Interior Lighting for Designers

tiveby nature. Magnetic transformers are
relatively large and heavy. Properly sized for
the lamp load, they have a long life expec-
tancy. They sometimes cause a noise prob-
lem by producing an audible 60-cycle
“hum.” Toroidal (doughnut-shaped) mag-
netic transformers are quieter, but they also
hum when controlled by some kinds of elec-
tronic dimmers. The hum grows with the
number of luminaires in a room, and the
luminaires, if improperly designed, will reso-
nate with their transformers.
Electronictransformers use electronic
circuitry, which iscapacitiveby nature. Elec-
tronic transformers are compact, light-
weight, and quiet. But their life is shorter
than magnetic units, premature failures are
common, and they are incompatible with
some dimmers. The small dimensions of
electronic transformers outweigh many of
these flaws, however. The designer must
consult the dimmer manufacturer to ensure
the compatibility of transformers and dim-
mers.

Ballasts

All lamps, except incandescent ones, require
aballast. Every discharge source has nega-
tive resistance characteristics. If the arc
discharge is placed directly across a non-
regulated voltage supply, it will draw an

unlimited amount of current almost instantly

and will be quickly destroyed. Therefore, a

current-limiting device called a ballast is

inserted between the discharge lamp and

the power supply to limit the electric current

flow through the arc discharge (figure 8.1).

Besides limiting the current flow, the

ballast also provides the correct voltage to

start the arc discharge. It transforms the

available line voltage to that required by the

lamp.

Traditionally, ballasts have not been

interchangeable. Most ballasts are designed

to provide the proper operating characteris-

tics for only one kind of lamp. For example,

175 W M57 metal halide lamps are inopera-

ble on ballasts intended for 175 W H39 mer-

cury vapor lamps, and vice versa.

Newer electronic ballasts are designed

to operate more than one connected load.

For example, several manufacturers offer

ballasts that can operate either one, two, or

three fluorescent lamps.

Lamp wattage is controlled by the bal-

last, not by the lamp. If a 100 W HPS lamp is

operated on a 400 W ballast, it will operate

at 400 W, to the detriment of the lamp’s per-

formance. This may also lead to premature

ballast failure.

Unlike incandescent lamps, the rated

wattage of a discharge lamp is the wattage

INTERIOR LIGHTING FOR DESIGNERS

Figure 8.1Typical F32T8 120 V ballast.