SCR switching systems can generate considerable noise - both acoustic and electromagnetic, a result of the abrupt and frequent interruptions in the current wave. The frequent switching causes the filament to vibrate, creating a hum known as
lamp
singing, it also reduces the
lamp
life. Another unsolicited by-product of the switching, is electromagnetic noise which can cause disturbances to various electrically powered systems which are susceptible to electromagnetic interference. This is especially apparent in sound systems which respond audibly to radio frequency interference. One of the solutions commonly implemented is the connection of sound systems to separate power lines which are not part of the lighting system.
In order to reduce these two types of noise, a
choke
is inserted in each dimmer, between the
SCR
and the mains supply. The
choke
is a magnetic core around which copper wire is coiled. It acts as an inductive filter which slows down the abrupt switching action of the
SCR
and eliminates a large part of the noise.
A good
choke
slows down the rise time, which is the time it takes for the current to reach from 10% to 90% of its full value at the designated point in the current cycle, by 300 to 500 microseconds.
In order to function properly the
choke
size has to match the maximum
electric load
that the dimmer handles. Large capacity dimmers need large chokes, which are heavy and cumbersome, and dictate the physical size and structure of the dimmers. The
choke
itself produces a muffled noise (caused by eddy currents in the
choke
which produce tiny mechanical movements). As a result, the dimmers have to be located at a distance from the
stage
, preferably in a separate room. In addition the
choke
dissipates power and reduces
SCR
efficiency to 98%.
Noise reduction requirements, and thus the required rise time, vary depending on the intended use of the dimmer. The minimum recommended rise time for theatre dimmers is 300 microseconds, while the recommended rise time for television studio dimmers is 500 microseconds.
