A two-wire sensor sits in series with the load and must draw a small current even when off — that residual current is what causes false-on PLC inputs. A three-wire sensor has its own supply and a clean switching output. A four-wire sensor adds a second, complementary output. For new DC designs, three-wire is almost always the right answer.
Quick reference
- 2-wire — simplest wiring. Residual (leakage) current when off, typically 0.5–1.7 mA. Voltage drop when on, typically 3–8 V.
- 3-wire — separate supply, output isolated from the load current. No meaningful leakage, minimal drop.
- 4-wire — three-wire plus a second output, usually the complement (NO and NC together).
- The classic 2-wire fault: PLC input reads permanently ON. Fix with a bleeder resistor across the input, or change to 3-wire.
- Series and parallel connection multiplies these effects — voltage drops add, leakage currents add.
Two-wire: elegant wiring, awkward electronics
A two-wire sensor is wired exactly like a mechanical limit switch: one cable, in series with the load. That is genuinely convenient, particularly when retrofitting into existing conduit where pulling a third core is expensive.
The difficulty is that the sensor’s own electronics have to be powered from the same two wires. When the output is off, no current should flow — but the oscillator still needs to run, otherwise the sensor cannot detect the next target. So a two-wire sensor always draws a small residual current, typically between 0.5 and 1.7 mA, through the load.
When the output is on, the sensor is not a perfect switch either. It drops a residual voltage across itself, commonly 3 V and up to 8 V on some units, which is subtracted from what reaches the load.
Both of these are unavoidable consequences of the topology, not manufacturing shortcomings. They only become problems when the load is a modern, high-impedance PLC input rather than a contactor coil.
The false-ON problem, and how to fix it
A 24 V DC digital input on a typical PLC has an input impedance in the region of 3–5 kΩ and guarantees an OFF state below roughly 1.5 mA. Feed it from a two-wire sensor leaking 1.5 mA and the input sits right on the boundary. It may read correctly on the bench at 20 °C and read permanently ON in a hot panel, because leakage rises with temperature.
The standard remedy is a bleeder resistor wired in parallel with the PLC input. It provides an alternative path for the leakage current so that the voltage across the input falls below its OFF threshold.
Choose R so that: Ileakage × (R ∥ Zinput) < Voff-threshold
In practice, for a 24 V DC input, a resistor in the range of a few kΩ to around 30 kΩ solves it. Start around 5 kΩ and verify with a meter across the input in the off state. Check the power rating — at 24 V a 5 kΩ resistor dissipates about 115 mW continuously when the sensor is on, so a 0.5 W part is sensible.
The resistor is a workaround, not a design. It burns power continuously, it is one more thing to fall off a terminal rail, and the next engineer will not know why it is there. If you are designing rather than repairing, use a three-wire sensor.
Three-wire: the sensible default
A three-wire sensor takes brown (+), blue (0 V) and black (output). Its electronics run from the supply, not through the load, so:
- Leakage current in the off state is negligible — microamps, not milliamps.
- Voltage drop in the on state is small, typically well under 2 V.
- The output can be optimised purely for switching, which allows better short-circuit and reverse-polarity protection.
- Switching frequency is generally higher, because the oscillator is not starved for supply.
The output polarity — PNP sourcing or NPN sinking — becomes a separate choice, covered in our guide to PNP versus NPN outputs.
Four-wire: when you need both states
A four-wire sensor adds a white core. It is normally one of two things:
- Complementary outputs — one NO and one NC of the same polarity. Useful for safety and diagnostic logic, because the control system can check that exactly one of the two is active. If both are on, or both off, something has failed.
- Antivalent or dual-polarity outputs on some ranges, offering a PNP and an NPN output from the same head. Less common, and usually more expensive than simply ordering the right sensor.
Complementary outputs are worth the extra core wherever a false reading has real consequences. They are a poor man’s line-monitoring — genuine fault detection at the sensor level requires a NAMUR sensor and isolating amplifier.
Side-by-side comparison
| 2-wire | 3-wire | 4-wire | |
|---|---|---|---|
| Cores required | 2 | 3 | 4 |
| Residual current when off | 0.5 – 1.7 mA | Negligible | Negligible |
| Voltage drop when on | 3 – 8 V | < 2 V | < 2 V |
| Works on AC and DC | Yes — AC/DC universal types available | DC only | DC only |
| Suits high-impedance PLC inputs | Often needs a bleeder resistor | Yes | Yes |
| Polarity choice | Not applicable | PNP or NPN | PNP or NPN, both states |
| Best for | Retrofits, AC loads, limit-switch replacement | Almost all new DC machine building | Safety and diagnostic logic |
Series and parallel connection
Connecting multiple sensors together compounds every effect above, and it is where two-wire installations most often come unstuck.
In series (logical AND), the voltage drops add. Four two-wire sensors each dropping 5 V consume 20 V of a 24 V supply, leaving the load 4 V — it will not operate. Three-wire sensors in series drop far less, but you still need to check the total against the load’s minimum operating voltage.
In parallel (logical OR), the leakage currents add. Four two-wire sensors each leaking 1.5 mA present 6 mA to the input, which will read permanently on regardless of any bleeder resistor you fit.
There are also limits on how many three-wire sensors can be paralleled before the combined off-state leakage matters, and rules about protecting each output. Our note on series and parallel connection of proximity switches covers the arithmetic in detail.
Choosing for a real installation
- Is there a 24 V DC rail available? If yes, use three-wire. This decides most cases immediately.
- Are you replacing a mechanical limit switch in existing two-core conduit? A two-wire AC/DC sensor is the pragmatic choice — just check the load’s minimum operating current and voltage.
- Is the load a contactor coil or a PLC input? Coils tolerate leakage happily. PLC inputs do not.
- Do you need to distinguish “no target” from “sensor failed”? Four-wire complementary outputs give you a partial answer, NAMUR gives you a full one.
- Will sensors be combined? Do the voltage-drop and leakage arithmetic before ordering, not during commissioning.
Frequently asked questions
Why does my PLC input stay on with a two-wire sensor connected?
The sensor’s residual current — typically 0.5 to 1.7 mA — is enough to hold a high-impedance digital input above its OFF threshold. Fit a bleeder resistor of a few kΩ across the input to divert that current, or replace the sensor with a three-wire type. The problem often appears only once the panel warms up, because leakage increases with temperature.
What value bleeder resistor should I use?
Size it so that leakage current times the parallel combination of the resistor and the input impedance stays below the input’s OFF-state voltage. For a 24 V DC input, values from a few kΩ up to about 30 kΩ are typical; around 5 kΩ is a reasonable starting point. Verify with a meter across the input, and check the resistor’s power rating for continuous operation.
Can I use a two-wire sensor on 230 V AC?
Yes — two-wire AC and AC/DC universal sensors exist precisely for this, and they are the standard replacement for mechanical limit switches on AC circuits. Confirm the minimum load current the sensor needs to operate correctly; a very light load such as a small indicator lamp may not draw enough.
Does a three-wire sensor have any leakage current at all?
A small amount, but it is measured in microamps rather than milliamps, because the output transistor’s leakage is not carrying the electronics’ supply current. In practice it is far below any PLC input threshold and can be ignored for normal design purposes.
Is a four-wire sensor worth the extra core?
Where a wrong reading has consequences — interlocks, position confirmation on a press, end-of-travel on something heavy — yes. Being able to check that exactly one of the complementary outputs is active detects a large class of sensor and wiring faults. For ordinary presence detection it is an unnecessary expense.
Fighting a leakage current problem on an existing line? Send us the sensor part number, the input card details and how the sensors are combined, and we will tell you whether a bleeder resistor is enough or whether the sensor needs to change. Accent Controls builds two, three and four-wire proximity switches in Mumbai.