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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?

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  • \$\begingroup\$ Just buy a self-ballasted mercury-argon vapor lamp. They include the tungsten, all nicely built in and designed for each other. \$\endgroup\$
    – jonk
    Mar 5, 2020 at 7:34
  • \$\begingroup\$ thank you for your time to answer my question but my goal is to operate any hid bulb, i should of been more specific, my bad. \$\endgroup\$
    – jwr
    Mar 6, 2020 at 0:39
  • \$\begingroup\$ No problem. Then you need to know this: tungsten increases its resistance by a factor of 10 when going from room temp to about \$3500\:\text{K}\$. Conversely, mercury-vapor lamps have a glow-phase and a glow-to-arc phase transition which has a very high "negative resistance" response. Tungsten (except for its long time constant) is a good fit for these applications. But it has to be designed, too. You can't just slap a random incandescent together with a random mercury-argon bulb. \$\endgroup\$
    – jonk
    Mar 6, 2020 at 1:54

2 Answers 2

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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

enter image description here

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  • \$\begingroup\$ In practice, the tube needs to be short enough that the peak AC voltage is high enough to start an arc without the aid of a dedicated starter. 230V AC peaks at about 330V, but 120V AC only goes up to 170V. \$\endgroup\$
    – Simon B
    Mar 5, 2020 at 13:11
  • \$\begingroup\$ thank you for your answer, however, i kind of already figured that they are connected in series just like the ballast is, but what im wondering is what wattage incandecent bulb do i use? does wattage have to match up with the hid bulb wattage? \$\endgroup\$
    – jwr
    Mar 6, 2020 at 0:26
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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

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