No-Load Current vs Load Current in Sensors

What is the difference between no-load current and load current?

Consider an illustrative sensor specified for a maximum 20 mA without load and a 100 mA switching output. Connecting a PLC input that draws 3.7 mA does not make that output draw 100 mA. If the 20 mA figure bounds internal consumption in the required operating state, the simplified shared-supply budget is 20 + 3.7 = 23.7 mA, before other equipment and allowances.

Read the qualifier next to “current consumption.” A value explicitly stated as “without load” can be combined with external load demand. A value already including loads must not have those same loads added again. “Typical” consumption is also different from a guaranteed maximum.

The circuit explanations below cover ordinary DC sensor interfaces. Personnel-protection systems require their specified safety devices, controller compatibility and validation.

Where does current flow in PNP and NPN sensors?

A conventional three-wire sensor has L+, 0 V and a switching output. Its electronics are powered between L+ and 0 V. The output controls another path through the load. The drawings show conventional current with the output ON; they are conceptual circuits, not terminal assignments.

PNP output: current goes from the sensor to the load

The load connects between the output and 0 V. When ON, the PNP stage supplies current from L+ into the load. The sensor's L+ lead therefore carries both its internal current and the current it sources to that load.

Swipe the circuit diagrams sideways to see both current branches.

NPN output: current returns through the sensor to 0 V

The load connects between L+ and the output. The NPN stage completes its return path to 0 V. In this arrangement the sensor's L+ lead feeds its electronics, while its 0 V lead returns both internal and load current. Measuring one lead cannot be interpreted without the circuit.

For the input-group common, use the exact sensor and PLC wiring diagrams. The NPN vs PNP guide and PLC sinking/sourcing explanation cover those connections. NO/NC describes switching logic; it does not set the permitted current.

Which datasheet ratings must be checked together?

Find the complete part number, supply range and output topology first. Then read current consumption, per-output and total-output limits, voltage drop, leakage, load type and temperature conditions as a set.

Published examples: similar labels can have different conditions

Sources: SICK data sheet, Electronics section; Balluff guide, English ordering table and technical data; OMRON E3S-GS3 specifications. These are reading examples, not interchangeable product ratings.

Resolve the condition before using the number

• Maximum or typical: use the documented upper consumption for the relevant operating state when budgeting capacity.
• Per device or per channel: a two-output product may have both individual and combined limits. A sensor pair may list combined or separate consumption.
• Continuous or transient: a surge specification includes a time condition; it does not replace a continuous rating.
• Mode and temperature: communication, indicators, auxiliary outputs and cabinet temperature can change the applicable data.

Do not connect active outputs together to increase current capacity. Any permitted signal-combining arrangement needs the manufacturer's wiring instructions; it is not a general current-sharing method.

How do you size a 24 VDC supply without double-counting loads?

Draw a boundary around one power supply. Count the equipment and external loads actually fed by it, using their maximum simultaneous demand. Include the PLC, interfaces and other panel devices only once.

Base current = Σ sensor internal current + Σ simultaneous load current + other same-supply demand

This is a circuit budget, not simply a sum of output ratings. If the PLC module specification already includes its field-input demand, do not add that demand a second time. When separate supplies are used, assign each load to its source and check the required references, isolation and startup sequence.

Six sensors and six PLC inputs: a worked example

Assume six identical sensors, each budgeted at 20 mA unloaded, and six inputs drawing 3.7 mA each when ON. All six inputs can be ON together and use the same 24 V supply. These are illustrative inputs, not a universal PLC specification.

• Sensor electronics: 6 × 20 mA = 120 mA.
• Simultaneous input loads: 6 × 3.7 mA = 22.2 mA.
• Group subtotal: 120 + 22.2 = 142.2 mA.

Adding a chosen 25% planning allowance gives 177.8 mA, rounded to one decimal place. This still excludes any unentered PLC electronics, interfaces and other equipment. The percentage is an example assumption, not a required standard.

If the power supply is labeled in A or W: divide mA by 1,000. The 142.2 mA subtotal is 0.1422 A, equivalent to about 3.41 W at 24 VDC using P = V × I. With the chosen allowance, the group budget is about 0.178 A or 4.27 W. Compare with the supply's usable DC output rating, not its AC input current; this calculation does not include conversion losses or unlisted equipment.

For a mixed sensor group, calculate a subtotal for each type and add the results. Verify the field voltage during the worst operating and startup states; a sufficient current rating cannot compensate for excessive cable or distribution voltage drop.

Can the output directly drive the PLC input or relay?

Compare the actual load against the output's current, voltage and load-type conditions. Also confirm that the receiving circuit resets when the output turns OFF.

ON-state voltage can reject a load that passes the current check

For this calculation, start with the minimum voltage at the source point and deduct the losses from that point to the load. If starting from a measured field-terminal voltage, avoid subtracting upstream loss again. For a resistive load, I = V/R may be useful; electronic inputs and coils need their own operating data.

Relay coils and electronic loads need different startup checks

A plain DC coil's inductance limits its initial current rise; do not automatically assign it the large turn-on surge of an AC coil or capacitor-input module. Still check coil resistance tolerance, supply extremes, pickup voltage, temperature and turn-off energy. A coil assembly with electronics may have a different current profile.

Do not read a relay's contact rating as its coil current. The relay interfaces below carry markings for different circuits. If the sensor drives the coil, determine its load from the coil or interface-input data, not the contact's switching-current rating.

Suppression is part of the application: a diode across a DC coil can protect a switching path but can also delay release. Follow the sensor and load documentation for the permitted suppression and timing. OMRON's relay engineering explanation and relay precautions describe coil behavior and suppression effects.

OFF-state leakage must remain below the receiver's reset limits

The output LED going out does not prove that the external circuit sees an OFF state. Leakage, diagnostic pulses or coupled signals may leave a sensitive input active. Compare the actual OFF current and voltage with the receiver's guaranteed OFF limits. See the sensor leakage-current guide.

Short-circuit protection is not extra operating capacity. It describes a fault response under specified conditions. If repeated switching enters protection, establish and correct the cause rather than accepting the retry cycle as normal operation.

What changes for two-wire, analog and IO-Link sensors?

The simple addition of separate internal and output branches must match the actual architecture. These devices require a different interpretation of the same current labels.

OMRON's proximity-sensor terminology distinguishes two-wire leakage from ON-state voltage loss. Use the actual model limits, not the numerical values from a generic example. For a wiring comparison, see two-wire vs three-wire sensors.

How should you measure and troubleshoot sensor current?

Choose the operating state and branch before connecting the meter. A no-load reading, a loaded supply reading and an output-current reading answer different questions.

Use a controlled current measurement

1. Prepare the circuit and instrument.

   Prevent machine movement and switch off power before changing test connections. Follow the equipment procedure; use a correctly fused meter, input jack and range for the circuit.

2. Insert the ammeter in series.

   Open the branch to be measured and connect the meter into that path. Never place a meter in current mode directly across L+ and 0 V.

3. Reproduce the stated no-load condition.

   For a compatible three-wire device, disconnect and insulate the external output load, then record supply current at the specified voltage and mode. Do not apply this procedure to a two-wire sensor.

4. Measure the real load in its ON state.

   Restore connections with power off, then measure output-branch current. Check voltage across the load and at the sensor terminals using the meter's voltage mode and correct jack.

5. Check OFF state and peaks separately.

   Record OFF levels against receiver reset limits. Use suitable bandwidth and resolution for short pulses or startup events; an ordinary averaged reading may miss them.

Measurement principle: Tektronix/Keithley, How to Measure Current. Consult the instrument manual for its current-input protection, limits and test procedure.

Interpret the reading at the point where it was taken

The meter can influence the circuit: its current range introduces a voltage burden. If a marginal input stops operating only during measurement, check that burden and the changed connections. A clamp used for milliamps must have suitable DC sensitivity; follow its DC zeroing and conductor-placement procedure, and measure the intended conductor rather than the complete outgoing-and-return cable.

• LED changes but PLC stays OFF: check common wiring, loaded input voltage, input current and filtering.
• Output heats or repeatedly resets: inspect the actual load profile, wiring, temperature and protection response.
• PLC remains ON after the sensor switches OFF: check leakage, pulses, coupling and receiver reset limits.

Keep the tested model, wiring drawing, supply condition, state and measurement point with each result. For voltage-loss analysis, use the residual-voltage guide.

What should you verify before replacing or ordering a sensor?

A replacement can have lower internal consumption yet a weaker output. Review the complete electrical match before approving the substitution, even when the housing, supply voltage and sensing distance match.

Use the load-current guide for a detailed output review and the sensor replacement guide for the broader mechanical and sensing match.