Replies

1. 
   BillHartmann | Feb 26, 2005 09:29pm | #1

   "Is a ballast just a transformer for a flourescent light?"

   Sorta, but not exactly.

   The flourescents are a arc discharge type of light. It takes a high voltage to strike the arc (which produces UV light which excites the phosurs coating to produce visiable light).

   After the arc is struck it the bulb wants to draw more and more current and the main purpose of the ballast is to limit the current.

   so it is more of an inductor that is in series with the bulb than a transformer.

   But they also have transformer windows to generator low voltage (3 volts IIRC) for the filamets that are on some types of bulbs (the onew with more than 2 contacts). The filamets are used to help the bulb strike sooner and with less flicker.

   This is for a basic magnetic ballast.

   There are electronic ones that end up doing the same functions, but in a much different way.

1. 
   4Lorn1 | Feb 28, 2005 03:12am | #2

   Good explanation.

2. 
   Pierre1 | Feb 28, 2005 03:57am | #3

   I don't see starters on residential fluorescents anymore. Could you answer a few basic questions for me?

   - If the starter's function is to boost initial voltage, does it do so by acting as a capacitor that gets dumped upon start-up?

   - Do the new electronic ballasts combine this function with that of limiting the current once the tube has warmed up?

   Thanks.

1. 
   DanH | Feb 28, 2005 06:50am | #4

   The starter is a time-delay switch. It cuts off the heater current after a brief time.

2. 
   BillHartmann | Feb 28, 2005 07:59am | #5

   "- Do the new electronic ballasts combine this function with that of limiting the current once the tube has warmed up?"I have not really looked into the details of how the electronic ballast works.But they are a type of electronic power supplies that can have controls to easily adjust there outputs. So unlike a magmetic ballast which just puts in a current limiter then can adjust the voltage to better match the lamp.So like the difference between an engine with a governer, that limits the engine rpm, and crusie control which maintains the desired road speed.

1. 
   4Lorn1 | Feb 28, 2005 08:10am | #6

   As an aside.Most of those electronic ballasts operate at high frequency. A regular ballast makes and breaks the arc 120 times a second causing the flickering strobe effect common to older units.The better electronic units operate more efficiently, less heat, and largely eliminate the strobing that can make florescents hard on the eyes around some moving machinery and CRTs.

2. 
   DanH | Feb 28, 2005 06:00pm | #7

   I believe that the high frequency and wider ability to adjust voltage with an electronic ballast eliminates the need for a "heater" in some cases.

3. 
   junkhound | Feb 28, 2005 06:21pm | #8

   Have not torn down any of the commercial electronic ballasts to see their exact operating mode, but can offer the following, papers I've seen on the commercial parts look to be similar. 

   On aircraft and the space station, the electronic ballasts typically generate an open circuit voltage of 500+ volts by way of a high frequency (200+kHz) ferrite transfomer driven by Mosfets (fast switches). After the lamp arc is established, the fast switches perform a current limit function by turning on for a much shorter period of time (typically in a pulse width modulation  type topology) to set the lamp current for the appropriate wattage.

    Aerospace electronic ballasts are super light weight (< 5% the weight of commercial 60Hz ballast types )

1. 
   CPopejoy | Feb 28, 2005 07:43pm | #9

   Hmmm...Helluva service call, to re-ballast a fixture on the space station.

   I'd go, though. Probably even waive the trip charge.[threadjack alert!]Always wanted to see earth from space, ever since I saw that image of the crescent earth taken by one of the Apollo crews (Apollo 11 in lunar orbit at the time, if memory serves).I don't think it's a coincidence that the modern environmental protection movement really bloomed at about the same time we were getting images of our little blue planet out there in the vastness of space. Sure provided the perspective that we're in a closed and finite biosphere. Musta even had an effect on Nixon; he took some bold steps to protect the environment (NEPA, creation of the U.S. EPA).Cliff

2. 
   billyg | Mar 02, 2005 10:23pm | #10

   That's right, they use either pulse width modulation or they use a pair of MOSFETs to generate overlapping pulses -- the greater the overlap of the pulses, the more power is delivered to the load/lamp.  Light weight and low power is important not only for space stations, but also for laptop computer backlights in the LCD screens.  ;-)

   Billy

1. 
   junkhound | Mar 03, 2005 12:24am | #11

   FWIW: Had the chance to see and handle a few of the fluorescent fixtures for the station back in '98 or so before they went up. Triple containment of the tubes on the off chance of a broken tube - a little mercury sure plays heck with aluminum structure.

2. 
   billyg | Mar 03, 2005 12:36am | #12

   Incredible.  It is amazing what goes into those things. 

   OT -- I saw the Challenger blow up when I lived in Florida in the mid-80s.  I was jogging at lunchtime and ran toward the beach in Melbourne to watch the launch.  Suddenly it happened and people started pulling off the highway to get out of their cars and stare at the sky in disbelief.  Then you could see people driving down the road crying.  They grew up with the space program in that part of Florida and it hit them hard.

   Billy