VHO Ballast question

[StirCrazy]

Originally Posted by Rikko

Steve, can you explain that in a little more detail? I've heard that bit before but it never made sense to me. If the ballast can provide 110W, what difference does it make if it's a 110W, 12' long NO or a 110W, 4' long VHO? With Watt's Law (err.. I think it's Watt's Law) power is just the product of current and voltage, and I'm betting that the voltage between all of the fluorescent species is standard. Where does this disconnect in logic with the VHOs happen? I certainly don't doubt it, but I can't grasp where it's happening.

I will quote a paragraph from my write up in the newbie area, if you want to read it all the link is
http://www.canreef.com/phpBB2/viewtopic.php?t=7740

"the difference between NO, HO and VHO ballast, basically in a simple system it is the maximum current that the ballast is designed to output. This means that a ballast made for NO lighting will be able to put out a max current of 0.4Amps, a HO ballast outputs around 0.8Amps (also the same make up as a ballast for Power compacts), and a VHO is approximately 1.5Amps. All this doesn't tell you how much power the actual tube consumes though, as this is also a factor of the resistance of the tube. To make different wattages of tubes the manufactures build a specific resistance value into the tube its self, so that when this resistance is added to the resistance of the ballast you can then use the current to figure out the actual tube power output. Lucky for us this is labeled on the tubes and we don’t have to do the math. "

so basically the transformer is a constant current generator that will supply its current rating up to a maximum wattage. so a 80 watt NO ballast will supply 0.4amps constant current up to a maximum power rating of 80 watts. So a HO ballast only puts out a current of 0.8amps but to the rated wattage, but a VHO bulb is designed to run on a 1.5amp current.

some times a VHO bulb will run on a HO ballast if the starting method is right, but it will only run at HO levels. so if you have a 110 watt VHO (if my math is correct) it has a resistance of about 73 Ohms, so running this bulb on a HO ballast will give you a power output of 58 watts (which I think is the approximate rating for a 4 foot HO bulb.

Steve