So now you may be asking yourself, "so what all does a ballast do in general"? Well, here is a little bit of info on how flouresent ballast work and their basic simplicity. The same somewhat applies to automotive ballast. Our automotive ballast take in your cars DC power and converts it to AC current.
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how stuff works.com wrote:
The simplest sort of ballast, generally referred to as a magnetic ballast, works something like an inductor. A basic inductor consists of a coil of wire in a circuit, which may be wound around a piece of metal. If you've read How Electromagnets Work, you know that when you send electrical current through a wire, it generates a magnetic field. Positioning the wire in concentric loops amplifies this field.
This sort of field affects not only objects around the loop, but also the loop itself. Increasing the current in the loop increases the magnetic field, which applies a voltage opposite the flow of current in the wire. In short, a coiled length of wire in a circuit (an inductor) opposes change in the current flowing through it (see How Inductors Work for details). The transformer elements in a magnetic ballast use this principle to regulate the current in a fluorescent lamp.
A ballast can only slow down changes in current -- it can't stop them. But the alternating current powering a fluorescent light is constantly reversing itself, so the ballast only has to inhibit increasing current in a particular direction for a short amount of time. Check out this site for more information on this process.
Magnetic ballasts modulate electrical current at a relatively low cycle rate, which can cause a noticeable flicker. Magnetic ballasts may also vibrate at a low frequency. This is the source of the audible humming sound people associate with fluorescent lamps.
Modern ballast designs use advanced electronics to more precisely regulate the current flowing through the electrical circuit. Since they use a higher cycle rate, you don't generally notice a flicker or humming noise coming from an electronic ballast. Different lamps require specialized ballasts designed to maintain the specific voltage and current levels needed for varying tube designs.
With that being said, you now know the basics of what all is going on inside a ballast. The DC power from your car is being turned into AC power to supply the charge needed to power up the HID bulbs. The ballast throws out 23k +/-1-2k of volts to the HID bulbs upon start-up often refered to as warm-up. This is when you seeing HID trun on and start to change colors and get brighter as they warm. This usually lasts only around 25 seconds or so on OEM ballast. Cheaper aftermarket ballast tend to warm-up longer thus causing premature bulb life loss.
Sometimes when people first get HID, they tend to show boat infront of their friends turning their HID off/on rapidly. Is this good some say? The answer is no. If you've ever seen HID turned off and on you would of noticed a 4100k turns redish-orange for a second. This is the bulbs way of saying OUCH! What happens is the bulbs have already created Xenon gas to for the light but hasn't cooled back into salts and then when the bulbs are turned back on, the ballast are sending out a start-up of 23k volts which IS NOT a good thing. The bulbs already had enough Xenon in them to supply light and didn't need the 23k shot to them. This kills bulb lifespan.
So you've learned about ballasts and bulbs now. Lets move on to the wiring now shall we....
Some people out there just aren't aware of the dangers with wiring HID straight off of your existing oem wiring. Should a relay be used to power HID, yes and always needs to be used. Why you ask perhaps? Your oem halogen equiped car was never designed or intended from the manufacturer to use or run high voltage/high current/ high amperage HID ballasts. Ballast draw a imense amount of amps upon start-up and could very seriosuly damage your wiring and not just at where its connected. We are talking serious damage to fuse boxes, ecu's, or worse could short and cause fires on very old cares that even have a hard enough time trying to power halogen. The reason why is, that when the ballast "demand" power, your car has to supply it from somewhere. Lets say its tapped into your oem headlight wire ok. Now you power up the ballasts, the draw current from your wiring, your wiring might not be up to the task so its needs help, t searches for a source and before you know it, you've now weakend not only one source but two now just to try and supply the ballast good clean power. This is why a relay harness is needed. A relay harness gets its power straight from the battery via relays. These relays are then wired to go to your ballasts now.
To understand how a relay works, go here:
http://www.mgcars.org.uk/electrical/body_relays.html<----Excellent link
More about relays!
http://www.bcae1.com/relays.htm
-or this one-
http://electronics.howstuffworks.com/relay1.htm<-------link