Besantek BST-3PT5, Инструкция по эксплуатации

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7. Principle of how it work
As this instrument takes it's power from the circuit
under test, it has a power supply circuit which derive
it's power from the system. That supply circuit
regulate a +5V for the circuitry. This is why you
need a minimum of 40Vac before this instrument is
operational.
The first circuit is the 3 Phase presence indicator,
which is shown by the individual Phase Presence
LCD Indicators and the second circuit is the three
phase sequence indicator by LCD indication.
3 Phase Presence Indication circuit:
The voltage measured between phases is utilized
to trigger the corresponding Phase Presence LCD
indication. Once that voltage is high enough, it's
presence is shown on the LCD Indicator.
3 Phase sequence indicator circuit:
The sequence of these measured voltages are feed
into a digital circuit which compare which phase
sequence to indicate and indicate it on the LCD.
An oscillator clock the LCD to increase it's life
duration.