Thermistors and varistors sit inches apart on schematic diagrams, yet they guard circuits in opposite ways. One watches temperature; the other watches voltage. Choosing the wrong part can quietly erode reliability or explode on the first surge.
Engineers who treat them as interchangeable “black blobs” often discover smoky evidence of the difference. This guide dissects their physics, specs, and real-world traps so you can place the right bead every time.
Core Physics: How Each Component Reacts to Its Trigger
Thermistors change resistance as heat alters the mobility of charge carriers inside a metal-oxide lattice. NTC beads lower their resistance exponentially because thermal energy spawns more available electrons. PTC chips do the reverse: crossing a Curie temperature collapses the conductive path, driving resistance up by orders of magnitude within milliseconds.
Varistors ignore heat unless it arrives as a side effect of excessive voltage. Their zinc-oxide grains form back-to-back p-n junctions that remain high-impedance below a clamping threshold. Surge energy forces the junctions into avalanche, creating a low-impedance shunt that diverts current away from fragile silicon downstream.
Put differently, a thermistor behaves like a thermometer that also limits current, while a varistor behaves like a pressure-relief valve that only opens when voltage spikes.
Resistance Curves: Reading the Shapes That Matter
Pull the datasheets and the first graphic you see is R-T or V-I. NTC curves drop 10 kΩ to 100 Ω between 25 °C and 100 °C, a 100:1 span that lets inrush limiters tame 100 W transformers with parts no larger than a pea. PTC curves resemble a hockey stick: flat until 120 °C, then a near-vertical rise to 10 kΩ within 5 °C, a self-reinforcing trip that turns motor windings off faster than a fuse.
Varistor curves show current on the Y-axis and voltage on the X-axis. Below the varistor voltage (V1mA), leakage stays under 1 mA; above it, current rockets from 1 A to 1000 A while the device adds only 20–40 V. That clamping ratio—40 V at 1000 A versus 3 V at 1 mA—is the figure that decides whether your 400 V MOSFET survives a 2 kV telecom surge.
Overlay the two plots and you see why a varistor cannot do thermistor work: its resistance is flat until catastrophe, then collapses, whereas a thermistor modulates continuously across everyday temperatures.
Accuracy, Tolerance, and Calibration Expectations
Thermistors come in ±1 % to ±10 % tolerance bands, but that number only applies at 25 °C. Drift terms—β value tolerance, self-heating, and wire resistance—can shift a temperature reading 3 °C in a 12-bit ADC circuit unless you calibrate with a two-point fit. Medical probes routinely use a 0.1 °C window, achievable only with laser-trimmed beads and a lookup table of 64 points.
Varistors publish ±10 % on V1mA, but that is a room-temperature spec. Clamping voltage climbs 0.1 % per °C, so a 470 V part can clamp at 510 V inside a 70 °C enclosure. More importantly, each 1 J surge permanently nudges V1mA downward by 0.5 %; after 100 surges you may be 25 V closer to the rail of your 650 V MOSFET, silently eroding margin.
Designers who ignore these drifts often blame “mysterious” field failures that were actually predictable physics.
Calibration Workflow for Thermistors
Freeze and boil: immerse the probe in an ice bath at 0.0 °C and a stirred oil bath at 100.0 °C. Record the ADC counts, solve the Steinhart-Hart equation for the three coefficients, and store them in EEPROM. One-time calibration holds ±0.1 °C for five years if you derate self-heating current to 0.1 mA.
Varistor Degradation Check
Apply a 100 mA DC pulse, measure V1mA, then compare to the virgin part. If the shift exceeds 5 %, schedule replacement at the next service interval. This 30-second test catches cumulative damage that a hipot tester misses.
Energy Handling: Surge Ratings That Look Alike but Aren’t
Thermistors absorb energy only briefly during turn-on. A 10 Ω NTC wired in front of a 100 µF, 400 V bus sees 8 J at cold start; the disk heats to 150 °C in 100 ms, then presents 1 Ω steady-state. Do the math: 8 J in a 20 mm disk raises temperature 80 °C, well within ceramic limits, but repeat that start every 30 s and the average power becomes 0.27 W—enough to cook the epoxy coat and crack the disk.
Varistors swallow surges that would vaporize a thermistor. A 20 mm 471 K part is rated 6 kV, 3 kA, 70 J for a 8/20 µs pulse. That energy compresses into a volume smaller than a sugar cube, producing a 400 °C local hotspot. If the pulse arrives before the previous heat has dissipated, cumulative temperature rises 50 °C per shot; after three rapid pulses the varistor can enter thermal runaway and ignite the coating.
Always pair varistors with a 30 A series fuse or a thermal cut-off so that catastrophic failure becomes a controlled shutdown instead of a firework.
Response Time: Nanoseconds vs Milliseconds
Varistors begin clamping in under 25 ns; the limiting factor is lead inductance, not the zinc-oxide grains. Mount a 5 mm surface-mount varistor with 2 mm copper planes and you can clamp a 1 kV/µs ESD strike before it reaches the gate of a GaN FET 30 mm away.
Thermistors are thermal creatures; their resistance cannot follow sub-cycle events. A 5 mm bead needs 200 ms to change 90 % of its resistance after a 50 °C step. Therefore, never try to use an NTC as a real-time over-current sensor—it will report yesterday’s news.
Combine both: let the varistor stop the 500 V spike in nanoseconds, then allow the NTC to throttle the follow-on current milliseconds later.
Self-Heating: Friend or Foe?
Self-heating is a nuisance for temperature sensing but the operating principle for inrush limiting. Run 10 mA through a 10 kΩ bead and you dissipate 1 W; the bead stabilizes at 90 °C, shifting its own set-point by 15 °C. Smart firmware compensates by pulsing the bias at 1 % duty, reducing average heat to 10 mW while keeping resolution.
Varistors only self-heat during surges, yet that heat lingers. A 40 J strike raises core temperature 120 °C; the epoxy surface stays above 100 °C for 30 s. Packing three varistors in 5 mm columns without airflow can cascade failure: the middle part starts 30 °C hotter and clamps lower, so it hogs the next surge and dies first.
Rule of thumb: leave 10 mm air gap or use thermal pads to share the heat with the PCB copper.
Environmental Limits: Humidity, Salt, and Altitude
Thermistors use glass-coated beads that laugh at 85 % RH, but the silver-palladium terminations corrode in salt fog within 500 h. Specify immersion-tinned or gold-plated leads for marine EV chargers. At 3000 m altitude, convection cooling drops 30 %, so derate steady-state current 20 % or choose a larger bead.
Varistors rely on epoxy faces that breathe. Moisture ingress lowers V1mA 3 % after 1000 h at 85 °C/85 % RH, but the bigger threat is silver migration inside the termination layer. Use conformal coating or switch to phenolic-encapsulated parts for outdoor LED drivers in Phuket or Miami.
Both components survive –55 °C, yet NTC resistance climbs 5×, so reset your PFC enable thresholds for cold-start in Siberian telecom huts.
Failure Modes: Open, Short, or Sneaky Drift
Thermistors fail open after thermal fracture; the circuit simply stops, making the failure obvious. PTC versions fail short if the carbon chain burns, but then trip to high resistance, so a motor stalls instead of catching fire. Either way, the symptom appears immediately.
Varistors fail short when zinc-oxide grains melt into a conductive bridge. If the upstream fuse is a slow 10 A type, the varistor can draw 100 A for seconds, carbonizing the PCB before anyone notices. Always specify a thermal fuse glued to the varistor body; it opens at 115 °C and prevents the charcoal phase.
Sneakier is the 5 % V1mA drift that accumulates for years. Your 460 V clamp becomes 420 V, and one lightning strike far away punches through the MOSFET. Schedule a bi-annual clamp voltage check on any surge protector that guards irreplaceable equipment.
Cost Matrix: Pennies at Scale, Dollars at Risk
High-volume NTC discs cost $0.02 for 5 mm, $0.08 for 20 mm. Precision medical probes jump to $1.20 because of laser trim and 4-wire leads, but the sensor is still cheaper than the stainless sleeve that shields it.
Varistors start at $0.04 for 7 mm 471 K, but UL-approved 20 mm parts with thermal fuse hit $0.45. Add $0.20 for coated versions that meet IEC 62368 outdoor pollution degree 3. Compare that to a $3.50 gas discharge tube or a $2 TVS diode array, and the varistor remains the cheapest path to 6 kV combo-wave protection.
Factor in field replacement cost: a $0.50 varistor can save a $1200 motor drive, yielding a 2400:1 ROI. Skimping on rating to save three cents is a false economy that accountants eventually notice.
PCB Layout Tricks That Pass EMC on the First Spin
Place the NTC in series with the line, but keep it 10 mm away from electrolytic caps; its 100 °C steady-state temperature halves cap lifetime. Use kelvin pads for sense thermistors: run two 0.2 mm traces to the ADC and a separate 1 mm trace for the heater current so that IR drops don’t masquerade as temperature errors.
Varistors demand short, fat loops. Route 2 mm copper from line to varistor to fuse to neutral, then sprinkle 0603 100 nF ceramics across the same nodes to shave off the 100 MHz ring that follows a 1 kV surge. Position the varistor on the edge of the board so that if it bursts, soot ejects away from low-voltage control circuits.
Never place a varistor directly under an NTC; the hot thermistor preheats the varistor, lowering its clamp voltage and inviting a feedback loop of thermal stress.
Practical Design Recipes
Motor Inrush Limiter
Select a 5 Ω NTC that drops to 0.5 Ω hot, size for 8 A steady-state, and verify that the resulting 4 W self-heating stays below 100 °C inside the enclosure. Wire a relay that shorts the NTC after 500 ms so that steady losses drop to zero and efficiency gains 2 %.
SMPS Primary Clip
Use a 471 K varistor (470 V) after the fuse but before the bridge, add a 2 Ω cement resistor in series to limit follow-through current, and bolt a 115 °C thermal cut-off to the varistor disk. This trio passes 2 kV common-mode surges per IEC 61000-4-5 while protecting the 650 V CoolMOS with 90 V margin.
Battery Pack Temperature Sense
Pick a 10 kΩ NTC with β = 3435, mount it under the nickel strip of the hottest cell, and run two 30 AWG PTFE wires to the BMU. Calibrate at 15 °C and 45 °C with a climate chamber; store the coefficients in flash. Set overtemperature cutoff at 55 °C, hysteresis 5 °C, to prevent chatter.
Regulatory and Standards Maze
Thermistors for mains limiting must be UL 1434 recognized, IEC 61051 compliant, and pass 100 surges at max temperature. If you bury an NTC inside a sealed plastic box, add a pressure vent; exploding epoxy during a fault can crack the enclosure and void UL.
Varistors fall under IEC 61051-2, but also trigger IEC 62368 if they protect IT equipment. A single 20 mm disc may need 6 kV/3 kA combo-wave, 1000 surges, followed by 1000 h at 85 °C/85 % RH, then still clamp within ±10 %. Choose parts with UL 1449 Type 1 listing to avoid extra agency fees during NRTL submission.
For medical body probes, ISO 10993 biocompatibility overrides everything; even the epoxy must be USP Class VI, adding $0.30 to BOM but preventing a 510(k) rejection.
Future-Proofing: SiC, GaN, and 800 V Buses
Wide-bandgap switches push bus voltages to 800 V and switching edges to 100 V/ns. NTC inrush limiters rated 250 VAC now see 900 VDC, so select 1000 V recognized parts or stack two 5 mm discs in series to share voltage. The faster edge also means varistor lead inductance matters more; choose 5 mm SMD cases with copper-plane footprints that drop inductance below 1 nH.
At 150 kHz switching, self-heating in the NTC increases 20 % because harmonic content grows. Run thermal simulations with worst-case waveforms, not just sine waves, or the bead will creep past 120 °C and trigger PTC action in the middle of a desert afternoon.
GaN FETs fail at 650 V absolute max, so downgrade the varistor from 471 K to 431 K (430 V) to preserve 220 V surge margin after 1000-hour aging. The tighter clamp pays an extra $0.03 but saves a $5 transistor, a trade-off that scales across million-unit server PSU ramps.