Xenon lamp operation
Once discharge has started after the ignition impulse, the ballast ensures that the bulb receives the proper operating conditions as regards current and voltage. The ballast produces running voltage in a DC/AC converter, where 12 V DC is converted to approx. 85 V AC with a frequency of about 300 Hz. Upon start of a cold bulb, running voltage, running voltage just after ignition impulse tends to be relatively low, approx. 20 V. As long as B+ is supplied to the ballast, it produces a running voltage and the bulb lights.
If the bulb goes out during operating, the ballast detects this by current monitoring of running voltage and the ignition procedure is started. If the ignition procedure fails, the internal protective functions switch off the ballast.