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**4 to 20mA signal products**

- Differential Pressure Transmitters
- HART® Liquid Level Transmitters
- Natural Gas Pressure Transmitters
- Low Differential Pressure Transmitters

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## User Guide

This tool will calculate the maximum load resistance, minimum power supply or transmitter supply that can be used, in order for a 4 to 20mA current loop circuit to have sufficient power to operate.

Determine the lowest voltage power supply that you will be able to use with a 4-20mA current loop circuit by entering the total load resistance in the loop, and the specified manufacturer’s minimum supply voltage for the transmitter.

If you know what power supply you have available, and what transmitter you want to use, you can determine the maximum load resistance that will be allowed in the circuit without affecting the performance of the transmitter.

You can use this tool to determine what transmitter can be used, based on the minimum required supply voltage to maintain a 4-20mA signal over the 0 to 100% full scale measurable range.

### Formulas

The formulas used by this 4-20 mA maximum load, minimum transmitter and power supply calculator to determine each individual parameter is as follows:

V_{minTx} = V_{minPS} – (I_{20mA} · R_{maxL})

R_{maxL} = (V_{minPS} – V_{minTx} ) / I_{20mA}

V_{minPS} = V_{minTx} + (I_{20mA} · R_{maxL})

#### Symbols

- V
_{minTx}= Lowest allowed transmitter supply voltage - R
_{maxL}= Maximum allowed load resistance of current loop circuit - V
_{minPS}= Lowest required power supply voltage - I
_{20mA}= Highest current signal of 20mA

### Parameters

#### Minimum Transmitter Supply Voltage (V_{minTx})

This is the minimum supply voltage in volts (V) that is required or available to power the transmitter and maintain a 4-20mA signal.

#### Maximum Load Resistance (R_{maxL})

This is the maximum load resistance specified in ohms (Ω) that can be added in series to the power supply and transmitter without affecting the operation of the current loop circuit.

#### Minimum Power Supply Voltage (V_{minPS})

This is the minimum voltage measured in volts (V) which is required or provided to power the current loop circuit and produce a 4-20mA signal.

**4 to 20mA signal products**

- Anaerobic digestion methane gas 2bar pressure transmitter
- Dirty water, sludge & sewerage tank level 400 mbar pressure sensor
- High Temperature Pressure Transmitters
- Vacuum Pressure Transmitters