line voltage to the area and connecting it directly to the transformer, the voltage drop problem is eliminated. When using this type of power unit make sure that all installation requirements are met, particularly with regard to access and drainage.

It should be noted that not all enclosed transformers are suitable for use indoors. This may sound strange, indoor use having more protection from the weather, but it has to do with heat. This is not a problem when the transformer is in the open air, however if not designed for indoor use, the transformer could overheat. If the transformer is to be located indoors make sure that it is rated for that type of location.

Quality transformers are made with protection from over load and short circuit situations. This is an important safety feature. The transformer is protected on the line voltage side by the fuses or breakers in the electrical service panel for the building. But the secondary side of the transformer is not connected to that system. It is energized by induction not direct contact so the breaker does not protect it. Quality transformers protect the secondary side by running the low voltage through a breaker or fuse.