SX440 is a half-wave phase-controlled thyristor type Automatic Voltage Regulator(AVR) and forms part of the excitation system for a brushless alternator, such as Stamford or Stamford technical alternator.

Technical specification
Input
       Voltage        190-264V ac
       Frequency     50-60Hz
Output
       Voltage         Max 90V dc at 207V ac input
       Current         Continuous 4A dc
                          Intermittent 6A for 10 secs
        Resistance    15 ohms minimum
Regulation     +/-1%