Horn -GPM 12V DC- Lambretta LI, LIS, SX, TV (series 2-3)

AC stands for alternating current = alternating voltage = ratchet
DC stands for direct current = direct voltage = horn

ratchet
Many old vehicles without batteries use alternating voltage as the on-board power supply. Only so-called buzzers can be connected to it. These are recognizable by a tone pitch which increases and decreases with engine speed.

Horns
Due to the non-constant frequency, buzzers have not been in conformity with the registration of German vehicles since approx. 1980. At this time, most vehicles were equipped with a battery and a DC on-board power supply. Here the real horns are used, recognizable by a constantly constant pitch, independent of the engine speed.

Exceptions
There are some exceptions where a DC horn was used, despite the AC on-board power supply. Piaggio used so-called horn rectifiers from 1984 onwards, with the introduction of the Lusso series. Here, a special bridge rectifier with downstream capacitors, which is only responsible for the horn, converts the AC on-board power supply for the horn into DC. This rectifier can also be retrofitted to all vehicles with a 12V AC on-board power supply if, for example, the TÜV does not accept a ratchet.

(US)