A customer recently asked if SKBOWE was comparable with AC and DC HID ballasts, a confusing question because every HID ballast I’ve ever seen has been marked for 12V DC in, 85V AC out (bulb side). Turns out there actually are “DC” output HID ballasts, and they should be avoided as D2S bulbs are specified for AC drive only.
AC vs DC
As discussed on the issues page, HID bulbs are arc lamps that run a fixed current through two electrodes in a sealed quartz capsule. The voltage drop is about ~85V after ignition (20kV) and warmup. Like any arc lamp, electrons jump off the negative (-) electrode, and impact the positive (+) electrode, causing the (+) side to run hotter and wear out faster. To counter this effect, all OEM ballasts and most aftermarket designs send AC current to the electrodes, alternating each electrode between (+) and (-), so that both electrodes wear evenly.
Because the input power from the car electrical system is nominally 12V DC, there are two stages required to output the necessary power to an HID bulb:
- Step up 12V DC -> ~120V DC, using a boost converter with a flyback transformer, rectifier diode, and filter capacitor,
- Chop the 120V DC into 85V AC (RMS) using a set of push-pull MOSFETs (H-Bridge)
However, Chinese cost-cutters, in an effort to produce increasingly cheaper ballasts, figured they could get rid of the second step and feed DC directly to the bulb. The result is lower component count, but a bulb that will burn out very quickly due to rapid erosion of the positive electrode. It will also result in a lopsided, unstable arc that will “dance” and not locate at the center of the reflector foci.
Some Ebay Sellers are starting to market the difference, but price is almost always the most reliable indicator.
How to avoid DC ballasts
Price is usually a giveaway, though some manufactures have figured this out and are now up-pricing DC ballasts to make huge profits. Because DC ballasts are cheaper manufacture without the DC to AC stage they sell for very low prices, say, below $35 for a kit. AC ballasts require a large inductor and HV regulator for DC to AC conversion and are more expensive to manufacture. High-end Digital 35W AC Ballasts can’t be produced at that price, let alone sold for anything close. Without special equipment it’s hard to test at home, so make sure to order from a reputable company that specifies an AC output.
Also it doesn’t help for online shopping, but AC ballasts are significantly heavier than DC ones:
The what NOT TO BUY post has an example of a DC ballast. The Morimoto XB is an example of a good AC ballast. When I get some free time, I will update the ballast selection page with measurements from the ballast collection.