Sometimes an ass-kicking is needed to force you to figure a system out. In this case it was an annunciator dimming problem. The crew reported that the green flow bars and all "white" annunciator switch-lights would not dim. We were called 30 minutes to "go" time and there was no MEL relief.... we lost the flight. (Although, this represented absolutely no "safety-of- flight" issues. It would of been a nuisance but, for a short set of flights... the aircraft "should of "departed for revenue flight and the problem worked at next ground time.)
Anyway, we dug into the problem. I'm sure the light dimming systems installed on other Airbus types are similar to what we found on this A300.
One minor note (no major,actually): Even though the dimming system interacts with the light test system, every effort was made NOT to go down the test system road. Personally, I find the light test system on this aircraft to be an absolute joke. A RATLU (Reprogramable Annunciator Light Test Unit) sequences through "every" bulb individually and stops on each defective bulb when its encountered. A neat little light show, but come on now..... how hard would it be to use a test switch to turn them "all" on and "scan" them with your eyes (like everybody did in the old days)? The auto-test can be by-passed to do just that, but most crews go the auto route.
Back to the dimming, the Dimming Unit is located on the right avionics rack. It's a plain blue box with no lights or switches on the face. Inside are seven modules. Each module controls a "set" of lights. White, blue, amber, green flow-bars are just four of them. All the modules control their own set of relays (probably more than 50 relays for all the modules). These relays are located behind the RATLU under the maintenance test panel in the cockpit. It is through the relays and two module amps in which dimming is accomplished.
Cockpit warning or control switch-lights are looking for power on one side and ground on the other..... that's it. They can be turned on/off by the addition or subtraction of either of those two items. On the A300, both methods are used. Some lights receive voltage from the system being controlled. Some receive a ground.
We can use a fuel pump "off" switch as an example (although I'm not digging back into the prints to verify if its a ground or power.... but we'll pick power). If the left inner pump is off, a fuel pump relay supplies 28VDC to one side of the bulb (two bulbs to be exact). The bulb needs to see a ground to turn on. That ground is supplied by the dimming box. It's a hard ground if the system is set to bright. It's a fixed resistance (or variable ground in the case of flow-bars) if the system is in dim. In dim, the ground is routed through a "engaged" dimming relay to a dimming amp (DIM-) in one of the seven modules. The resistance provided by the amp is what causes the bulb to be dim.
The same can said for bulbs that have a ground supplied by the controlling system. In bright they receive a full 28VDC from the dimming box module. In dim, that voltage is reduced by another amp (DIM+) in the associated module.
Three of the modules have an extra input. Two modules receive a variable resistance input from light sensors in the cockpit. The other module receives a variable resistance input from the dimmer knob.
Please excuse my explanation. I'm sure I left out specifics, but hopefully "the big picture" of system operation can be deduced.
Digging back into a failing memory bank, I had shot the light sensors from the Dimmer Unit rack plug during our troubleshooting.... I "think" that resistance was around 32K-33K ohms.
The system schematics can leave your head spinning (as do the majority of Airbus prints). Most, if not all the dimming relay coils receive voltage from the (turned on) transistor in each module. Some relay coils are hard grounded. Some have the ground "controlled" to actually engage the relay.
Again..... to much shit to dig back through to provide a better explanation of system operation. Should I scramble my brain on prints or have a beer??? The beer (maybe a lot more than one) is a much more pleasurable scramble experience.