I have seen some YouTube videos of some folks operating mercury vapor, metal halide and high pressure sodium bulbs on a incandescent bulb to limit current, but they never explain how.
I can't find anything about how this is done. What's the secret? For clarity: secret = what wattage incandescent bulb with for what hid wattage?
The 'secret' is to wire them in series. The incandescent lamp acts as a resistor to limit current, the same reason a ballast is used. The only difference is a ballast wastes much less power because the inductor is reactive (power is dissipated only in resistance). 'Self-ballasting' mercury vapor lamps have an internal filament which makes them less efficient, but more convenient.
There are also small UV lamps that have a filament inside, but dtill need a ballast. They commonly use a capacitor as the reactive element, but can use a small incandescent lamp or a resistor on lower voltage. Here's an example:-
UV-C Germicidal Mercury Vapor Lamp
For example, a 400 Watt H33 mercury lamp operates at 3.25 Amps of current. You need 220-240V to start the lamp therefore you need 240 V circuit to run it. It won’t work from 120V unless you have a ballast that increases voltage to about 240V. When the lamp starts it only drops a few volts across the arc tube which increases to about 130 V or thereabouts. I would experiment with various 130V incandescent bulbs in parallel to achieve around 400 Watts incandescent and when the mercury lamp is at full brightness, measure the line current and adjust your incandescent lamp wattage accordingly to get 3.25 Amps delivered to your mercury lamp. It may seem like an inefficient way of powering a mercury lamp, but the incandescent lamps will color correct the mercury lamp by adding red light to the spectrum… so it’s a win win situation. The incandescent ballast lamps will be stressed by over voltage during starting due to the low voltage drop in the mercury lamp start up negative resistance characteristic
