A polymeric positive-temperature-coefficient (PPTC) device is a resettable overcurrent protector: when excess current heats it, its resistance rises and limits current. After power is removed and the device cools, it returns toward its low-resistance state. PPTCs can help protect motors, transformers and industrial controls, but unlike a conventional fuse they do not fully disconnect the circuit during a fault.
How a PPTC resettable fuse works
A PPTC contains a conductive polymer matrix. During an overcurrent event, heat causes the matrix to expand and disrupt conductive paths, shifting the device from low resistance to a much higher-resistance, current-limiting state. To reset it, remove power and allow it to cool; its resistance then moves back toward its normal low-resistance state. The device limits current rather than acting as a complete electrical disconnect. Littelfuse explains the polymer PPTC mechanism.
Where PPTCs can protect motors and transformers
Littelfuse says its PolySwitch line-voltage-rated (LVR) devices are intended to help protect motors and transformers in commercial and home appliances from mechanical overload, overheating, stall, lost neutral and other potentially harmful conditions. The LVR datasheet also lists electromagnetic loads, industrial controls, motors, fans and blowers, transformers, intelligent appliances and robotic machines as applications. These are family-level application statements, not a guarantee that every LVR part suits every motor or transformer; match the part to the circuit and fault conditions. See the Littelfuse LVR datasheet, revised December 20, 2020.
Littelfuse LVR line-voltage options
The cited LVR datasheet describes 120 VAC/VDC and 240 VAC/VDC classes, with operating-current options up to 2 A in its table. It lists UL 1434, CSA, TÜV and CQC approvals for the family. Confirm the exact voltage, current and agency markings for the specific part: family-level ratings and approvals should not be assumed to apply identically to every model.
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Other PPTC product formats
Bourns lists transformers, small and medium electric motors, and industrial controls among Multifuse applications. Its product family includes radial-leaded, surface-mount, axial-leaded battery-strap and uncoated-disk packages. The required package depends on the board or assembly, thermal environment and electrical ratings. Bourns describes Multifuse resettable fuses and applications.
Using a PPTC in an industrial controller
A PPTC may suit a controller’s low-voltage supply rails, I/O or connected loads when those circuits can experience overloads or short circuits and automatic recovery is useful. It can reduce routine fuse replacement after a recoverable fault. Littelfuse and Bourns both identify industrial controls as an application, but that does not make a PPTC a universal substitute for a fuse or other protection scheme. Consider how much fault current remains after it trips, how the fault is cleared, and whether the device’s thermal behavior is compatible with nearby components.
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How to choose a PPTC rating
Use the selected manufacturer’s datasheet for the exact part and the real operating environment. Littelfuse’s LVR documentation includes selection fields such as maximum working voltage, hold and trip current, interrupt capability, time-to-trip and steady-state resistance; its data also provides temperature derating information. Work through these checks before choosing a device:
- Match voltage. Confirm the circuit’s normal operating voltage is within the device’s maximum working voltage. For line-voltage use, verify the precise AC or DC rating and the applicable safety approvals for the individual part.
- Set hold current for actual conditions. Choose a hold-current rating above the circuit’s normal steady-state current at the expected ambient temperature. Apply the manufacturer’s temperature-rerating curve; a nominal current figure alone may not be suitable at elevated temperatures.
- Check trip current and trip time. Compare the fault current and the time the circuit can safely tolerate it with the device’s trip behavior. Account for startup or other normal current surges to avoid nuisance trips.
- Verify fault and interrupt capability. Check the maximum credible fault conditions against the device’s interrupt rating and ensure the circuit can tolerate the residual current that remains after the PPTC enters its high-resistance state.
- Review resistance and physical fit. Compare initial or steady-state resistance, package and lead style, thermal placement, and required creepage and clearance in the assembly.
- Confirm approvals on the exact part. Where relevant, verify the part’s actual UL 1434, CSA, TÜV or CQC markings rather than relying only on a family-level description.
PPTC versus a conventional fuse
The main difference is what happens after the protection operates. A conventional fuse is generally preferred when the design requires a genuinely open circuit and zero fault current. A PPTC instead remains a high-resistance path, so leakage current can continue through the load during the fault. Bel highlights this distinction in its discussion of PTC fuses. Read Bel Fuse’s technical article on PTC resettable fuses.
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Choose a PPTC when automatic recovery is valuable, access is difficult, or recoverable overloads are expected—and only when its residual current, thermal conditions and clearing behavior are acceptable to the equipment. Choose a conventional fuse when fault isolation is essential. Neither choice removes the need to diagnose the underlying fault before restoring operation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why a single PPTC rating cannot fit every circuit
Ratings are specific to the product and test conditions. For example, Bel’s 0ZCM examples have specified hold-current values from 50 to 200 mA and trip-current values from 150 to 450 mA; the article reports a maximum 0.1-second trip time under its stated test conditions. Those figures describe those examples, not a general PPTC capability or a rating for a motor, transformer or controller without further design checks.
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