Siemens SITRANS P Bedienungsanleitung PDF herunterladen (Seite 68)

Installation UMSITRPDS3-1

November 2010

4-16

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Loop Resistance, Ohms

Power Supply vs. Loop Resistance

Power Supply, Vdc

675

250

15.5

FIGURE 4-10 Power Supply vs. Loop Resistance

4.3.5.1 Point-to-Point Network

The graph in Section 4.3.5 defines an analog mode transmitter’s operating region for the allowable ranges

of supply voltage and network resistance. Perform the following calculations to ensure that the power

supply output voltage permits the transmitter to remain within the indicated operating range.

1. Calculate the minimum power supply output voltage.

The minimum network power supply voltage requirement is a function of Network Resistance and

full scale current (20 mA), and is calculated by the following formula:

Minimum Power Supply Output Voltage = 10.5 volts + (0.02 × Network Resistance in ohms)

Power supply output voltage must be greater than the calculated value. The minimum voltage across

the input terminals of a transmitter is 10.5 volts.

2. Calculate the maximum power supply output voltage.

The maximum network power supply voltage is a function of Network Resistance and zero scale

current (4 mA), and is calculated by the following formula:

Maximum Power Supply Output Voltage = V

max

+ (0.004 × Network Resistance in ohms)

Power supply output voltage must be less than the calculated value. The maximum voltage (

max

)

permitted across the input terminals is:

• Non-intrinsically safe transmitter: 45 volts

• Intrinsically safe transmitter: 30 volts

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