Want to learn about the ballast circuitry used inside moving head lights

[Yamin]

Hi there guys,
I would like to learn more about the ballast circuitry used inside the moving head lights. Lately I have been getting these kind of units in for service. I couldn't find any resource online schematics or workings of these ballast circuits.
I have noticed that the ballast circuit destructs if the power is applied and the lamp's filament happens to be open. I couldn't check the filament of these bulbs with a DMM prior :S. Would a new bulb become faulty if the ballast circuit had any issues?
Anyway I was hoping that you could direct me to some info on these types of ballast circuits.

Thanks so much for the help

[rvalente]

Check that:

https://www.infineon.com/dgdl/irplhid2.pdf?fileId=5546d462533600a4015356a011722c87

I'm pretty sure I have one or two demo boards here, we could do some business

[cozza]

1) Lamps used are metal halide arc lamps (HMI), they don't have a filament or continuity between the electrodes

2) The ballast won't self destruct if it can't strike the lamp, even if it is a basic design. If a lamp is not fitted and the igniter voltage (igniter is a separate module in the head or yoke usually) arcs from the lamp socket to chassis, then the igniter may become damaged after repeated starts with no lamp fitted.

In a good design there will be three active parts to the circuitry.

Active Power factor correction stage  (choke, diode and mosfet(s) plus control chip ) - a boost converter which increases the DC bus voltage to between  340 to 400 V DC - this may be not in the unit if it is not power factor corrected.

Current regulator stage (choke, diode and mosfet(s) plus control chip )- this regulates the power supplied to the output H-bridge and sets the lamp power

Output bridge - usually an IGBT (sometimes mosfet) H bridge of four devices, converts the regulated DC output of the current regulator to square wave AC for the lamp.

All stages operate at mains potentials with no isolation from the line, so an isolation transformer and isolated scope are mandatory to troubleshoot these units.

Additionally you will find a capacitor pre-charge circuit and igniter timer in these units typically.

If you find failed mosfets or IGBTs when inspecting with DMM, then the gate driver circuitry is usually compromised as well (not always but often). This may be discrete transistors or an IC package.

[cozza]

If the ballast self destructs at power on, check the lamp wiring in the moving head has not rubbed through and shorted to chassis through the pan / tilt swivel points first. Usually this is well protected. Also check the igniter module is not shorted between the lamp connections.

[Kjelt]

Be carefull the ignition voltage can range from 2kV to 10kV and if they are hot restrike discolights perhaps even 25kV as car HID ballasts. So always work isolated behind an isolation transformer and no secondary path to earth and as stated above work with differential HV probes AFTER ignition and lamps starts to give light. Then also add a safetycircuit like a sparkgap, if the light goes out the ballast will try to reignite, make sure you have an emergency stop circuit so you can kill the power.
Some have an igniter as seperate little box but modern ballast generate the high voltage by sweeping an oscillation circuit and detecting ignition by the current and low voltage. Then the ballast will run up the power (current because lamp voltage will remain the same) till the preprogrammed wattage is reached and keep it there.
After age the voltage if the lamp will rise and when it reaches a safety threshold voltage the ballast will refuse to continue to light the lamp.
So if your lamp ignites , runs up and is then shut fown within minutes this is normal and time to replace the bulb.

[Yamin]

@rvalente - thanks, yes look similar to the ballast circuit I have.

@cozza & @Kjelt - thanks so much for the detailed explanation of the circuitry. Really do appreciate it.

I actually couldn't find the ignitor module you mentioned, probably this model does not have them? Would it be possible to share a photo of the ignitor module.
Is there any way to check HMI bulbs?
With regards to self destruction I mentioned - I'll share the experience I had. I got two faulty moving heads in for repair. The client said that it just wouldn't turn on. I had a look inside and there were some fault on the main power supply which feeds the fans/micro controller board. I repaired it and before powering up I did check at the terminals for the power input for the ballasts - no shorts. Powered on, the microcontroller circuitry fans everything started to work. Ran it for sometime and afterwards turned on the lamp and the moment I did that the moving head died. Opened up and discovered the fuse has blown. I checked the input power terminals for the ballast circuit and it showed that it was shorted.
I had a similar experience with the second light as well.
So in the end I replaced the ballast and the lamp and they both are working now. I troubleshooted to see exactly what shorted in the faulty ballast circuit and it turns out one of the MOSFET had shorted in both the faulty ballasts.
That's why I thought that a faulty bulb takes out the ballast.
Do you have any suggestion for further references material I should check to learn more about it.

Thanks for the help really do appreciate it.

[Kjelt]

Is there any way to check HMI bulbs?

After Ignition and it is burning connect isolated differential probes over the bulb and measure the Voltage. Compare to new bulb.
Depending on the Wattage the voltage of a new bulb might be 90-100V and the old bulb will be 120V but that Voltage depends on the type of hid bulb and the wattage.

What brand is the ballast znd can you see a microcontroller on it ?

[Yamin]

Is there any way to check HMI bulbs?

What brand is the ballast znd can you see a microcontroller on it ?

Thanks, sorry can I ask what you mean by znd?

[Kjelt]

Sorry iPad keyboard, just "and".

[cozza]

Follow the wiring from the lamp socket, if the unit has an external igniter then it will be inline between the lamp base and the ballast.

Moving head lights normally have an external igniter as the cabling from the ballast to the lamp is fairly long. It may be hidden under one of the yoke side covers, or else it may not have one.

[Yamin]

Is there any way to check HMI bulbs?

After Ignition and it is burning connect isolated differential probes over the bulb and measure the Voltage. Compare to new bulb.
Depending on the Wattage the voltage of a new bulb might be 90-100V and the old bulb will be 120V but that Voltage depends on the type of hid bulb and the wattage.

What brand is the ballast znd can you see a microcontroller on it ?

Yes there seem to be a atmel micro-controller, and some more IC which I'm guessing is for MOSFET driving.

Follow the wiring from the lamp socket, if the unit has an external igniter then it will be inline between the lamp base and the ballast.

Moving head lights normally have an external igniter as the cabling from the ballast to the lamp is fairly long. It may be hidden under one of the yoke side covers, or else it may not have one.

I couldn't find any external igniter. The two wires from this board goes directly to the lamp.

I tried uploading some photos but its not going through, if you have a look at the photo attached in post 1 the connector on the right side of the board directly goes to the lamp. Are there any service manual schematics available which I can have a look for reference.

Thanks for the help.

[Kjelt]

Then the ballast probably uses a sweeping frequency to resonant ignite, like an oscillation.
Its common in modern uC driven ballasts.

The ignition voltage can be increased drastically under the resonant ignition through a frequency sweep.

[Yamin]

Hi guys, thanks very much for the responses I was wondering if I could clear one more thing.

I was able to find the model number/type of lamp replacement that I was provided with.
https://www.osram.com/ecat/SIRIUS%20HRI-Discharge%20lamps-Entertainment-Specialty%20Lighting/com/en/GPS01_1028465/PP_EUROPE_Europe_eCat/ZMP_1022817/

It says its a High Pressure Discharge lamp, and I was wondering whether it was different from HMI lamp HID lamp. Or are they the same. By referring to some online material I can't figure out how they differ.

Thanks again for the help

[Kjelt]

The difference is mainly the higher wattage, smaller package and lower lifetime.
These kind of bulbs practically operate the same as a hid streetlamp or shoppingmall lamps but the latter work for 20000 hours and have less Lm/W.
Here you have a bulb that produces extreme Lm/W in a very small package.
You can compare them with video projector bulbs.

[Yamin]

Thanks @Kjelt, so they are pretty much similar. So they also do not need the ignitor from what I read, am I right?

[Kjelt]

The modern electronic ballasts for these bulbs have the resonant igniter integrated on the pcb , so indeed no seperate igniter.

[Yamin]

Nice one thanks! I wasn't able to read the paper you suggested as I am not a member, but it seems a lot of people have done research on resonant ignition. Very interesting indeed.