Hi Steve,
Quote:
Victor, a ballast is a constant current source it will not vary the power to the bulb but maintain a constant amperage, the resistance caused by the bulb is what determines the power difference, and no using a shorter tube will not burn it out faster.
also the ballast wiring diagram clearly shows a parallel set up so using two wires per bulb should be fine.
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These aren't constant current power sources at all. You are probably confusing these magnetic ballasts with electronic ballasts (which are constant current). Ballasts are just devices that can start and keep a light glowing - nothing about constant current or how it starts the lamp.
Simply, a magnetic ballast is big resistor (which is frequency dependent - but it looks like a resistor at 60 hz). The magnetic ballasts with the lamp form a big resistor divider. Since lamps have a known ANSI characteristic (i.e. F40T12 have a specific characteristic that is published in ANSI specs), they can design a ballast (aka a resistor) to light it properly. Some lamps have very similar characteristics, so they can use a single ballast for several types of lamps. However, some are so dissimilar, that its impossible to meet. The point is that these ballasts are design to work with a specific set of lamps with similar characteristics. Once you deviate from these lamps, the ballast will operate in unknown territory. Lamps will glow brighter or dimmer depending on the electrical characteristics of the lamp and how it interacts with the resistor (aka ballast). Electronic ballasts are different since they are constant current and not simply a resistor formed by a big coil. Also, that's why you can easily overdrive with electronic (and predictably), but its not so predictable with rapid start magnetic ballasts.
Why am I saying this ballast can damage shorter tubes? Well, it all has to do with the fact that this ballast is designed for long 8 foot tubes. It takes a very high voltage to ionize the gas in the 8 foot tube such that the tube can conduct current and glow. The longer the tube, the large the high voltage strike. Unfortuntely, these strikes damage the electrodes on the lamp....larger voltage....more damage. The 8 foot tubes are specifically designed to withstand these larger voltages, but 4 foot tubes are not as well designed. So, these 4 foot tubes may have reduced lifetimes because of this effect.
As for the parallel thing, I thought he was going to try to hook up 4 tubes on one ballast. Two tubes on a ballast is alright (its designed for that), but 4 tubes is not reliable or safe. This is a rapid start magnetic ballast and I would be worried about what it will do with 4 tubes -- especially 4 4 foot tubes.
But...its all IMHO
- Victor.[/quote]