A CLICK PLC can classify a cable by resistance, but the measurement front end must convert resistance into an analog value the PLC can read. The stated measurement objective is repeatable pass/fail testing with approximately ±3 ohm accuracy. The acceptable resistance limits, cable length, connection method, and calibration requirements remain unspecified and must be defined before selecting hardware.
Resolve the no-voltage requirement
An electrical resistance measurement requires test excitation. A meter or signal conditioner applies energy to the device under test and measures the resulting electrical response. If the requirement prohibits any applied voltage or current, the PLC cannot determine resistance electrically. Clarify whether the actual restriction is no external operating voltage, while permitting the limited excitation generated by a resistance-measurement circuit.
Select the measurement architecture
| Architecture | PLC interface | Engineering decision |
|---|---|---|
| Resistance signal conditioner, such as the identified SCU-1400 | Conditioner's analog output connects to a CLICK analog input | Confirm that its resistance range, output range, accuracy, and calibration support the required limits and approximately ±3 ohm accuracy. |
| Custom voltage-drop circuit | Analog voltage input; a 16-bit card was proposed for resolving small changes | A voltage reading alone is insufficient. The circuit must establish or measure current so resistance can be calculated from R = V / I. Validate the complete circuit before relying on PLC calculations. |
The custom approach proposed a stable 10 V DC source, low ripple, and a long PLC smoothing filter. These are design suggestions, not a complete resistance-measurement specification. Source stability and filtering can reduce variation, but filtering cannot correct lead resistance, contact resistance, offset, or an undefined test current.
Configure the pass/fail test
- Define the lower and upper acceptable resistance limits and specify whether approximately ±3 ohms refers to total measurement accuracy, allowed process variation, or both.
- Document cable length, conductor arrangement, switching devices, and the connection method, such as alligator clips, pogo contacts, or spring-loaded pins. These elements can contribute series resistance and affect repeatability.
- Select the conditioner or measurement circuit and map its analog output range into resistance units in the CLICK software.
- Compare the scaled resistance with the defined lower and upper limits. Declare pass only when the reading is within the acceptable window.
- Apply smoothing only after observing the unfiltered signal. Confirm that the chosen filter stabilizes the result without hiding intermittent connector faults.
Verify accuracy and repeatability
Test the complete fixture with known resistance references near the lower limit, nominal value, and upper limit. Repeat measurements after reconnecting the cable so the test includes connector and fixture variation. Record the raw analog value, scaled resistance, and pass/fail result. If traceability, calibration, or test documentation is required, include those requirements in the hardware-selection decision rather than assuming the PLC scaling alone provides them.
FAQ
Can a CLICK PLC measure resistance directly?
Not from the described arrangement. Use a resistance signal conditioner with an analog output or a defined excitation circuit that converts resistance into a measurable voltage.
Can resistance be measured without applying voltage to the cable?
No electrical resistance value can be derived without test excitation. Clarify whether limited meter or conditioner excitation is acceptable even though external operating voltage is prohibited.
How do I program resistance pass/fail in a CLICK PLC?
Scale the analog input into ohms, then pass the device only when the measured value is between the documented lower and upper limits. Verify the complete system against known resistances and the stated approximately ±3 ohm accuracy objective.