Siemens Sitrans 사용자 설명서
7ML19985GE01
SITRANS LC 500 – INSTRUCTION MANUAL
Page 23
m
mmmm
Int
erc
onnect
ion
Interconnection: SITRANS LC 500
Wiring
Supply
The SITRANS LC 500 uses a switched power supply circuit, which makes the most
efficient use of the available power present on the terminals. If the signal current is low,
(4mA), the terminal voltage will be high, and if the signal current is high, (20 mA), the
terminal voltage may be low, due to all the resistive elements in the loop, such as the
barrier and sense resistor.
efficient use of the available power present on the terminals. If the signal current is low,
(4mA), the terminal voltage will be high, and if the signal current is high, (20 mA), the
terminal voltage may be low, due to all the resistive elements in the loop, such as the
barrier and sense resistor.
Examples:
• With a 250 Ohm sensing resistor, no barrier and negligible cable resistance, the
overall supply voltage should be at least 15.0 V.
• With a 250 Ohm sensing resistor, a barrier of 280 Ohm, and 20 Ohm cable
resistance (500 m), the total resistance is 550 Ohm, so the overall supply voltage
should be at least 20.5 Volts.
should be at least 20.5 Volts.
• For a multi-drop application, where the measuring supply is fixed to 4 mA, the
voltage on the terminals of the SITRANS LC 500 should be at least 12 Volts.
The loop circuit is completely isolated from the measurement circuit. It is designed so that
the internal capacitance and inductance on the terminals are isolated and do not factor
in safety calculations.
the internal capacitance and inductance on the terminals are isolated and do not factor
in safety calculations.
Notes:
•
The transmitter is powered by the current loop and needs at least 9.5-13 Volt on
the terminals: 9.5 V at 22 mA or 12 V at 3.6 mA.
the terminals: 9.5 V at 22 mA or 12 V at 3.6 mA.
•
The maximum supply is 33 Volt. If the voltage is higher the device will shut down.
•
The loop-circuit will withstand voltages up to 250 Vac/Vdc without any damage.
Voltage drop versus mA for current transmitter operation
mA
V-
su
pp
ly
voltage drop over 250 ohm
measuring resistance
voltage drop over 280 ohm in
barrier
voltage drop over blocking
diode in barrier
margin or voltage drop over
margin or voltage drop over
instrument cable
operation voltage,
transmitter