Like traditional fuses, PPTC fuses can protect electronics from overcurrent conditions but with the added advantage of automatically resetting themselves once the fault has been removed. They are pretty cheap and fairly easy to use if you understand how they work.
PPTC fuses are not really fuses, rather they are non-linear thermistors. PPTC stands for Polymeric Positive Temperature Coefficient, and this gives a clue as to how they work. They are constructed from an organic polymer material laced with conductive particles. At low temperatures the polymer assumes a crystalline structure and the conductive particles are forces into the spaces between the crystals, forming conductive pathways through the bulk material. Typical PPTC fuses have a cold resistance in the milliohms to low ohms range.
If the current through the material is sufficient, the material heats up and the polymer transforms to the amorphous state breaking up the conductive pathways and raising the resistance considerably. Typically, the hot resistance can be three or more orders of magnitude higher than the cold resistance. The current therefore reduces to a much lower level (although not zero) keeping the device in the amorphous state.
Once the current is removed, the PPTC fuse cools and reverts to its low-resistance state. Figure 1 shows the resistance-temperature characteristic for a typical PPTC fuse. The line represents the thermal equilibrium state where the internal I2R heating is balanced with the heat loss from device to ambient. The curve is relatively flat in both the low and high resistance regions, but transitions steeply between them over some critical temperature region.
This diagram shows the resistance-temperature characteristic of a typical PPTC fuse. In the on-state, the PPTC has a resistance in the milliohm to ohm range, while in the tripped state this increases three or four orders of magnitude. This change in resistance occurs due to physical changes in the structure of the polymer material of the device.
When selecting PPTCs and designing circuits with them it is necessary to be aware of a few critical parameters: IHOLD is the maximum continuous current that the device can conduct without tripping, ITRIP is the minimum continuous current that is guaranteed to cause a trip and VMAX is the maximum operating voltage.
Let’s consider a practical example. Figure 2 shows a simple reverse-polarity protection circuit for a typical “wall-wart” powered device. Under normal conditions, the diode is reverse biased, and the voltage drop across the PPTC is very small. If the supply is reversed, the diode provides a short-circuit path and the PPTC limits the reverse current to a safe level. Let’s assume that the load requires a maximum of 400mA under normal operating conditions, the input supply is a nominal 12V, the maximum ambient temperature inside the device is 55°C.
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This is a simple reverse-polarity protection circuit used as an example in the text. Under normal conditions the diode is reverse-biased and the PPTC exhibits a low resistance for minimal voltage drop. If the input polarity is reversed, the diode conducts and the PPTC fuse trips to limit the diode current to a safe level.
In broad strokes, therefore, we need a PPTC with VMAX ≥ 16V and IHOLD ≥ 400mA. Looking at the selection guide from Littlefuse (one of the several manufacturers of PPTCs) we see that the 2016L range looks to be in the ballpark.
Figure 3, extracted from the data sheet shows the IHOLD values for the devices in this range versus ambient temperature. Using the 60°C column (to give a little headroom above our 55°C spec) we see that the 2016L075/60 (a nominal 750mA part) is the lowest-current device that will meet our 400mA requirement. It is really important you take the operating temperature into account in this step. You can see from the table that the 500mA part (2016L050) does not have a high enough IHOLD at temperatures above 40°C.
This chart shows the IHOLD current vs temperature for various members of the Littlefuse 2016L family of PPTC fuses. We chose the 2016L075/60 device because it has an IHOLD just greater than the 0.4A we need at 55°C.
The 2016L075/60 has a VMAX of 60V, so it has more than enough voltage headroom for our purposes. Figure 4 shows that the on-resistance (Rmin) of our chosen device when cold is of the order of 0.13Ω, so the voltage drop under normal circumstances is ~50mV. Note that the PPTC will not return to this low resistance immediately after a reset. It will fall to Rmax once cooled but may take several hours to reach Rmin again.
This chart, extracted from the manufacturer’s data sheet shows that the 2016L075/60 device has a VMAX of 60V and an on resistance of 0.13Ω when cold (note it will rise to 0.9Ω after being reset and may take many hours to reduce to the original level).
We now need to check the trip characteristics to make sure we understand what will happen in the event of a short circuit. Figure 5 shows the trip current vs time characteristic of this device family; it shows that the 2016L075/60 should trip in about 0.7 seconds with a 3A source, about 2 seconds with a 2A source. We know the device is guaranteed to trip at some point (probably 10s of seconds) with 1.5A as this is the rated ITRIP. We need to choose a diode that can cope with this – I would opt for at least a 2A continuous rating since its possible that a fault current of just less than 1.5A could exist indefinitely.
This chart shows how long the PPTC will take to trip based on the current. Because PPTCs are thermal devices the trip time depends on current level and may take several seconds at currents close to the ITRIP level.
This example has, I hope, shown that PPTC fuses are very handy devices and fairly easy to use in practice. You do need to select your device carefully, especially with regard to your operating temperature range. Given the 2016L075/60 costs about 80 US cents at the time of writing so its pretty cheap insurance in an application like this.
References
“PolySwitch Resettable PTCs Devices – Littelfuse.” Accessed May 4, 2023. https://www.littelfuse.com/products/polyswitch-resettable-pptcs.aspx.
“Bourns® Multifuse® Fuses | PPTC Resettable Fuses.” Accessed May 4, 2023. https://www.bourns.com/products/circuit-protection/resettable-fuses-multifuse-pptc.
“Resettable Fuse.” In Wikipedia, April 23, 2023. https://en.wikipedia.org/w/index.php?title=Resettable_fuse&oldid=1151362860.
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Andrew Levido (andrew.levido@gmail.com) earned a bachelor’s degree in Electrical Engineering in Sydney, Australia, in 1986. He worked for several years in R&D for power electronics and telecommunication companies before moving into management roles. Andrew has maintained a hands-on interest in electronics, particularly embedded systems, power electronics, and control theory in his free time. Over the years he has written a number of articles for various electronics publications and occasionally provides consulting services as time allows.
