Magnetic ballasts use a large spool of wire wrapped around a set of steel sheets to generate the high voltage and wattage required by HID lamps.
Most magnetic ballasts use a metal or plastic canister known as a capacitor as part of the regulating system. These ballasts are heavy, rugged, and built-to-last for many years in harsh weather conditions.
Magnetic ballasts are certified by the Canadian Standards Association (CSA) for electrical safety and fire protection standards. They are designed to meet lamp operating standards set by the American National Standards Institute (ANSI). These standards ensure the lamp and ballast will operate correctly together. Metal Halide and High Pressure Sodium (HPS) lamps typically require their own separate ballasts for operation. There are switchable magnetic ballasts that can operate both types of lamps. The user must flip a switch prior to operation. Switchable ballasts are a compromise because they typically operate the HPS lamp to specification, but operate the Metal Halide lamp out of specification.
Electronic ballasts use semiconductors and microchips to provide the high voltage and wattage required by HID lamps.
The inside of an electronic ballast looks very similar to the inside of a computer. One or more circuit boards house mini capacitors and mini wire wound magnetic transformers. The use of these circuit boards allows for light weight and produces less heat than a magnetic ballast.
High wattage electronic ballast (anything above 250W) are not manufactured to meet lamp operating standards set by ANSI. Standards have not yet been written by ANSI for electronic ballast compatibility with HID lamps.
Because there is no ANSI standard, all electronic ballast are made differently. This has resulted in inconsistent lamp performance on electronic ballasts. Many electronic ballast are not properly ETL or UL listed for fire and electrical safety.
Most, if not all, electronic ballasts on the market today are not digital even though they may be marketed that way. Digital would imply that the ballasts would have a microprocessor inside.