Selecting TVS Diode Mounting for PLC Proximity Inputs

At the panel, the proximity input may flicker, register false transitions, or remain vulnerable even though the added red components look correctly wired. Start here: identify the components, trace the two shunt paths, and decide where an induced transient must be diverted. For the shown circuit, mount the protection close to the PLC input terminals, with short connections to the correct DC common.

The drawing calls for two devices labeled 26 VDC and a 1.5 kΩ resistor. The supplied round red parts look more like disc metal-oxide varistors (MOVs) than the axial TVS diodes commonly pictured in catalogs, but appearance alone does not identify their electrical ratings.

Start with the panel symptom

Watch the PLC input indication while the machine operates. Record whether the problem occurs when a nearby load switches, when the proximity-switch cable moves, or whenever the sensor changes state. Those observations separate an induced-transient problem from a steady-state wiring or logic fault.

Observed symptom	Likely cause or next check
Brief input transitions when nearby equipment switches	Transient voltage may be coupling into the field cable. Inspect routing, shielding, common connections, and clamp placement.
Input always on or always off	Check the sensor output type, the 1.5 kΩ pull-up path, input-module compatibility, and basic wiring before changing suppressors.
Input works until a suppressor is connected	The part may have the wrong continuous-voltage rating, a polarized TVS may be reversed, or the suppressor may be connected to the wrong reference.
False transitions remain after suppressors are installed	Measure at the PLC terminal. Long suppressor leads, the wrong common point, or conducted disturbance through the power supply can leave voltage across the input.
No input problem is visible	The parts may be preventive protection. Verify the circuit against the modification drawing rather than creating a fault test.

A transient is not the first suspect when the input has a stable, repeatable wrong state. Check the normal DC levels first. Adding larger suppression parts will not correct an incompatible sensor output or a missing pull-up.

Confirm the supplied component first

Read every marking on the red body and packaging. Use the manufacturer datasheet for that exact marking to identify whether it is an MOV, a unidirectional TVS diode, a bidirectional TVS diode, or another component. Record its continuous working voltage, breakdown or varistor voltage, clamping voltage at the specified test current, pulse rating, and polarity.

• An MOV is normally bidirectional and has no installation polarity.
• A bidirectional TVS also clamps both polarities and normally has no polarity-sensitive orientation.
• A unidirectional TVS is polarized. In a positive DC circuit, it is normally connected reverse-biased across the protected nodes.
• Body color and disc shape do not prove a voltage or energy rating.

If the markings cannot be matched to a datasheet, stop. An unidentified suppressor can conduct during normal operation, fail to clamp below the PLC limit, or fail when exposed to the actual pulse energy.

TVS diodes are commonly selected for fast, lower-energy electrical transients. MOVs can absorb larger events when their physical construction and published ratings support that duty. Neither category is automatically correct: compare the exact device data with the circuit voltage and the transient measured at the PLC terminals.

Trace the two protection paths

The schematic shows one suppressor from the 24 VDC supply conductor to ground and another from the proximity-switch output to ground. The 1.5 kΩ resistor appears to pull the signal toward the positive supply when the sensor output is not conducting, which points to an open-collector or open-drain style output.

Confirm that interpretation with power removed:

1. Trace the 1.5 kΩ resistor from each end. One end should follow the supply node shown in the drawing; the other should follow the sensor-output and PLC-input node.
2. Trace both suppressors to the reference labeled ground.
3. Determine whether that reference is the DC 0 V common, protective earth, or a drawing-specific reference point.
4. Compare the field wiring with the input-module and sensor connection diagrams.

Do not substitute protective earth for DC common merely because both are drawn with a ground-like symbol. A clamp diverts current into the node connected to its return lead. Using the wrong node can create a poor transient path or an unintended connection between control common and protective earth.

Next, power the circuit and measure the supply-to-common voltage and input-to-common voltage in both sensor states. If the input switches between valid module levels, continue to the transient check. If it does not, correct the sensor, pull-up, common, or module wiring first. That is not a suppressor-placement fault.

Mount the clamps at the PLC input

Place both suppressors close to, or directly at, the PLC-side terminals shown in the circuit. The field cable can collect capacitively coupled, inductively coupled, or common-mode disturbance along its run. A clamp at the sensor leaves the PLC end of that cable exposed to voltage developed along the remaining conductor.

Placement is part of the suppression circuit. Transient current flowing through lead and trace inductance creates additional voltage before the suppressor can hold the protected node. Keep the connection from the signal terminal through the suppressor to the selected common short and direct. Avoid looping the suppressor leads through a remote terminal strip.

The supply-conductor suppressor protects the local supply presented to the field circuit. The signal suppressor protects the PLC input node. Fit both where the drawing requires them; one does not automatically replace the other.

Some PLC input modules include internal input protection, but its rating and topology come from the module documentation. The external parts may supplement that protection by diverting current before it enters the module. Do not omit a specified external component solely because the input already operates normally.

Check the 26 VDC designation

Do not interpret 26 VDC from the drawing as a complete selection specification. Determine whether it denotes a required continuous working voltage, a nominal varistor value, a breakdown threshold, or only a circuit annotation. Those quantities are not interchangeable.

Use two boundaries when checking the datasheet:

• The suppressor must remain nonconductive, apart from its specified leakage, at the highest normal circuit voltage.
• Its clamping voltage at the applicable pulse current must stay below the PLC input module's permitted transient voltage.

Measure the highest steady supply voltage at the panel, including normal power-supply tolerance and operating conditions. Then read the PLC limit from the module manual and compare it with the suppressor's clamping curve, not only its nameplate or nominal voltage.

If a unidirectional TVS is installed forward-biased across 24 VDC, it behaves like a conducting diode rather than a high-voltage clamp. Power the modified circuit through its normal protective device and check for unexpected current draw before declaring the installation complete.

Check the pull-up and input loading

The 1.5 kΩ resistor is likely a pull-up. At an assumed 24 VDC across it, the ideal current is:

I = V/R = 24 V / 1500 Ω = 0.016 A = 16 mA

The corresponding ideal resistor dissipation is:

P = V²/R = (24 V)² / 1500 Ω = 0.384 W

Those results apply only when the full 24 VDC is across the resistor. Use the measured voltage and actual sensor states to calculate the installation value. Check the resistor's power rating and allowable temperature rise from its markings or datasheet; the evidence gives the resistance but not its wattage.

Also account for the PLC input current and suppressor leakage. If the sensor is open-collector or open-drain, it must sink the pull-up current plus the module's input current when active. In the inactive state, excessive suppressor leakage can lower the signal or approach the input threshold. Read the sensor output rating and PLC on/off thresholds from their respective documentation.

Install the resolving branch

Use this procedure after the component identity, ratings, reference node, and normal signal levels pass their checks:

1. Remove power and apply the site's electrical isolation procedure.
2. Mount the 1.5 kΩ resistor between the two nodes specified by the drawing.
3. Mount one verified suppressor from the 24 VDC conductor to the drawing's verified common node.
4. Mount the second verified suppressor from the proximity-output/PLC-input node to that common.
5. Observe polarity for any unidirectional TVS. Do not assign polarity to an MOV.
6. Keep suppressor conductors short and route their diverted-current path away from the protected signal conductor.
7. Inspect for loose strands, accidental common-to-earth bonds, and clearance problems around the added parts.
8. Restore power and measure the supply and input voltage in both sensor states before running the equipment.

Mounting the parts at the sensor because it is mechanically convenient wastes time when the disturbance is induced along the cable between that point and the PLC. Replacing an unidentified red disc with a guessed TVS wastes time as well. Identify and rate the part before changing technology.

Verify the correction at the terminals

Test at the protected node, not only through PLC logic. Connect a suitably rated measuring instrument between the PLC input terminal and the same common used by the clamp. Use a probe arrangement that does not add a large loop or exceed the instrument's input rating.

1. Record the steady input voltage with the proximity switch active and inactive.
2. Operate the equipment under the condition that previously produced the false transition.
3. Capture the minimum and maximum input voltage at the PLC terminal.
4. Confirm that the PLC indication follows only the intended sensor state.
5. Check that neither suppressor conducts continuously or heats during normal operation.
6. Repeat the test at the supply conductor if the input disturbance remains.

If the signal-terminal disturbance is controlled but the supply still moves, inspect the power-distribution and common-return path. If neither waveform changes, investigate cable routing, shielding, input filtering, sensor compatibility, and the PLC program instead of fitting additional clamps blindly.

Frequently Asked Questions