Interfacing 24VAC Proximity Sensors to Omron CP1L DC Inputs

Overview: The 24 VAC vs 24 VDC Input Mismatch

The Omron CP1L series (CP1L-L, CP1L-M, CP1L-EM, CP1L-EL, and the CP1L-J variants) is a compact modular PLC whose CPU units and all expansion I/O units use 24 VDC sourcing inputs only. There is no AC input module in the CP1L catalog. When a panel migrates from a controller that previously accepted 24 VAC two-wire inductive proximity sensors (for example a Logo! 230RC or similar), the existing sensors cannot be wired directly to the CP1L terminal block. The mismatch is electrical, not logical: a CP1L input provides a wetting voltage of approximately 24 VDC and detects threshold currents in the 5–8 mA range, while a 24 VAC two-wire proximity switch is a series-load device designed to switch an AC load with an off-state leakage of typically 2–5 mA.

This reference documents the four practical conversion paths, quantifies leakage and minimum-operating-current constraints for each, and gives a commissioning procedure suitable for a 12-sensor retrofit. The recommendation for small sensor counts (≤ 24 points) is replacement with DC three-wire sensors. For larger counts or where the AC cable plant must be preserved, electromechanical relay interposing is the proven method. SSR interposing works only when the AC SSR's minimum operating current is below the proximity sensor's off-state leakage. Remote I/O with an AC input slice is viable for ≥ 32 points where the comms and power supply cost is amortized.

CP1L Input Electrical Specifications

The official CP1L CPU Unit Operation Manual (W471) and the CP1L Specifications page define the input characteristics used in every calculation that follows.

The 5 mA ON current and 1 mA OFF current are the two thresholds that determine whether any interposing device will register reliably. Any leakage path that injects more than 1 mA into a CP1L input when the field device is OFF will produce a false-ON.

CP1L Expansion I/O Catalog: AC Input Availability

The CP1L expansion bus supports only the modules listed in the operation manual. No 24 VAC or 100/240 VAC input module exists for the CP1L.

Conclusion: any AC sensor retrofit on a CP1L must use either (a) replacement sensors, (b) interposing devices that convert the AC presence into a 24 VDC contact closure, or (c) remote I/O distributed over CompoNet, EtherCAT, or serial with an AC input slice.

Method 1 — Direct Replacement with 24 VDC 3-Wire Sensors (Recommended)

For a 12-sensor migration, the lowest total cost and the highest long-term reliability come from replacing each 24 VAC two-wire proximity switch with a 24 VDC three-wire PNP (sourcing) proximity switch. The CP1L's onboard 24 VDC sensor power supply (typically 300 mA on CP1L-M, 600 mA on CP1L-EM) can power the sensors directly.

Selection criteria for the replacement sensor:

• Sensing range ≥ existing sensing range (commonly 4 mm for M12 flush, 8 mm for M18 flush)
• Operating voltage 10–30 VDC
• Output type: PNP NO (matches CP1L sourcing input)
• Output current rating ≥ 100 mA
• Off-state leakage ≤ 0.1 mA (well below CP1L 1 mA OFF threshold)
• Voltage drop at rated load ≤ 2 VDC

Common drop-in equivalents to the Omron E2E-X series AC variants are the E2E-X5MY1 (M12, 4 mm, PNP NO) or E2E-X10MY1 (M18, 10 mm, PNP NO). Verify the sensing distance, housing thread, and connector pinout against the existing installation.

Method 2 — Electromechanical Relay Interposing

When the existing AC two-wire sensors must be retained, a 24 VAC coil relay converts each sensor's switching state into a volt-free contact that drives a CP1L DC input. The relay coil is wired in series with the AC proximity sensor; the relay contact is wired to the CP1L input and the 24 VDC wetting supply.

Coil current is the deciding parameter. A typical 24 VAC relay with a 0.5 W coil draws about 21 mA at 24 VAC. The AC proximity sensor must source this current plus its own leakage budget. If the sensor's rated load-current minimum is ≤ 5 mA (typical for compact two-wire AC prox), the relay will not pick up reliably. Use a low-power 24 VAC relay coil with rated current ≤ 5 mA, or use an interposing 24 VAC pilot relay driven by the sensor that switches a separate 24 VDC load.

Two practical relay choices for low coil current:

Method 3 — Solid-State Relay Interposing

SSR interposing replaces the electromechanical relay with an AC-input, DC-output SSR. The AC sensor switches the SSR's input; the SSR's DC transistor output drives the CP1L input. This method has no moving parts but introduces two constraints: SSR minimum operating current and SSR off-state leakage.

Verify both constraints against the AC sensor's specifications.

Minimum Operating Current

An AC-input SSR (for example the Omron G3NA-205B or G3F-203SN) requires a continuous input current above its minimum-operating-current spec, typically 5–10 mA for a 10 A output SSR or 1–2 mA for a low-current SSR. The AC proximity sensor must be able to source this current when its target is present. The Carlo Gavazzi EI1204-TBOSL cited in the original application has an off-state current < 2 mA and a minimum load current of about 5 mA. This means:

• ON-state (target present): the sensor sinks ≥ 5 mA, sufficient to drive a low-current AC SSR input.
• OFF-state (target absent): the sensor leaks ≤ 2 mA, which is below most AC SSRs' 5 mA minimum operating current, so the SSR reliably drops out.

The interposing chain is feasible for this specific sensor. Always validate against the actual sensor datasheet.

Off-State Leakage into CP1L

The SSR's output stage leakage flows into the CP1L input. For an Omron DC-output SSR rated at 10 μA max leakage at 24 VDC, this is well below the CP1L's 1 mA OFF threshold. A margin of two decades separates the worst-case leakage from the OFF threshold.

Leakage margin calculation:

Method 4 — Remote I/O with AC Input Slice

For installations with 32 or more AC sensors distributed across a machine, deploying a remote I/O network preserves the AC cable plant. The CP1L-EM (with built-in EtherCAT) or a CP1L-M with a CP1W-CRT21 CompoNet master can host distributed slices. Omron's GRT1 series CompoNet slices include AC input variants (for example GRT1-ID8-1 for 100–240 VAC, 8-point). Each slice requires 24 VDC field power for the bus interface plus the AC mains for the inputs.

Cost amortizes poorly below 32 points because the slice, the master, and the configuration work exceed the cost of either sensor replacement or relay interposing. Reserve this method for greenfield panels with ≥ 64 AC field devices or where safety-related diagnostics of the AC sensor circuit (open-load detection) justify the slice's price.

Method Comparison

Wiring Topology (Method 2 Example)

Method 3 Leakage Calculation

For an SSR-based interface using the Carlo Gavazzi EI1204-TBOSL and an Omron DC-output SSR (G3F-203SN or equivalent), the worst-case current flowing into the CP1L input in the OFF state is:

This is the current that would flow through the sensor's OFF-state leakage into the SSR input (which does not pass through to the output) — only the SSR's own 10 μA output leakage reaches the CP1L. The CP1L OFF threshold is 1.0 mA, so a margin of 100× remains. The interface is robust.

However, a critical SSR constraint is the minimum operating current at the AC input side. If the chosen SSR has I_min_op ≥ 3 mA and the sensor only guarantees I_load_min = 5 mA at 24 VAC, the sensor has no problem sourcing the SSR's minimum. If the SSR requires ≥ 8 mA, the sensor cannot reliably drive it and Method 2 (EMR) or Method 1 (DC sensor) becomes mandatory.

Commissioning Procedure

1. De-energize the panel and lock out the 24 VAC supply to the sensors and the 24 VDC supply to the CP1L.
2. Verify the AC sensor's off-state leakage with a clamp meter on the AC line: read < 5 mA (typical 1–2 mA).
3. Install the interposing device (EMR, SSR, or replacement DC sensor) per the topology diagram.
4. Restore 24 VDC power to the CP1L only. Using CX-Programmer, force input 0.00 OFF; read the input status in the I/O memory monitor. Expected value: 0.
5. Apply 24 VAC to the sensor circuit. With target removed from sensor: input 0.00 must remain 0 (no false ON).
6. Present target to sensor. Input 0.00 must read 1 within 50 ms (well under the default 0.5 ms ON response plus 10–15 ms for EMR pull-in).
7. Repeat for every sensor point. Log the response time for each in the commissioning sheet.
8. Run a 24-hour soak test on all 12 inputs before placing the machine in production.

Verification Checklist

Troubleshooting Matrix

Replacement Sensor Cross-Reference

FAQ