Remote-processing RPC-220 Manuel d'utilisateur télécharger pdf (Page 36)

ANALOG INPUT SECTION 11

Page 12-3

MEASURING HIGHER VOLTAGES

Voltages higher than + 5V are measur ed by inserting a

series resistor to the input. A r esistor can be connected

directly to the STB-20.

The table below shows resistor values for typical input

voltages using the 0-5V range.

Maximum

Input Voltage Resistor

6 20K

12.5 150K

24 380K

Use the following formula to determine the series

resistance necessar y for a m aximum voltage input:

Rs = Vi * 20000 - 100000 0 - 5V range

Rs = Vi * 40000 - 100000 0 - 2.5V range

Rs is the resistor value in ohms in ser ies with the input.

Vi is the maximum input voltage. When the result of

your calculation is negative or zer o, a series r esistor is

not necessary.

Adding a series resistor increases the source impedance

beyond recommended values. A way to compensate for

this is to put a capacitor at the line at J1. This

effectively makes an RC filter. A 0.001 mfd or greater

capacitor is adequate. This does limit the systems

frequency response.

NOTE: When an input voltage exceeds the input range,

other channel values are affected.

Measuring 4-20 mA Current Loops

Current loops are a convenient way to transmit a value

and still assure the integrity of the signal. If the line

should break, a 0 volts (or nearly so) is returned.

A 4-20 ma current loop is converted to 1 - 5V by placing

a 250 ohm resistor across the input of the chan nel to

ground.

Since the measurement range is 1 to 5V, the count range

is reduced by 20% to 819.

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Remote-processing RPC-220 Manuel d'utilisateur Page 36