#519 PPTC Performance

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

Build

Notes

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

Construction

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!)

PPTCPerformance_test

Breadboard

Schematic

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

Conclusions

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

PolySwitch 0.2A 30V 1206 - a “no brand name” from aliexpress
PolySwitch 0.5A 30V 1206 - a “no brand name” from aliexpress
Resettable fuse - wikipedia
LittelFuse PPTC information
PTC Fuses - Weird Component #3 - Elektor issue 3/2014