Metal Halide Starting Circuit

[Y2KEDDIE]

I was looking at the cicuit diagram for a Metal halide Lamp fixture and curious how it works.

I see a step down auto tansformer with its tapped winding coneected in series with a 24ufd capaitor, then another winding (inductor), and then the lamp itself with the series return back to the low side of the auto transformer.

Both widings , autotransformer and inductor share the same laminated core. Do these two windings interact?

I realise the autotransformer is to lower the line voltage; in this case 480V, and the inductor (ballast) is for current limiting the arc once established.

How does the capacitor and inductor create the starting pulse to ionize the gas in the lamp. Is there a series resonance effect?

I calulae the 24ufd reactance to be 111 ohms at 60 Hz, how does this come into play?

 

[john monks]

No. The transformer you are looking at is more likely an inductor with windings that are in series with the Metal halide Lamp and this is to limit the current that goes through the lamp. Now there is another winding that is configured at 90 degrees from the normal way of magnetically coupling power from the core. This core typically has direct current thats only purpose is to saturate the current and thereby increase the current to the lamp. This is also a good description of a "magamp" that was used in theaters in the early 20th century for controlling the brightness of the lights.
This may help:
http://www.google.com/patents/US5432406

 

[Y2KEDDIE]

Interesting reply John, but no, this ballast/transformer is from a working light fixture. There is no dc circcuit.

I was just looking for an explanantion how the series capacitor, inductor , bulb circuit creates the HV pulse required for ionization.

 

[john monks]

Some do contain a dc circuit and some don't. But for those that don't like a common mercury vapor street light the ballast is sort of a combination transformer and inductor. It acts like a high voltage transformer of several hundred volts and a series inductor. The inductance limits the current for the bulb by dropping the voltage going to the bulb to a safe level. In the case of a street light there is a separate starting element with a series resistor built into the bulb for causing the gas inside the bulb to start ionizing. In the case of a projection TV a high voltage pulse is placed across the two electrodes inside the bulb to start the ionization. Then a constant current keeps the gas ionized and the voltage will drop somewhere around 5 to 30 volts. This is true for car headlights also. The high voltage pulse comes from a solid state circuit with a step-up transformer.
If I knew the make and model of your fixture and the bulb I might be able to answer the question more specifically.

 

[Y2KEDDIE]

Holophane PETL400MH48 FIXTURE using a 400W M59 base, Metal halide bulb.

Does the capcitor and inductor keep making a HV pulse until the gas ionizes? What stops the pulsing?

In this current fixture the capacitor opened up, but I replaced the Xfmr/Inducuctor combination as well, as it came in a kit.

 

[john monks]

Based on the description you gave I'm assuming that the bulb is a high pressure sodium in an explosion proof housing. So I think it is safe to assume that the ballast is the type of transformer/inductor I described. So I believe that the capacitor you are looking at is only to prevent RFI (radio frequency interference). In this circuit there should be a separate starter similar to a starter for the old fluorescent lighting except much bigger. This device places a short across the lamp connections and them opens up causing an inductive kick of thousands of volts causing the sodium gas to ionize and start glowing. When this happens the starter is out the circuit thus the high voltage pulses cease. The starter works by voltage. When the voltage is high it starts placing a short across the lamp at random times causing high voltage pulses to occur. When the lamp turns on the voltage across the lamp decreases causing the starter to act like an open circuit. Does yours have such a starter? It is probably cylindrical in shape and is called an IGNITOR.

 

[Y2KEDDIE]

No, this fixture does not have a separate starter. There is only the two winding Xfmr/balast , capacitor, and bulb, all in series as indicated on the schematic printed on the core.

Other fixtures I've worked on have the said starter, usually an encapsulated block with three leads.

This circuit doesn't have a separate starter, which I find unique. I'm guessing the intial surge charges the capcitor fast and as it dischargest hrough the inductor/ballast it creates the HV pulse neded to ionize the gas. This might be a corect explanation, but I was looking for more detail. I'm curious: if this is how it works, what stops the pulse, or does it kep pulsing as long as there is voltage applied from the mains.

The schematic clearly shows one section of the xfmr/ballast is a tapped autotransformer, its output is connected to one side of a 24ufd capacitor, the other lead of the capacitor connected to one side of an inductor (same core) and the other lead of the inductor connects to the bulb tip, the outside (threaded) base of the bulb conects to the low (common) side of he transformer.

 

[Y2KEDDIE]

Maybe the low resistance of the ionzed gas dampens the pulse.

I was thinking the capacitor inductor cobination is resonating (Hi Z) creating the HV pulse, and the ionized gas ( Lo Z), lowers the Q (dampens) extiquishing the pulse. (Just a guess).

 

[john monks]

I'm sure that bulb you are using has an internal resistor and a separate spark gap for igniting the arc to start the lamp. The transformer/ inductor is as I described before. The only thing I can conclude from what you describe is that the series inductance capacitance is to increas the effective voltage driving the lamp and this helps keep the off time to a minimum and also keeps the phase angle of the current and voltage from the line at a low angle. Apparently there is no separate pulsing circuit to start the lamp and this is very common for street lights. Each bulb has it's own internal starting circuit comprised of a resistor and a separate internal spark gap that never turns off.