2-Wire Sensor Leakage Current Calculator
A two-wire DC proximity sensor still draws a small current when OFF. That leakage may keep a PLC input above its guaranteed OFF threshold. Enter the sensor leakage and PLC input ratings to calculate the input voltage, possible bleeder resistance and resistor power.
This check models the PLC input as a resistance. The sensor's ON-state ratings must be checked separately.
Check OFF voltage and bleeder resistance
Use maximum sensor leakage, the PLC input impedance and its guaranteed OFF threshold. Enter input impedance tolerance if specified, and the OFF-current limit if listed. The starting values are an example. Leave the bleeder blank until you select a resistor; enter its specified tolerance.
Values changed — recalculate to update the result.
Assumptions: constant maximum OFF leakage current, a resistive PLC input and the entered component tolerances. The bleeder connects across the PLC input. For nonlinear PLC inputs, use the module manufacturer's test method.
Where the bleeder resistor sits
This example uses a two-wire DC switching sensor feeding an input referenced to 0 V. Confirm the polarity and common connection on the actual PLC module. AC switches and 4–20 mA transmitters use different circuits.
Calculation method
Vin,OFF,max ≈ Ileak × Rin,maxmA × kΩ gives volts. At low impedance the PLC branch current can be larger: IPLC,OFF,max ≈ Ileak (without a bleeder, before any supply-voltage limit).
Req = Rin × Rb / (Rin + Rb)The bleeder lowers the equivalent input resistance. Use Rin,max to find maximum OFF voltage, and Rin,min to find maximum PLC OFF current. Use the highest possible bleeder resistance for both checks.
Rb,V ≤ Rin,max × Vtarget / (Ileak × Rin,max − Vtarget)Rb,I ≤ Rin,min × Itarget / (Ileak − Itarget)Set Vtarget = (1 − margin) × VOFF and Itarget = (1 − margin) × IOFF,max. Apply only bounds whose unbleeded checks fail; omit the current bound if no OFF-current limit is given. Divide the lower applicable bound by (1 + bleeder tolerance) to obtain the nominal upper limit.
Pb,actual ≤ VCC,max² / Rb,minUse the minimum possible resistor value, Rb,min = Rb,nom × (1 − t), in ohms (kΩ × 1000). The supply-voltage square gives an upper bound on DC dissipation. Omron uses a 4× factor in its rating examples. Check working voltage and temperature derating.
Worst-case OFF voltage uses high PLC input impedance and high bleeder resistance. Worst-case PLC OFF current uses low input impedance and high bleeder resistance. Power uses low bleeder resistance. Enter tolerance data from the relevant datasheets; for nonlinear PLC inputs, use specified current–voltage curves.
Check ON operation before fitting a resistor
With the bleeder fitted, the extra ON-state current is approximately Vinput,ON/Rb. Include it when checking the sensor's load-current range and resistor heating.
PLC input stays ON when the two-wire sensor is OFF
| Observed result | Check next |
|---|---|
| Input LED remains lit, no target present | Record measured input voltage and sensor OFF leakage; compare with the module's guaranteed OFF limits. |
| LED off but software bit ON | Check input filtering, software mapping, inversion, diagnostics and output program logic. |
| Input does not reach ON with a target | Check sensor residual voltage, minimum load current, module ON threshold and supply at load. |
| New resistor becomes excessively hot | Disconnect safely; recheck voltage, actual resistance, thermal conditions and worst-case dissipation. |
Measure the input voltage with a suitably rated meter, following the site's isolation procedure.
Manuals and related pages
Reference manuals: Omron CP1L CPU Unit Operation Manual, W471, section 3-4, “Precautions when Connecting a Two-wire DC Sensor”; Omron CJ2 CPU Units Hardware User’s Manual, W472, section 5-3, “Precautions when Connecting a Two-wire DC Sensor”. Use the ratings and wiring diagram for your exact module.
See also: 2-wire vs 3-wire vs 4-wire sensor wiring · PNP vs NPN outputs · Inductive sensor mounting.