This question has been nagging me since I started to experiment with them when using SOX lamps. Unlike any other ballast I've seen, they are quite universal in their application in the fluorescent space. Fulham has an entire webpage on how to wire them up to nearly any fluorescent lamp available. I have not seen this with any other ballast. The whole thing about how many wires to use and why some of the same output rated ballasts have more wires than others, etc. is interesting. Also, the fact that it seems to "figure out" how to run the lamp properly is intriguing as well. For example on a WH3 using both red wires I can run a 35w or an 18w SOX lamp up to its rated output wattage with no change other than the lamp.

I've done tons of searching and found no information as to how these things work. It would clear up much as to how to implement them in applications for which they were not originally designed as well as fun knowledge. Thoughts?

Lamp families (e.g. PL-S, PL-C, T5HE, T5HO, T8 standard, T12HO, ....) have voltage/current characteristic either like :

 - Voltage according to lamp length, same current through the family

 - Voltage according to lamp length, higher current rating for the lower voltage lamps

This is done either for uniformity in the lamp family, or to be able to run them on the same ballast : nearly constant current for Electronic ballasts, or some inverse relation between current/voltage for Magnetic ballasts

So just making the ballast follow the rules will allready enable it to run a complete lamp family or atleast part of it



Some lamps are approximately double current version of a similar standard current lamp. So for example, it is possible that a 800mA HO lamp would work about right on parallelled outputs of a 400mA/output ballast



Finally, if some lamp stands out from the rest by its arc voltage (this is not precise at all ! for example a 36W T8 with nominal 103V can be in reality anywhere between 90..110V, or maybe as low as 70V when warming up), the ballast may contain logic to detect this lamp by arc voltage and do some correction for it

So the red wires could each be a regulated current output that can be parralled to adapt to the various lamp needs and the ballast just adjusts arc voltage to bring the current up to a steady state?

The arc voltage is determined by the lamp and its temperature. With the exception of few electronic MH and SDW ballasts, most electronic ballasts face the arc voltage as a given fact and cannot do anything to control it

So the ballast just limits current and the voltage rises and falls to maintain the constant current supplied?

The voltage is determined by the lamp (the lamp type and its temperature). It is fairly independent of the current i.e. it will stay fairly the same regardless of changes in the current (within some limits)

The ballast controls the current. Some ballasts have really nearly constant output current, some others have current that depends on the voltage the ballast sees on the lamp, but even there the range is narrow. For example a current range of 155..180mA (magnetic PL ballast) or 320..370mA (magnetic T8 ballast that also can run some PL and 2D lamps)