Here's the skinny on bombarders and chokes: I'm using a power company transformer known as a "PT" or potential transformer. I don't know exactly what they use it for – I think it is used as a source of 120 VAC control voltage in switchyards or something. The unit is rated as follows:
– turns ratio: 100 to 1
– "primary" voltage: 12,000 vac
– "secondary" voltage: 120 vac
– KVA rating; 1.5 KVA
Note that the KVA rating is a continuous duty, 24-hours-a-day-in-Death-Valley-in-the-summer-type rating. For short duty cycles, as found in neon work, I rate it at 6.0 KVA (i.e., 500 ma tube current), which works fine for tubes up to 15mm. I also run it on 240 vac, which also works well.
Here's the kicker: the cost, from SDG&E (San Diego Gas and Electric) is $51.20. No, I'm not kidding; see the attached invoice (it's true, folks) The deal is that SDG&E pulls these units out of service, cleans and fully tests them, and then puts them in storage for future use.
At this point, you can (hopefully) buy them for ten cents on the dollar as scrap (newbook price: $512). Heck, even full list ain't bad compared to real bombarder prices, but at fifty bucks, well, I wound up buying two to have a spare. You can't even buy a new 15,000/30 for that!
This deal is probably too good to last. Sooner or later, SDGAE is going 1o discover it's giving away the store on this one, so don't be surprised if the price increases or availability dries up...
Next problem: a control choke.
After some experimental verification, I determined that a standard Sears-type home arc welder (which I already had) can be used as a choke. This unit conlrols the welding "heat" output by sliding a laminated steel magnetic shunt between the welder's primary and (step-down mode) secondary windings.
Cnnect it in series with the bombarder transformer, as per the drawing (adding a 240 volt/50 amp receptacle where shown allows the welder to be disconnected and used for its intended purpose). It goes without saying that everything should be well grounded, as shown, for safety's sake. Short the arc welder* s ground cable to the electrode holder.
On my unit, setting the welder at the lowest heat allows about 100 mA of tube current. At the highest heat setting, the tube current is about 650 mA. To test this setup, I used an old eight-foot fluorescent tube as a load, in place of where neon would be connected (bombarding current for neon is normal operating current for a fluorescent tube).
I'm not completely sure if the welder appreciates this, but nothing gets overly hot, and there are no bad smells or anything. I've used this setup for over a year with no problems.
