> I saw your post on the aquatic plant list. I was wondering if you
>would be willing to suggest an electronic ballast from graingers that would
>run 4 - 36" bulbs or 2 - 36" bulbs. I've found magnetic ballasts that will
>run both T-12 and T-8 bulbs; are there electronic ballasts that will do this
>or no. Thanks for your time and know how.
I don't know, I'll look Monday - I've had several requests so I'll post a
chart on the APL.
Flourescent bulbs are negative impedence devices, i.e. when the mercury
vapor is cold, the resistance is high and it takes a lot of voltage to
cause ionization. (they have little incandescent filaments in the tube ends
to pre-ionize the mercury). Once ionized, all the active electrons buzzing
in the mercury vapor conduct electricity much better and the resistance
goes down. If hooked to a constant voltage, the current would go up the
hotter the vapor got making the vapor even hotter - a runaway condition
which could lead to an exploding lamp. (Don't try this at home. I warned
A conventional ballast uses an inductor (or transformer/inductor if the
voltage needs to be changed too) to limit the current. My 24 inch bulb runs
at about 80 volts. Assuming the same mercury gas density, the longer the
tube is, the higher the voltage is that must be applied to excite it (the
mercury gas column is simply longer so it takes more voltage to force the
electrons through it).
The output voltage required for the lamp depends on the effective length of
the bulb. Electronic regulation is used to limit the current to the desired
operating level. Theoretically, if the electronic ballast provides a
constant current, you could use a shorter bulb with no consequences.
However depending on the real world design of the electronic regulator, it
might not be happy delivering this current at a lower voltage or it might
increase the current for a lower voltage load (shorter tube). The design
margins may be there or they may not. I'll talk to Magnetek-Triad and pose