PPTC Fuse Selection: Hold Current and Ambient Derating

Quick answer

A PPTC resettable fuse is selected by its hold current at the hottest ambient inside the enclosure, not by the headline number on page one. The 0.50 A RXEF050 holds 0.50 A at 20 °C, 0.36 A at 50 °C and only 0.20 A at 85 °C. Pick a part whose derated hold current covers the load with at least 20% margin, confirm the trip current and trip time for the fault you expect, and check that the voltage drop and post-trip resistance are acceptable for the rail.

Executive takeaways

• Hold current is a temperature-dependent number. Littelfuse's own rerating table for the RXEF050 falls from 0.78 A at −40 °C to 0.20 A at 85 °C — a 74% swing across the rated range.
• Trip current is not the same as hold current. The RXEF050 trips at 1.00 A but holds 0.50 A; between those two values it may sit for a long time without tripping, or trip slowly.
• Trip time is measured at a specified current. The RXEF050 is specified at 2.50 A with a maximum of 4.0 s; at 1.2 × I_T the trip can take much longer.
• PPTC is a low-cost, self-resetting overcurrent device, not a precision current limiter. For precise limits or fast faults, use an eFuse or a fuse; for overvoltage, use a TVS.

1. What a PPTC actually does

A polymeric positive temperature coefficient device is a conductive polymer loaded with carbon particles. At normal temperature the particles form conductive chains and the part behaves like a low-value resistor. When current heats the polymer above its switching temperature, the material expands, the chains break and resistance rises by orders of magnitude, which drops most of the supply voltage across the device and limits the current.

When the fault clears, the polymer cools, the chains re-form, and the part returns to a low-resistance state. That is the whole value proposition: no replacement, no service call. The trade-off is that the reset is thermal, so it is slow — seconds, not microseconds — and it depends on the ambient and on how well the part can shed heat.

One number is easy to miss: after a trip, resistance does not return to its original value immediately. The datasheet lists a post-trip resistance, measured one hour after the event; for the RXEF050 that is 1.17 Ω maximum versus 0.77 Ω before the trip. A circuit that was marginal on voltage drop before a fault can become worse after one.

2. The five ratings that matter

A concrete example is the RXEF050: 0.50 A hold, 1.00 A trip, 72 V maximum, 40 A maximum fault current, 0.77 W dissipated when tripped, and 0.50–0.77 Ω initial resistance. Those are the numbers that go into a selection decision — not the 0.50 A headline alone.

Approvals are part of the same page. The RXEF series carries UL, CSA, TÜV and CQC recognition; confirm that the approval you need is listed for the exact ordering part number, not just the family.

3. Ambient derating is the selection rule

PPTC hold current falls as ambient temperature rises, because the device needs less self-heating to reach its switching temperature. Manufacturers publish a rerating table or curve, and it is the single most important page in the datasheet.

For the RXEF050 the table reads 0.78 A at −40 °C, 0.68 A at −20 °C, 0.60 A at 0 °C, 0.50 A at 20 °C, 0.48 A at 25 °C, 0.41 A at 40 °C, 0.36 A at 50 °C, 0.32 A at 60 °C, 0.27 A at 70 °C and 0.20 A at 85 °C. At 50 °C the part delivers 75% of its 25 °C rating; at 85 °C, 42%.

The practical rule is to take the maximum ambient inside the box — not the room temperature — add the 20–30 °C rise that a sealed enclosure can add, and read the hold current at that point. Then require at least 1.2 × the load current. A 0.30 A load at 50 °C needs 0.36 A of hold current; the RXEF050 sits exactly on the line, with no margin at all, which is why the next size up is usually the right answer.

4. Trip time: how fast is “fast”?

Trip time is specified at a particular current, and it rises sharply as the current approaches I_TRIP. The RXEF050 datasheet gives a maximum time-to-trip of 4.0 s at 2.50 A. At 1.2 A the part may take much longer; at 1.0 A, it may never trip in a useful time, because that is the trip threshold and not a guaranteed clearing point.

This is the second most common selection error: treating I_TRIP as a fast-acting threshold. A PPTC is not a fuse. If the protected circuit cannot survive several seconds at, say, 1.5 × I_TRIP, the PPTC is the wrong device — an eFuse or a fast fuse is the correct choice.

5. Worked example: a 5 V, 0.30 A port at 50 °C

Take a 5 V auxiliary port that draws 0.30 A continuously, inside an enclosure that reaches 50 °C. The target is a self-resetting device that does not trip in normal operation and that limits a short circuit.

• Hold current: 0.30 A × 1.2 = 0.36 A at 50 °C. The RXEF050 is exactly 0.36 A, so the margin is zero. The RXEF065 (0.53 A at 50 °C) gives 77% margin and is the better fit.
• Voltage rating: 72 V for the series, comfortably above a 5 V rail.
• Voltage drop: at 0.30 A and 0.77 Ω maximum, the drop is 0.23 V, or 4.6% of a 5 V rail. Acceptable here; on a 3.3 V rail the same part would cost 7% and should be reconsidered.
• Dissipation: 0.30² × 0.77 = 0.07 W in normal operation — small, but it adds to the enclosure heat that caused the derating in the first place.
• Fault behaviour: at 1 A, the RXEF050 is around I_TRIP and may take a long time; at 2.5 A the specified maximum is 4.0 s. The downstream circuit must tolerate that.

That last point is the design decision, not the part number. A PPTC protects wiring and connectors from sustained overcurrent; it does not protect a sensitive device from a fast fault. If the load is a hot-plug connector with a large capacitor, the inrush itself can trip a PPTC that is correctly sized for steady state — another reason to check the load's startup profile, not just its average current.

6. PPTC vs fuse vs eFuse vs TVS

These four parts are often listed as alternatives, but they answer different questions.

A fuse is faster and cheaper per unit, but it is one-shot and must be replaced. An eFuse gives precise current limits, fast response and often voltage/thermal monitoring, at a higher cost and complexity. A TVS clamps voltage, not current, and is the right device for ESD and surge — a different fault class. The PPTC sits at the low-cost, self-resetting end: good for ports, adapters and battery packs, not for precision current limiting.

7. Package choice: radial, SMD or strap

The same polymer physics comes in three mechanical families, and the package decides where the part can be used.

The electrical selection does not change with the package, but the thermal environment does. An SMD part sitting on a dense board with a regulator nearby will run hotter than the same rating in a radial package with free air around it, and the derating table assumes still air at the stated temperature. Treat the board as part of the thermal system.

8. Two applications where the details matter

A USB charging port. A 5 V, 0.5 A port is often protected with a 0.50 A-class PPTC. At 25 °C that looks fine, but inside a plastic enclosure at 45–50 °C the derated hold current is already below 0.40 A, and a tablet that pulls 0.5 A will trip the part after a few minutes. Either move up one size or improve the airflow around the connector.

A battery pack. Strap devices are chosen for low resistance, but the inside of a pack is one of the hottest places in a product. A part rated 2 A at 25 °C may only hold 1.4 A at 60 °C. The pack's thermal test, not the datasheet headline, is the number that matters.

9. When not to use a PPTC

• Ambient above the rated range. Beyond 85 °C most parts derate to a fraction of their rating or are simply not specified.
• High inrush loads. Motors, incandescent lamps and large input capacitors can trip a PPTC that is correctly sized for steady state.
• Safety-critical circuits. A self-resetting device that reconnects automatically is not a substitute for a fuse or a breaker where the fault must stay isolated.
• Mains and high-energy faults. Use a fuse or a proper circuit breaker rated for the available fault current.
• Precise current limits. If the limit must be within ±10%, use an eFuse or a current-sense loop.
• Very low rails. The voltage drop across the part (and its post-trip resistance) can be a large fraction of a 1.8 V or 3.3 V rail.

10. Layout and thermal coupling

Because the trip is thermal, the board layout changes the trip point. Keep the PPTC away from hot components — regulators, power inductors, diodes — and give it copper and airflow where possible. A part placed next to a 2 W regulator will trip earlier than the datasheet curve suggests.

Do not put PPTCs in parallel to share current: the two parts will not share evenly, and the first to heat will take more resistance and push current to the other. The datasheet and application notes are explicit about this. If more hold current is needed, use a larger single device.

Keep the fault path short and wide. The PPTC limits current only after it heats; any series resistance or inductance between the source and the device delays the fault current and changes the trip time.

11. Five mistakes we see

1. Selecting at 25 °C. The datasheet's headline hold current is a room-temperature number; the enclosure is not 25 °C.

2. Confusing I_HOLD with I_TRIP. A part that holds 0.50 A does not trip at 0.50 A; the trip threshold is 1.00 A on the RXEF050.

3. Expecting fuse-like speed. Seconds are normal for a PPTC; if the circuit needs milliseconds, it is the wrong device.

4. Ignoring inrush. A motor or a large capacitor can trip the part at power-on even when the steady-state current is well inside the hold rating.

5. Forgetting post-trip resistance. After a fault the part may sit at 1.17 Ω instead of 0.77 Ω, which changes the voltage drop and the heat for the rest of its life.

12. Selection checklist

• Maximum load current, including tolerance and transients.
• Maximum ambient inside the enclosure, with the enclosure rise added.
• Derated hold current at that ambient; require ≥ 1.2 × load.
• Trip current and trip time at the fault current you expect.
• Maximum voltage and maximum fault current.
• Initial and post-trip resistance; voltage drop on the rail.
• Inrush profile of the load.
• Thermal placement: distance from hot parts, copper area, airflow.
• Whether the fault must stay isolated (fuse/breaker instead).
• Verification: trip test at the real ambient, not on the bench.

FAQ

Can I replace a fuse with a PPTC?

Only if the fault is a sustained overcurrent that the circuit can tolerate for seconds, and if automatic reconnection is acceptable. For safety-critical isolation or fast faults, keep the fuse or use an eFuse.

Why does my PPTC trip at a lower current than the datasheet says?

Because the datasheet hold current is specified at a reference temperature, usually 20–25 °C, in still air. A part inside a warm enclosure or next to a hot component sees a higher effective ambient and trips earlier.

What is the difference between hold and trip current?

Hold current is the maximum the part passes without tripping; trip current is the minimum that will switch it to the high-resistance state. Between the two, behaviour depends on time, ambient and heat sinking.

How long does a PPTC take to reset?

It resets when it cools below the switching temperature — seconds to minutes depending on the part, the current and the airflow. It is not instantaneous.

Can I put two PPTCs in parallel for more current?

No. They will not share current evenly, and the first to heat pushes more current into the other. Use a single larger device.

How do I validate a PPTC in the real enclosure?

Run the product at maximum load in the final enclosure until temperatures stabilise, then measure the current through the part and the temperature next to it. If the part is already warm at steady state, derate again — the datasheet curve assumes still air, not a closed box next to a regulator.

Does the trip current derate as well?

Yes. Datasheets usually publish the hold-current rerating table; trip current and trip time also shift with temperature. Do not assume the 23 °C trip current applies at 60 °C — read the curve or ask the supplier for the hot trip data.

Does a PPTC protect against overvoltage?

No. It responds to current and temperature. Overvoltage protection is a TVS or a clamp; the two are complementary, not interchangeable.

Glossary

I_HOLD

Maximum current the device passes without tripping at the reference temperature.

I_TRIP

Minimum current that switches the device to its high-resistance state.

V_MAX

Maximum continuous voltage the device can withstand at rated current.

I_MAX

Maximum fault current the device can interrupt.

P_D

Power dissipated by the device in the tripped state.

Post-trip resistance

Resistance measured one hour after a trip; higher than the initial value.

Sources

• Littelfuse RXEF series datasheet — RXEF050 ratings and the temperature rerating table.
• Eaton, PTC resettable fuse application guidelines (Technical Note 11055) — hold/trip definitions, derating, parallel-device warning, 12 V PLC example.
• Littelfuse PolySwitch product family — device series and applications.