I have the tentative specs of 35W automotive XMH lamps being 85V at 0.45A. If you have the specs of other wattages definitely let me know. Anyway this is not super duper similar to the specs of an F40T12 but it's sorta close (100V at 0.43A). So I decided to try it out.

A while ago I acquired the internals of a 35W HID flashlight, which uses an XMH lamp and corresponding ballast. Tried running an F40T12 on it and it seemed to work, the power supply showed a little over 40W but I didn’t bother to actually measure the specs. At this time I had concluded that the tube was probably underdriven a little bit, just an inference based on the input wattage and the warmth of the ballast. Of course it is instant-start so not great for lifespan, and the >10kV ignition spikes would be very dangerous for a conventional fluorescent fixture but whatever, proof of concept.

Anyway I killed that ballast by doing something quite stupid before I got the chance to take measurements, so I got a pack of two more for cheap on Ebay. Tested one on the old XMH lamp and it worked fine. Tested on an F40T12 and similar results (~42W input power to the ballast). But this time I took measurements, and I was completely surprised.

Started with the voltage measurement on a true RMS meter of course, which only read like 2-3V! So I switched it to DC mode… and yeah it outputs DC. Specifically 114V at 0.312A for this particular F40T12, which makes for 35.5W (simple to calculate now cause PF=1 with DC). Just as a sanity check I revered the tube polarity, and the dark sock at one end of the tube which I thought was cathode wear instantly switched sides! Of course this was just a Faraday dark space, which is only seen on the negative end when running tubes on DC.

And no, output polarity does not automatically switch each time the ballast is powered on. How can they get away with this when running an XMH lamp? I would assume one electrode is getting rapidly beaten away while the other remains relatively intact. Is this only an artifact of cheaper ballasts?