Pulse start MH lamps

[bumclouds]

Hey guys,

 

Could somebody please explain to me what role the ballast plays in the pulse starting of Metal Halide lamps? From what I understand, the ballast is a current regulator - and so I cannot understand why you need a current regulator if you're supplying quick 1-5kV pulses.

 

Thanks heaps

Andrew

 

PS: if somebody could dig up a tutorial or paper of some kind to read on the topic, that would be great. I have been googling for about 5 hours now and I cannot find a good one.

[Jivemaster]

First, every different lamp type is slightly different BUT all discharge lamps have some basic principles.

 

A discharge tube doesn't conduct at all until the arc is started, then it has a variable resistance according to the arc conditions, including a negative resistance portion of the operating curve.

 

SO, the initial strike is by HV pulse but the running current is limited by another device. The simple current limit is a large choke or ballast -these get heavy for big lights so high tech discharge lamps also use SMPS to provide both the HV/RF strike and the constant current run system. The SMPS also supervise lamp life indication and power factor improvement.

 

In a Ballast circuit the inductor prevents the strike pulses imposing spikes on the mains supply.

 

A concurrent post has been automatically merged from this point on.

 

Added;

The life of a discharge lamp is the point at which the light output has dropped to half it's original value. Most lamps will go past this even a long way past, but the light output is getting less all the time, and very importantly the mode of ultimate failure is a catastrophic failure of the envelope, spraying high speed glass in all directions. This is why discharge lamps need all the covers and interlocks.

[bumclouds]

Jivemaster - thanks. But I'm a little spun out by your explaination. What is RF strike?

 

To clarify what I'm confused about, I'll paste a paragraph I read on some PDF:

 

The ballast in an HID lighting system has generally two purposes: 1) to provide the proper

starting voltage to strike and maintain the arc; and 2) to provide the proper current to the lamp

once the arc is established.

 

How can you have a device that supplies 1-5kV voltages, and then also acts as a current regulator?

 

You say:

A discharge tube doesn't conduct at all until the arc is started

 

Does this mean - the ballast is providing a 1-5kV voltage for the pulse, but not supplying any current because the bulb doesn't conduct at that point? And then after that the ballast behaves as a current regulator?

[themadhippy]

Think of the ballast as a big coil of wire, when you put AC through it constantly it has a fixed impedance ( xL =2 pi FL if you want to calculate it were F= frequency L is the inductance and xL is the impedance) so it acts as a current limiter. when you turn the power off theres this stuff called back emf thats caused by the magnetic field collapsing ,the back emf can be a quite large depending on the coil size,so by turning the coil on and off very rapidly you can produce some quite impressive sparks,put these sparks in a vacuum with some gas and it might glow.

[Jivemaster]

OK you are an undergrad in the right sort of department to get a good answer, so go there and ask, I'm a chemist.

 

However;

 

A discharge tube is an envelope usually quartz or high performance ceramic, with two or three electrodes. The three electrode lamps are usually used for triggered applications so ignore then at the moment.

 

A lamp with two separated electrodes does not conduct. Add a filler chemistry (HMI CSI HID etc) and the lamp can be made to conduct.

 

Starting the discharge needs a pulse of a high voltage and sometimes RF to initially start the discharge, once the lamp is hot the discharge continues with a MUCH lower voltage. The early discharge lamps (think yellow sodium street lamps ) had a simple circuit, the more modern units such as moving heads have a high tech switch mode PSU which can supply all the voltages and currents and do lots more. They tend to be lighter and electrically better, and may give better lamp life BUT can cost a LOT more

[Smiffy]

They tend to be lighter and electrically better, and may give better lamp life BUT can cost a LOT more

 

They are also a LOT quieter!

 

Worth bearing in mind for Theatrical and TV applications...

 

Cheers

 

Paul

[bumclouds]

Oh okay.. So the ballast is a giant inductor? I've seen some schematic diagrams where the ballast is a transformer as well? Do the same basic principles apply?

 

So... the reactance of an inductor is xL = ωL.. so no matter what amplitude signal is applied to the input side of the ballast, the impedance is only dependant upon frequency?

 

I kind of understand your explaination about the generation of sparks.. Is the ignitor switching on and off rapidly like a square wave or something, at the output of the inductor?

 

EDIT: I think the ballast in the fixture I'm looking at right now, uses a transformer.. as I can see it has multi-voltage inputs.. and that would only be possible with a transformer and not an inductor, am I right?

[musht]

Some ballasts will be tapped for multiple voltages.

 

A transformer "transforms" one voltage to another, ballast dosen`t have a secondary winding in the sense that a transformer does.

 

About here my physics runs out..

 

Someone whose dosen`t, Don Kilipstein

 

http://members.misty.com/don/

 

If you want to ask more detail sci.engr.lighting usenet or access via google groups

[PaulDF]

Oh okay.. So the ballast is a giant inductor? I've seen some schematic diagrams where the ballast is a transformer as well? Do the same basic principles apply?

A transformers role is to provide a secondary voltage, the supply to it isn't usually pulsed as it is with ballasts. A transformer is connected in parallel whereas a ballast is connected in series with the load.

So... the reactance of an inductor is xL = ωL.. so no matter what amplitude signal is applied to the input side of the ballast, the impedance is only dependant upon frequency?

Yep, in a nutshell as frequency changes so does the inductive reactance and resulting impedance.

This impedance is important to control the current through the lamp after it has struck.

I kind of understand your explaination about the generation of sparks.. Is the ignitor switching on and off rapidly like a square wave or something, at the output of the inductor?

The high voltage required to generate a spark is from the back emf as the circuit is broken and the magnetism collapses.

This emf can be multiple times higher than the supply voltage.

Might be worth having a peep of the old switch start fluorescent lamp fittings as these use the same basic principles for starting and limiting current.

[Jivemaster]

An iron ballast will often have tappings for different mains voltages, an electronic ballast may cover a wide input range with no or few tappings.

[Dmills]

<Handwavium explanation follows>

 

Lets say the lamp when running drops say 95V (Fairly typical), and the supply is 230V, and the lamp running current is say 10A.

Wire an inductor in series with the lamp such that it drops 135V@10A at the appropriate phase angle.

 

Now, to get the lamp lit we need a pulse of a few KV, so if we connect a switch across the lamp and short it out then current will flow in the inductor, release the switch and V=Ldi/dt relation will result in the voltage rising until the lamp breaks down and the current can continue to flow.

 

In practice the switch is wired from a tap on the inductor so that it does not have to stand the full ignition pulse (And the switch is automatic), but that is a detail.

 

HTH.

Regards, Dan.

[bumclouds]

Now, to get the lamp lit we need a pulse of a few KV, so if we connect a switch across the lamp and short it out then current will flow in the inductor, release the switch and V=Ldi/dt relation will result in the voltage rising until the lamp breaks down and the current can continue to flow.

 

In practice the switch is wired from a tap on the inductor so that it does not have to stand the full ignition pulse (And the switch is automatic), but that is a detail.

 

HTH.

Regards, Dan.

 

Ohhh!!! I get it!!!! Thanks dan!

 

So with a pulse start, the switch goes up and down several times? How does the ignitor actually do this, in a non-mechanical fashion? (im assuming it doesn't have moving parts?)

[Dmills]

So with a pulse start, the switch goes up and down several times? How does the ignitor actually do this, in a non-mechanical fashion? (im assuming it doesn't have moving parts?)

Often they fire a single pulse and then repeat after a second or so.

 

There are electronic versions, but you also see variants with bimetalic strips with heaters wired so that while no lamp current flows the heater (which is in parallel with the bi metallic strip will eventually cause the strip to bend closing the circuit and bypassing the heater... Strip cools, contacts open, pulse is generated. Once the lamp is lit there is not sufficient voltage to heat the strip. This is often used in things like the MH lamps used in amenity lighting where the starter switch is thermal and is built right into the bulb (In this version the switch is closed when cool and heats up as the current flows, once the lamp is lit there is sufficient heat to keep it open from the proximity to the arc tube).

 

There is also a variant that discharges a capacitor via a spark gap into a small section of the inductor which then gets stepped up by autotransformer action. This one can get sneaky as by picking the correct sparkgap and by charging the cap via a high value resistor from output of the inductor, you can ensure that the thing will only pulse until the lamp is lit.

 

Modern electronic ballasts often use series injection igniter's that place a pulse transformer in series with the anode lead and use a capacitor discharge to produce ignition pulses. Some of these also produce a current limited voltage boost to quickly heat the cathode surface to encourage electron emission (Commonly seen in xenon and krypton arc lamps, and may be many hundreds of volts).

 

Regards, Dan.