NTC vs PTC vs LDR: what is the difference?

NTC and PTC thermistors respond to temperature; an LDR responds to light. An NTC’s resistance falls as its temperature rises. A PTC’s resistance rises within its positive-temperature-coefficient operating region. With common LDRs, increasing illumination lowers resistance. All three use a resistance change, but they sense different conditions and are not interchangeable.

For the individual operating principles, see what an NTC temperature sensor does and how PTC thermistors work. This comparison focuses on choosing the right device, understanding its circuit role and interpreting measurements.

Quick comparison

Read the arrows carefully: They describe resistance, not necessarily the voltage seen by a controller. The direction of the voltage change also depends on circuit arrangement.

NTC: temperature sensing and inrush limiting are different jobs

A sensing NTC allows a circuit to estimate temperature from resistance. A nominal value such as “10 kΩ” commonly refers to resistance at 25 °C, as specified in the datasheet. It does not mean the device stays at 10 kΩ as it heats or cools.

A power NTC can limit the initial current surge into equipment. Its relatively high starting resistance restricts current; self-heating then lowers that resistance. This is a power-handling application rather than precision temperature measurement. A small sensing NTC is not a substitute for an inrush limiter with the same nominal resistance.

Restarting equipment while the limiter is still hot can reduce its effectiveness because its resistance has not recovered. Selection therefore involves continuous current, surge energy and restart conditions, not just cold resistance.

PTC: identify the type before deciding what it does

PTC describes a temperature characteristic, not one universal protection device. Some types show a steep resistance rise around a switching temperature; others provide a more gradual sensing characteristic. A suitable PTC embedded in a motor winding can signal overheating to a protection relay or compatible input. The sensor does not itself operate as a power switch for the motor.

A polymeric PTC, or PPTC, is another device family used for overcurrent protection. Heating drives it into a high-resistance state that limits current. It does not necessarily create the open circuit associated with a conventional fuse, and some current can remain. Recovery depends on removing the fault and allowing suitable cooling and circuit conditions. Hold current, trip current, voltage rating and ambient temperature all affect selection.

A motor-protection thermistor, a resettable PPTC device and a PTC heater consequently need different specifications. The distinction between sensing and switching is also explained in temperature sensors versus thermostats.

LDR: resistance follows illumination

An LDR, or light-dependent resistor, normally has higher resistance in darkness and lower resistance under illumination. A dusk-control circuit uses that change to detect an ambient-light threshold. The LDR is the sensing element; a separate circuit must evaluate the signal and drive the load.

An LDR resistance specification needs its illumination conditions. Light level, spectrum and response time affect the reading. A flashlight test can show a response but does not establish an accurate lux calibration. An LDR also differs from a photodiode. Likewise, an industrial photoelectric sensor should not be treated as merely a two-terminal LDR.

Why can falling resistance produce either a rising or falling voltage?

A common measurement circuit combines the sensor and a fixed resistor as a voltage divider. Suppose the fixed resistor connects 3.3 V to the measurement node, while the sensor connects that node to 0 V. Assuming negligible loading by the measurement input:

Vout = 3.3 × Rsensor / (Rfixed + Rsensor)
With a 10 kΩ fixed resistor and a 10 kΩ sensor: 1.65 V.
When sensor resistance falls to 5 kΩ: 1.10 V.

In this arrangement, warming an NTC or illuminating an LDR lowers the measured voltage. Swapping the sensor and fixed resistor reverses the direction. A rising voltage alone therefore does not identify a PTC. The 5 kΩ figure is an illustrative resistance, not a specified temperature or light level. See electrical resistance for the underlying concept.

How to check the response with a multimeter

Disconnect power and check for stored voltage first. Never use resistance mode on an energized circuit. Parallel paths on the board can affect results; isolating one lead may be necessary and requires suitable rework skills. Follow the principles in measuring resistance with a multimeter.

For an NTC, a controlled temperature increase within its rating should reduce resistance. For a PTC, look for an increase in its specified operating region; a switching type may show little change during a small room-temperature test. For an LDR, compare shaded and illuminated readings. Do not use a flame or direct soldering-iron contact to heat an unidentified sensor.

A response in the expected direction is only an initial check. Correct nominal resistance, tolerance, curve and stability still matter. An unstable reading can also originate at a damaged connection. If results jump unexpectedly, inspect for cold or cracked solder joints before condemning the sensor.

Why matching the nominal value is not enough

Two 10 kΩ NTCs can produce different temperature readings. They may agree at 25 °C but have different B values or resistance-temperature tables. A controller calibrated for one curve can report the wrong temperature with another. Resistance tolerance, packaging, response time and mounting also affect suitability.

Self-heating introduces another error. Measurement current dissipates power in the sensor and can raise its temperature above the surface or medium being measured. Consider resistive power dissipation together with the sensor’s measurement-power limits and thermal conditions.

Three common questions

Do NTCs, PTCs and LDRs have polarity?
Bare two-terminal thermistors and ordinary LDRs are non-polarized. Powered sensor modules containing them may have a defined supply polarity and pin assignment.

A Pt100 increases resistance with temperature. Can it replace a PTC?
Not directly. A Pt100 is a platinum resistance temperature detector with a defined characteristic. Sharing a positive temperature coefficient does not make it compatible with an arbitrary PTC input.

Can a thermistor connect straight to a PLC analog input?
Only if the input supports that resistance sensor and its characteristic. A standard voltage or current input may require an appropriate measurement circuit or signal conditioner. Check the input specification and wiring diagram.