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Oct 19, 2015 7:03:00 AM

How to Troubleshoot 2 & 3 Wire Circuits

Electronic chip on circuit board. Macro close-up, shallow DOF.

The following steps clearly indicate the exact sequence to properly troubleshoot a Setra 2 wire, mil-amp circuit. You must work with the assumption that either the Setra product or the system & circuitry is bad.

1). Assure there is no pressure applied to the Setra product. . For extremely low (<0.1 psi) differential pressure range, place a “jumper tubing” across the S2 ports

2). The Setra product pressure port is disconnected from the piping system. (to avoid ground loops)

3). The mil-amp meter is working properly on the appropriate scale.

4). All wiring has proper continuity.

Setra's two wire products have electrical connections identified as:

Leads: red is positive input = 24 VDC & black is the 4 to 20 mA signal output.

Terminals: (+) is positive input = 24 VDC & (-) is the 4 to 20 mA signal output.

Data Acquisition System (DAS) having input terminals identified as (+) and (-). [With the assumption that the positive terminal has 24VDC and negative terminal is the input to a dropping resistor (see transducer data sheet for maximum allowed circuit resistance) and the DAS reads the voltage across the resistor.]

Step 1

Connect 24 VDC to the (+) terminal of the Setra product directly from the power source. Connect the positive lead from a mil-amp meter to the (-) terminal of the Setra product. Connect the negative lead from a mil-amp meter directly to the negative terminal of the power source. Turn 24 VDC power on.

Results: A) 3.84 to 4.16 mil-amp reading indicates Setra product is working properly and within designed specs. You may want to do a zero adjustment to return the unit to 4.00 mil-amp. (Go to step 2) B) Zero mil-amp reading; (confirm unit has 24VDC on (+) terminal) unit failed, return unit to factory for evaluation. C) Mil-amp reading greater than 4.16mA, unit out of spec. May be caused naturally by long term drift or by a positive overpressure. D) Mil-amp reading less than 3.84 mA, unit out of spec, may have been caused by a negative overpressure. You may want to do a zero adjustment to return the unit to 4.00 mil-amp. If unable, return unit to factory.

Step 2

Leaving the 24 VDC power on, relocate the negative lead from a mil-amp meter to the wiring leading to the negative terminal of your DAS.

Results: A) 4 mil-amp reading indicates Setra product and your circuit wiring is working properly. (Go to step 3.) B) Zero mil-amp reading, your DAS input exceeds Setra’s specification for excitation & resistive loading, OR your circuit does not return to same power source negative. CONSULT THE MANUFACTURER OF YOUR DAS.

Step 3

Relocate the positive lead from the Setra product to the positive terminal of your DAS.

Results: A) 4 mil-amp reading indicates Setra product and your circuit wiring is working properly. (Go to step 4.) B) Zero mil-amp reading, your DAS input exceeds Setra’s specification for excitation & resistive loading, OR your system does not have the proper voltage available at the positive terminal of the DAS. CONSULT THE MANUFACTURER OF YOUR DAS.

Step 4

With a Setra pressure transducer providing a 4 mil-amp output, be sure the DAS is displaying the proper value and performing the desired operation.

Three Wire

Follow these steps to properly trouble shoot a 3 wire, voltage signal circuit.

Step 1

Make sure there is no pressure applied to the Setra product by disconnecting the pressure ports from the piping system.

Step 2

Check that the volt meter is working properly on the appropriate scale.

Step 3

All wiring must have proper continuity. Setra products have electrical connections as terminals, connectors or leads identified (+ excitation), (- common) and (+ signal).

(+ excitation) or (+ exc) = connect directly to the power supply

( - common ) or (- com) = connect directly to the power supply negative & to the negative terminal of DC voltmeter

( + signal ) or ( + out ) = connect to the positive terminal of the DC voltmeter

With the power supply turned on, the voltmeter should indicate the appropriate voltage level specified for the model, with no pressure applied. NOTE: Orientation does play a factor in the actual output.

  1. If the output should be 0 volts DC you will need to lightly tap the port with your finger to observe an output.
  2. If the output should be 0.1 volts DC (or higher) then you should observe the out to be 0.100 volts DC

If no voltage signal is observed in either of the 2 cases, then the transducer should be sent back to Setra for evaluation ( confirm full exc. is across + exc & com ).

Note:
Lead identification :

red = positive excitation

black = negative excitation*

green = positive signal

white = negative signal*

*black & white are connected to the same terminal within the transducer – this is a true 3 wire unit

 

CLICK HERE for a more general guide to troubleshooting a pressure transducer, not just a Setra pressure transducer.

 

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