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#519 PPTC Performance

Testing the performance of a few PolySwitch polymeric positive temperature coefficient (PPTC) resettable fuses.



I’m planning to put a resettable fuse in a circuit, but it dawns on me I’ve never actually seen one fail. So rather than just take it on faith, let’s take a closer look at some resettable fuses and see how they behave under test.

SMD resettable fuses all seem to be of the polymeric positive temperature coefficient (PPTC) variety. They rely on the heat generated by a high current to raise the effective resistance of the fuse, thus throttling current.

I have some cheap fuses from aliexpress to test (200mA, 500mA 30V), but they are all “no brand” and without datasheets. Looking around at some of the leading manufacturers, I see that LittelFuse for one publish a very good set of information to go with their products.

Some key behaviours and characteristics that distinguish PPTCs from traditional fuses:

  • they are resettable (duh), but recovery may take time (for it to cool)
  • leakage current: when tripped, current is throttled but not completely cut unlike a fuse. This may or may not suit the application.
  • operating current: fuses are availbale to operating currents much higher than can be handled by PPTCs
  • packaging: available in smaller SMD packages (typically 1206)
  • operating temperature: PPTCs typically have lower maximum operating temperature than fuses, and performance changes much more significantly over the operating temperature range


I put a couple of PPTC fuses on a DIP adapter and wired up to a USB dummy load, all powered by a bench power supply with constant voltage/current and over-current limit (as a fallback in case the fuses don’t work as advertised!)




200mA PPTC Performance

With power supply set to 5V with 0.6A CC limit (OCP: 1A), engaging the 4.7Ω load triggers the 200mA PPTC.

The fuse cuts-in within 20ms, but during this time current is allowed to peks at about 400mA. It takes about 1.2s before current is throttled back to about 180mA.

At these voltage/current levels, the fuse resets in under a second.

scope_200_fine scope_200_coarse

500mA PPTC Performance

With power supply set to 5V with 1.5A CC limit (OCP: 2A), engaging the 2.2Ω load triggers the 500mA PPTC.

The fuse cuts-in within 20ms, but during this time current is allowed to peks at about 1.2A. It takes about 1.8s before current is throttled back to about 500mA.

At these voltage/current levels, the fuse resets in under a second.

scope_500_coarse scope_500_fine


So PPTC fuses - at least the ones I’m testing - are definitely not “fast blow”. They do allow currents in excess of their rating to pass for over a second before throttling back. This may be enough to prevent the worst damage/safety issues, but not enough to protect sensitive components.

The performance in these tests may have been affected by the relatively low power. The fuses are rated for 30V, so I presume they would “blow” faster if I was running at voltages higher than the 4/5V used in the test (i.e. higher power/more heat produced faster).

For better protection, a fuse and PPTC combination is probably a good idea:

  • the PPTC to provide resettable protection for moderate over-current situations
  • and a (higher value) fast acting fuse to quickly kill dead shorts

Credits and References

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This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.