Power supply advice

[Highway_Star]

Morning all,

I've aquired a Newarc TIG HF control box, you supply it DC and gas, and it provides the HF and gas solenoid.

I've a BOC Transarc MMA box which is AC only, so that's no good as it stand.

Either I build a rectifer to sit inbetween, I'm thinking this would also need smoothing caps and a hefty choke?

Or,

Due to equipment replacement in the cinema game at the moment (every ****** is going digital projection with 3D), I can probably lay my hands to a buckshee ex-cinema arc lamp switch mode supply (inverter to weldy types). This might work well, super smooth DC (typically no measureable AC ripple) and outputs variable between 30-200A. Ouput is controlled by a variable resistor so I'd be able to use a pedal, and switched on/off by a contact pair. Cinema PSUs are 100% duty cycle rated too and the switch mode variety are amazingly compact, can run single phase and amazingly light load on the mains. For example a 2kW xenon lamp running at 75A will pull 30A off the mains using a linear type power supply, you can comfortably run a 2kW xenon via a switcher on a 13A plug.

Now here's where I'm not sure as to the cinema supplies suitability. Typically before the arc lamp is struck, the open circuit voltage will be 100-140v. As soon as the lamp strikes (it's forward impedance drops to somewhere near 0 Ohms, it's a plasma arc contained in an inert gas filled bulb) this drops to somewhere in the 20-30v region. Current will be whatever the unit is asked for via the control pot. Does this sound any use for TIG, or do I need a PSU that maintains high voltage under load?

 

[SirDick Diodenob esquire]

I think 20 to 30V on load is ok , even 30 may be a bit high . A Murex Tradestig works on a very low voltage 12 to 16V on load with a 61V no load , a Murex Transarc which is not a dedicated tig machine but has dc output is still ok for tig is 22.5 to 25V on load and 41V no load . A Lincoln invertor 23.2v to 25v and 67V no load , all these machines are scratch start with no hf . Your Newarc controller will take care of the HF requirements and start voltages so I would think your power supply would be good . I would like to get hold of one of the switch mode supplies you mention as I like messing about with welding gear where can I get one ?

 

[eddie49]

Me too! Should we form an orderly queue outside our local Odeon?

 

[shenion]

Sounds like the PSU is constant-current; so it should be good for TIG.

You will need to add a choke to help maintain the arc. I have made several, 20-30 turns around an iron pipe with a frame around it.

 

[Highway_Star]

Happy days!

These PSUs have a hefty choke in there already, it's to help control inrush current when the Xenon lamp strikes, and minimise radiation of RF from the strike unit. Ignition in a Xenon arc lamp is provided by guess what? HF! So the PSUs are well protected from it. Typical arc lamp ignition voltage will be in the 50kV range. No idea on the actual frequency, some machines have a spark gap and capacitor/coil arrangement, some use capactive discharge.

There's actually not that many of the UK cinemas who were using these type of supplies, as most haven't bought any new projeciton gear for 30 years. One of the small circuits we fitted out over the last 15 years were using them and are starting to replace their 35mm kit. A mistake in my opinion, it's bought and paid for, why scrap it when it is still usable? However they've signed a deal with Sony, who are supplying their digital machines for next to nothing, part of that deal is that their must be no rival equipment in the place.

I'll see how many I can lay claims to!

 

[Highway_Star]

Right, I'ev got a pair of these beasties on their way to my gaff, so next week I'll need to make up some heavy DC leads etc. I'll need to get the connectors for thE TIG box, Dinse? Is that the right name? It's basically a round piece of brass with a lug/key on it.