Hey guys I am curious of this...could a 110v or 220v fluorescent preheat choke or an HID lamp choke for HPS or MV say...be wired right to mains and plugged in with no lamp at all...and sit there humming with no damage? Reason I ask this is because another member suggested this is a good way to test a choke out. I think that is brilliant idea and simple, the problem is will it overheat since it does not have the lamp as sort of a current limiter?  But I think I am thinking wrong and the ballast IS the ONLY current limit… And it would be perfectly fine just getting warm and humming...although useless LOL.  But it would sit there burning off energy for eternity with no harm considering it has it's recommended voltage and ventilation?  Thanks! :-)

The ballast coil is indeed the only current limited in the normal lamp circuit.
The problem is, when there is noextra drop in series with the ballast (= no lamp), the voltage acrossthe ballast become higher (e.g. 125W MV ballast have ~208V across it's terminals when the lamp). This higher voltage cause most ballasts core to become slightly saturated, so the current rise not only by e.g. the 15%, but usually up to ~40%. And that 40% higher current cause twice as much power dissipation than normally, so it would most likely overheat the ballast when applied for long term.
Many ballast designs are on purpose to increase the current more steeply, as it provide higher current for fluorescent preheat and/or higher current for warming up HID lamp.

So connecting it directly across the mains for test purpose is good (to test the curent, really severe overheating, hum,...), but leaving it like that for long time is not a good idea.

Usually the check is done so:
If the ballast distroyed instantly or a short time after connected to the mains, the ballast is EOL or defect.
If the ballast takes a lot of time to heat up when connected to the mains, the ballast is good.

Interesting!  Now i understand completely, Thanks!!! :-)

If the ballast takes a lot of time to heat up when connected to the mains, the ballast is good.

Or better to say "The ballast is safe to try with the real lamp".
The final say should always be on the performance with the real lamp (so current as function of arc voltage development over the lamp runup,...). But definitely, if the ballast "passed" the direct mains connection test, you could be pretty confident it won't damage the (e.g. rare) lamp at least in the short term during the measurements with the lamp.

Still better to test on a cheap lamp first

With HID ballasts that use semiparallel ignitor (where the ignition pulses are formed across the ballast coil), i do a second test in which i let it run with the ignitor without a lamp for a while, and see if it breaks down (same rule applies, use a cheap ignitor for testing a ballast of unknown condition)

Ballast specs often list a short circuit test current in their specs.

Ballast specs often list a short circuit test current in their specs.

Such information is very good guide to guess the ballast health...

Interesting! Thank you :-)