UFC 3-535-01
17 November 2005
15-5.1.3
Silicon Controlled Rectifier (SCR) (uses SCR type thyristors to convert
constant voltage source to constant current output, with solid state control circuiting).
15-5.2
The first two types have been reliable over the years, but fluctuations in
input voltage will cause corresponding changes in the output current. Most CCRs now
manufactured are SCR type.
15-5.3
The method of control for SCR regulators is very accurate over a wide
range of loads and varying input voltage, although the high precision produces
harmonics that can cause overheating and noise in the CCR. The degree to which
harmonics can be tolerated depends on the susceptibility of the load or power source to
the harmonics. The most susceptible types of equipment are those which require a
sinusoidal input.
15-5.4
Tests have shown that, as circuit load increases toward the rated capacity
of a CCR, harmonic distortion is reduced and the wave form approaches a sine wave.
This suggests that loading closer to rated capacity results in better circuit performance.
However, lamp failures and other factors may increase the output voltage, and this
needs to be considered when matching circuit load to CCR size.
15-5.5
CCRs have "taps" for adjustment to accommodate loads. Some
manufacturers have introduced taps which automatically adjust to the circuit load, while
others use manual taps which are typically set when the CCR is installed, but can be
changed later to accommodate changes in the load.
15-5.6
The allowable tolerances in CCR output current are shown in Tables 15-1,
15-2 and 15-3. The allowable tolerances are small, since small changes result in large
changes in light output. For a 6.6 amp circuit, reducing the current by 3 percent (to 6.4
amps) will reduce the light output over 15 percent, and reducing the current by 6
percent (to 6.2 amps) will reduce the light output by over 30 percent. The impact is the
opposite on lamp life, where a current lower than maximum will significantly increase
lamp life, and a higher current will reduce lamp life. The percent of full brightness
obtained at each of the nominal current levels for 3 and 5 step systems can be found in
paragraph 12-1.11.
Table 15-1. 5-Step 20-ampere Circuit (FAA Class 2, Style 2)
Brightness Step
Nominal Current
Allowable Tolerance
(amperes)
Current (amperes)
5
20.0
19.4 - 20.30
4
15.8
15.33 - 16.27
3
12.4
12.03 - 12.77
2
10.3
9.99 - 10.61
1
8.5
8.24 - 8.76
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