That light-bulb trick is handy in other stuff too like repairing amplifiers and such. But there's downside it reduces the power DUT takes, so all equipment won't work as intended (i'm no expert so this can be rubbish.....)Variac is one handy device, just remember output is not isolated. I have 500w variac in same ''circuit'' as my 2kw isolation transformer, and that isolation transformer(weights about 20kg) is fed throught self-made ICL, without it, it takes about 400A current worst case IIRC. That ICL is automatic; time relay that switches/shorts power resistor. In fact, time relay can't take 2kw power, so time relay controls bigger relay that shorts power resistor, resistor is in series with transformer (plus fuses)
If interested here's ICL as well as isolation transformer/variac combo (enclosure frame is microwave oven's frame)I prefer analogue meters, they tend to give better overall situation feedback
https://www.electro-tech-online.com...mprovement-for-isolation-transformer-icl.752/
https://www.electro-tech-online.com/blog-entries/isolation-transformer-variac-combo.726/
.....As there was talk about variac, so not entirely off-topic, and not bragging either (that isolation transformer-case is ugly, but it works)
and light-bulb current limiter, with bypass switch (e27 of course has safety issues.....)
https://www.electro-tech-online.com/blog-entries/helpful-piece-of-equipment.274/