ballasts

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billdozier

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Hey guys trying to understand the purpose of ballasts. I realize they control ampacity flow. Is this to save bulb life? What is thier main function? How do they work? Thanks guys
 
billdozier said:
Hey guys trying to understand the purpose of ballasts. I realize they control ampacity flow. Is this to save bulb life? What is thier main function? How do they work? Thanks guys
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They boost voltage to start the lamp and maintain a constant current to keep it running.

~Matt
 
Additionally, ballasts are current regulators. Fluorescents, for example, will conduct more current through the vapor as they get warmer, which can lead to current snowballing, or thermal runaway.

Ballasts cause the effect to reverse, so the increase in current results in an immediate current reduction, so the operating current is maintained at the proper level for light output and long life.
 
wishmaster68 said:
So does that mean if the lamp is snowballing the ballast is going out or a problem with the voltage?
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If it's not the lamp itself, then yes. I rarely replace lamps without a new ballast. It's not worth trashing new tubes within a month.
 
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billdozier:

To some extent you can classify gas discharge type lamps as constant voltage loads. This means the voltage drop across the tube is relatively constant independent of current. See page 457 of "Fundamentals of Engineering Electronics", William G. Dow, John Wiley & Sons, 2nd Edition, 1952.

Google Books has this page content restricted.

This page shows current vs voltage plots for glow and arc discharges.

When you have a constant voltage device as a load, then it is necessary to have a current limiting source to supply this load. The simplest means, but power wasting, is the use of a resistor between a voltage source and the constant voltage load.

A resistor in series with a neon bulb is the circuit of a typical neon bulb indicator. You can build a simple sawtooth generator with a resistor, capacitor, and a neon bulb.

.
 
billdozier said:
Hey guys trying to understand the purpose of ballasts. I realize they control ampacity flow. Is this to save bulb life? What is thier main function? How do they work? Thanks guys
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Yes, a Ballast's primary function is to limit the amount of Current flowing through the Discharge Lamp - actually the Current is "Regulated", throughout the processes of Lamp Starting, and normal operation.

If the Current was not "Ballasted" in some way - such as connecting a Discharge Lamp directly across the Line, the Lamp would allow as much Current the Power Source has available, to flow across the Lamp (from one Electrode/Cathode, through the inert Gas media, to the other Electrode/Cathode).
This will result in an almost instant destruction of the Lamp.

FYI: Discharge Lamps are known to exhibit "Negative Resistance" when connected to a Power Source, whereas all other types of loads will exhibit a Positive Resistance / Impedance, when connected to a Power Source.

The Ballast performs an additional function at the initial starting of the Discharge Lamp: it "sets-up" the necessary Voltage to start the Lamp.
If a separate starting aid, or "ignitor" is included, the Ignitor pulses Ballasted Current across the Lamp until the Arc is established; then the Ignitor drops out of the Circuit.

Once the Lamp is started & operating normal (initiating Arc becomes a solid "Plasma"), the Voltage required to run the Lamp is much lower than the "Cold-Starting Voltage".

For example, a typical F32T8 Fluorescent Lamp in normal operation has a rated Current of 430 ma (0.43 amps), with a _RATED WATTAGE_ of 32 Watts.
The Voltage across the Lamp (between the Electrodes) will be apx. 74 Volts.

Another example; for an F48T12-HO (High Output) Fluorescent Lamp in normal operation, the rated Current is 800 ma, and the _RATED WATTAGE_ is 60 Watts.
The Voltage between the Electrodes ("across the Lamp") will be 75 Volts.

Additional FYI: Standard "Reactor-Type" Ballasts (Electromagnetic) consist of Inductor Coils, which are wound to the core + connected together in a fashion which places the "Relative Polarity Points" of each coil in a "Series-Reduction" arrangement (AKA "Choke" arrangement).
This results in a higher Impedance reflected to the Line side of the Circuit, when the Current drawn from the Lamp increases.

Modern "Electronic" Ballasts incorporate a "Pseudo Inductor" component arrangement, to regulate the Lamp Current.
Electronic Ballasts also incorporate abilities to increase the Lamp(s) Performance; such as:
* High Frequency Lamp Operation (20kHz typical),
* "Program Start" for Hot Cathode Lamp operation (regulate the heating of the Lamp's Cathodes).

As mentioned by other Members, a Discharge Lamp may be "Ballasted" by any Current-Limiting device, such as a simple Resistor.

Discharge Lamps include:
* Neon Lamps,
* Fluorescent Lamps,
* HID Lamps (Mercury vapor, Metal Halide, High Pressure Sodium, Low Pressure Sodium),
* "Arc" Lamps.

Additionally, LEDs require some type of external Ballasting, typically done with a Fixed Resistor.
There are LEDs with Resistors "Built-In". These are normally rated for a specific Voltage, so as to keep the Current value within limits.

Hope this information is of assistance.

Scott
 
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