HID Ballast Buzz

[Kesawi]

Just installed a pair of Lightforce 170 HID driving lights and noticed the ballasts constantly buzz after the light has warmed up when testing during the daytime.

Is this normal?

Also, the light output seems underwhelming, but I haven't been able to find a long enough stretch of road without any traffic to test them properly.

[DJKOR]

The buzzing is completely normal. It is usually of a higher frequency during the startup phase, but then drops down a bit once the lamps are warmed up.

The reason for this is because the lamps require approximately 200 volts to maintain the arc that produces the light (after the initial ~23kV pulse used to establish the arc). Because you car only outputs 12v or there abouts, the ballast needs to step this voltage up. Only thing is that the input to the ballast is DC. To be able to step up the voltage, this needs to be turned into AC and then the voltage is stepped up.

In short, the process involved to step up the voltage creates that buzzing sound.