A lot of apprentices (and journeymen) have trouble understanding what makes HID lighting any different than regular lighting so troubleshooting them can be a bit confusing. Let's talk about it!
HID lamps are easy to understand once you wrap your head around the idea that the light inside of one is produced from an arc inside of a tube. There are no metal wires inside of an HID lamp, like you'd find in a standard incandescent. In fact this is the philosophy behind fluorescent tubes, neon signs, and compact-fluorescent as well. It's just an arc being struck in a tube, then the arc mixes with certain chemicals to form an arc-stream to produce visible light.
All HID lamps have a few things in common. 1) They use an arc to illuminate rather than a piece of metal. 2) They all use ballasts. 3) They are all used in relatively the same places - sports arenas/stadiums, street lighting, and parking lot lighting.
How these bulbs differ is where things get interesting:
Metal Halide lamps use halide salts on the inside of the arc tube. Halide salts such as fluoride, bromide, chloride, and iodide are deposited around the inside of the inner arc-tube and when an arc is stricken through the tube, the halide salts start to vaporize into the arc stream. Each halide salt has a certain color profile that it adds to the finished light output, and a specific temperature has to be maintained in order to keep the lamp at the same color consistency. This is achieved by painting the pinched edges of the arc-tube white to reflect light back into the chamber.
Rather than having any secondary elements in the arc tube to control the light color, a mercury vapor relies solely on the blue light emitted from vaporized mercury (which all 3 of these lamps have). What makes mercury vapor the most recognizable is the white coating around the inside of the lamp's outer envelope. Much like a fluorescent lamp, a mercury vapor lamp utilizes phosphorescence to control the color. Phosphor glows when introduced to UV light, so this coating helps to make the color of the glowing mercury less blue, and more white.
HPS lamps are pretty well known for their yellowish/orange color output. This lamp operates similarly to the MH and MV lamps, however the substances it uses to achieve its color and efficiency are hotter and require more pressure to ignite. Mercury is still inside of the inner arc-tube, however there is also sodium which is a very hot burning substance. The arc-tubes of HPS lamps are very skinny and made from an opaque ceramic rather than quartz or glass like a MH or MV lamp would be.
All 3 of these HID ballasts look similar, and for the most part, operate similarly. There are a few components to know when trying to troubleshoot these setups. Each has a lamp, a ballast, a capacitor, and sometimes an ignitor.
Ballasts are essential components in HID lighting. A ballast separates the primary circuit (incoming power from a panel) from the secondary circuit (outgoing power to the lamp). Ballasts use magnetic coupling, also called induction, to transfer power from one circuit to another through thin air. Much similar to how audio engineers will use an isolation transformer to clean up “noise” in speakers and amps by separating the power delivered, to the power consumed - an HID ballast keeps the lamp from “touching” the primary power feeding the fixture. So the lamp is really on its own internal circuit.
These ballasts are not stepping up or down the voltage like a typical transformer would. There’s still the same relative voltage in as there is out. It’s just an isolation technique which also helps the current not “run-away” or continually increase as the lamp heats up.
Capacitors are another type of isolation technique that helps to regulate current and keep the arc stable inside of the lamp. Capacitors also help balance the circuit out because it is a capacitive component, which acts inverse to the inductive element of the ballast. If you have too much inductance in a circuit it will run really inefficiently, so by adding a capacitor it counter-acts the ballast and brings efficiency back to a manageable level.
Only high-pressure sodium and pulse-start metal halide fixtures have ignitors. Neither mercury vapor nor standard metal halide fixtures need ignitors to strike an arc through the lamp. An ignitor is used when high pressure is necessary to strike an arc through certain materials and get the temperature up very quickly inside it. Since high-pressure sodium lamps have mercury and sodium inside it needs to utilize a high-voltage pulse to ignite the lamp, which the ignitor provides. Ignitors automatically remove themselves from the circuit once the arc is stricken, by use of a bimetal switch.
There’s a lot more I could talk about in the topic of HID lighting, and over the next few months I’ll start getting into troubleshooting techniques that will help you understand, a bit deeper, how these components work.
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**Disclaimer - These videos are for training purposes alone, all work done on electrical systems should be done by a licensed and insured electrical contractor. If you are not an electrician, do not attempt any of the work you are seeing in these videos.**