Loop-powered 4-20 mA Transmitter Circuit Voltage drop

Calculate the voltage drops in this loop-powered 4-20 mA transmitter circuit for the current conditions 4mA, 8mA, 12mA, 20mA.

Loop-powered 4-20 mA Transmitter

Calculate the voltage drop at different points in the above circuit, like V_CD, V_EF, V_FG, V_AB at different loop currents (4mA, 8mA, 12mA, and 20mA)

In order for a loop-powered transmitter such as this to function adequately, there must be a minimum DC voltage between its terminals (V_AB) at all times.

A typical value for this voltage is 12 volts (be aware that this minimum voltage level varies considerably between different manufacturers and models!). Identify what loop condition(s) may jeopardize this minimum supply voltage value, and how you as a technician would ensure the transmitter always received enough voltage to function.

Solution:

The Rosemount 3144 temperature transmitter, for example, requires a minimum of 12 volts at its terminals to function in the analog mode, and 18.1 volts in order to properly function while communicating using the HART digital-over-analog protocol.

Note how the operation of such a 3144 transmitter in a loop with a 24 volt power supply and 300 ohms worth of resistance would be jeopardized near the upper end of the signal range.

Questions for You:

1. If a technician happened to be measuring transmitter terminal voltage while the pressure applied to the “H” port of the transmitter suddenly increased, would the measured voltage increase or decrease?
2. This circuit shows two resistors, rather than just one. Identify a practical reason why a 4-20 mA loop circuit might have multiple resistances in it.
3. Demonstrate how to estimate numerical answers for this problem without using a calculator.

Share Your Answers for the remaining questions.

Credits: Tony R. Kuphaldt

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