P1-08TA AC Output Ghost Voltage: Bleeder Resistor Fix

Problem Overview

When an AutomationDirect Productivity1000 P1-08TA AC output module is commanded OFF, the field-side terminals can still measure between 15 V and 120 V AC across the connected load with a high-impedance digital multimeter. A low-current LED indicator wired directly to the output will frequently glow dimly, flicker, or fail to extinguish entirely. Replacing the LED with a small incandescent pilot lamp eliminates the symptom instantly, confirming that the problem is not a defective output transistor or wiring error: it is off-state leakage current interacting with a high-impedance load.

The leakage specification is documented at 4 mA maximum at 264 V AC per the official P1-08TA spec sheet and the module insert shipped with the unit (P1-08TA insert PDF). This level of leakage is normal for triac-based AC outputs and is not a warranty issue. The remedy is to either increase the load current draw or to install a parallel bleeder path that sinks the leakage before it can develop a measurable voltage across the indicator.

P1-08TA Module Specifications

The P1-08TA is an eight-point discrete AC output module that drops into any Productivity1000 base. The relevant electrical characteristics from the official datasheet and the catalog page are summarized below.

The 4 mA leakage figure is a worst-case number measured at the upper voltage limit. Real-world leakage scales roughly linearly with applied voltage, so on a 120 V system the typical leakage is closer to 1.5–2 mA per output.

Root Cause: Off-State Leakage Current and Voltage Divider Action

Triac and solid-state-relay (SSR) output stages cannot achieve infinite impedance in the OFF state. A small internal bias network, snubber, and the triac's intrinsic off-state leakage form a finite-impedance path to the AC line. The 4 mA value represents the current that flows from the AC source through this internal path, through the field wiring, and through the connected load back to the AC neutral when the output is commanded OFF.

The result is a classic voltage divider:

V_load = V_supply × Z_load / (Z_output_off + Z_load)

Where:

• V_load = voltage measured across the load (the ghost voltage)
• V_supply = nominal AC line voltage (120 V or 240 V)
• Z_output_off = the internal off-state impedance of the P1-08TA output stage
• Z_load = the impedance of the device wired to the output

Estimating the internal off-state impedance at 120 V:

Z_output_off = 120 V / 0.0018 A ≈ 66.7 kΩ

Typical indicator lamp impedances:

The 4 kΩ incandescent drops only a small fraction of the supply, so the indicator stays dark. The 24 kΩ LED and 240 kΩ neon drop most of the supply, which is why the LED appears as a dim glow and the neon as a steady, low-brightness arc. Field measurements of 15 V at the LED reflect a low-line condition or a meter-loading effect; readings near 120 V on a disconnected wire reflect the meter's own input impedance forming the upper half of the divider.

Why LED Lamps Expose the Problem More Than Incandescent

Modern LED replacement lamps include a switching power supply on the input that presents a high impedance until the line voltage exceeds the driver's bulk-capacitor threshold (typically 80–140 V peak for a 120 V design). Once the threshold is reached, the LED draws its full rated current. With only 1.5–2 mA of leakage available on a 120 V line, the input capacitor never reaches its conduction threshold, and the LED flickers or partially illuminates from the high-impedance charging current instead.

Older incandescent and neon indicators worked with AC output leakage by accident: a 30 mA incandescent had just enough cold resistance to clamp the ghost voltage below visible glow, and a neon bulb's series resistor limited current through the OFF-state leakage to a value below striking voltage. LED drivers do not include that clamping resistance, so the symptom is more visible today than it was on PLC-2, SLC 500, and PLC-5 systems from the 1980s and 1990s.

Bleeder Resistor Sizing

The simplest field-proven fix is a resistor wired in parallel with the LED indicator. The bleeder must satisfy two conditions:

1. Sink enough additional current to drop the off-state ghost voltage below the LED driver's turn-on threshold (typically 30–40 V peak for a 120 V LED).
2. Not draw excessive current when the output is commanded ON.

Target bleeder impedance for a 120 V system with ~2 mA leakage:

Z_bleeder = V_supply × 0.5 / I_leak_max = 120 × 0.5 / 0.004 = 15 kΩ

Recommended standard values:

For most LED indicator applications, a 10 kΩ to 22 kΩ, 1 W bleeder provides reliable OFF-state extinguishing with acceptable ON-state current draw. Derate the resistor power rating by 50% for continuous duty and 80% for enclosed cabinets without active ventilation.

For 240 V systems, double the resistance and the power rating:

Z_bleeder_240V = 240 × 0.5 / 0.004 = 30 kΩ → use 33 kΩ–47 kΩ, 2 W

Alternative Suppression Devices

When continuous power dissipation is a concern or the load includes SSR-driven devices, an RC snubber designed for AC output filtering is preferred. Commercial "anti-flicker" LED capacitors are widely available in 0.1 µF to 0.47 µF, 250 V AC or higher ratings, and present a low impedance at 50/60 Hz that absorbs the leakage.

Use X2 or Y2 safety-rated capacitors only. Unmarked or DC-rated capacitors can fail short and create a permanent hot-to-load short circuit.

Step-by-Step Installation Procedure

1. De-energize the field supply. Open the branch circuit breaker feeding the P1-08TA outputs and lock it out per NFPA 70E. Verify zero energy with a known-good tester on the field terminals.
2. Remove the P1-10RTB terminal block from the P1-08TA module. Document the wiring of the affected output point and any other points sharing the block.
3. Solder or crimp the bleeder resistor across the output terminal and the neutral/common terminal that the load returns to. Use heat-shrink tubing over the resistor leads and provide strain relief on the field wiring.
4. If using a snubber capacitor, observe polarity markings only on polarized DC-rated units. For X2 AC-rated capacitors, no polarity applies, but the marking band should face line side for service traceability.
5. Re-seat the terminal block onto the P1-08TA and torque the screws to the value listed in the Productivity1000 user manual (typically 0.5 N·m for the P1-10RTB cage-clamp terminals).
6. Restore field power and proceed to verification.

Verification Procedure

1. With the output commanded OFF from the ladder logic or task code, measure AC voltage across the load with a Fluke 87V or equivalent true-RMS multimeter. Expected reading: less than 10 V AC for a 10 kΩ bleeder, less than 20 V AC for a 22 kΩ bleeder.
2. Visually confirm the LED is fully extinguished. If a residual glow remains, reduce the bleeder value or parallel two resistors in parallel.
3. Command the output ON and measure the ON-state voltage drop across the P1-08TA output terminals. Expected reading: less than 1.5 V AC at 0.5 A. A reading above 2 V AC indicates a failing output stage that should be replaced.
4. Measure the additional current drawn from the output with the bleeder installed. The increase should match the calculated bleeder draw (for example, 12 mA at 120 V for a 10 kΩ resistor).
5. Cycle the output 100 times under load to confirm reliable operation. Monitor the bleeder resistor for overheating with a non-contact thermometer; surface temperature should stabilize below 70 °C above ambient.

Troubleshooting Matrix

Application Notes and Design Considerations

• The 4 mA leakage is part of the P1-08TA's published specification. Selecting a different module will not eliminate the symptom, since all triac-based AC outputs exhibit off-state leakage. Compare the P1-08TA spec sheet with the P1-15TD2 and P1-08NA datasheets if you are evaluating alternatives.
• For DC output modules, ghost voltage is not an issue. SSR-input DC devices driven from a triac AC output may still see residual voltage and could need their own snubber on the input side.
• For the Productivity1000 system as a whole, the 4 mA per output times eight outputs can deliver 32 mA of total leakage on a single module. If multiple modules share a common neutral back to the source, the cumulative return current must be within the rating of that conductor.
• For CE-marked installations, an X2 or Y2 capacitor is preferred over a bare resistor to meet conducted-emissions and immunity requirements; the resistor introduces a continuous loss that worsens the cabinet's thermal budget.